• switcher ringing noise

    From John Larkin@21:1/5 to All on Fri Mar 11 11:39:10 2022
    I used to love the LTM8078 dual switcher module. But it rings hard at
    around 400 MHz at every switch transition. This is called a "Silent
    Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any
    effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    --

    If a man will begin with certainties, he shall end with doubts,
    but if he will be content to begin with doubts he shall end in certainties. Francis Bacon

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jan Panteltje@21:1/5 to jlarkin@highland_atwork_technology. on Fri Mar 11 20:38:18 2022
    On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin <jlarkin@highland_atwork_technology.com> wrote in <h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>:

    I used to love the LTM8078 dual switcher module. But it rings hard at
    around 400 MHz at every switch transition. This is called a "Silent >Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any
    effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    Is the 10 nF 30 Ohm parallel to the diode a damping network?
    Use a series LC there tuned to 50 kHz to short it?

    That said I do not rememebr those oscillations
    tried a different make inductor?

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to John Larkin on Fri Mar 11 15:35:08 2022
    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings hard at
    around 400 MHz at every switch transition. This is called a "Silent Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any
    effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!


    In discontinuous current mode, an asynchronous switcher will produce EMI
    at the free resonance of the inductor. If you don't mind the
    efficiency hit at low current, a diode + RC snubber would probably fix it.

    We've started putting U.FL coax jacks on all out power supply outputs,
    so we can figure out what's conducted and what's pickup. Helps a lot.
    (They don't get populated except on first articles, but they're the size
    of a SOT23, so nobody cares.)

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Larkin@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Mar 11 13:12:41 2022
    On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings hard at
    around 400 MHz at every switch transition. This is called a "Silent
    Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any
    effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!


    In discontinuous current mode, an asynchronous switcher will produce EMI
    at the free resonance of the inductor. If you don't mind the
    efficiency hit at low current, a diode + RC snubber would probably fix it.

    The LTM is a synchronous switcher, and my 2576 is running continuous.

    Looking at the timings on by breadboard, the rings seem to start at
    the big di/dt current transitions in the schottky. But nothing we can
    do changes the ring frequency, so what's resonating?


    We've started putting U.FL coax jacks on all out power supply outputs,
    so we can figure out what's conducted and what's pickup. Helps a lot.
    (They don't get populated except on first articles, but they're the size
    of a SOT23, so nobody cares.)

    That's a good idea. We should also always include some way to measure
    currents.

    We have some cute little loop-antenna scope probes. We can park one
    above an LTM module and get a scope trigger from its internal
    inductor, then signal average our noisy signals and see which noise
    correlates to which switcher.


    Cheers

    Phil Hobbs
    --

    If a man will begin with certainties, he shall end with doubts,
    but if he will be content to begin with doubts he shall end in certainties. Francis Bacon

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Larkin@21:1/5 to pNaonStpealmtje@yahoo.com on Fri Mar 11 13:29:42 2022
    On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje
    <pNaonStpealmtje@yahoo.com> wrote:

    On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin ><jlarkin@highland_atwork_technology.com> wrote in ><h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>:

    I used to love the LTM8078 dual switcher module. But it rings hard at >>around 400 MHz at every switch transition. This is called a "Silent >>Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    Is the 10 nF 30 Ohm parallel to the diode a damping network?

    Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1.

    Use a series LC there tuned to 50 kHz to short it?

    The problem isn't at 50 KHz, it's the fast ringing on both switching
    edges.


    That said I do not rememebr those oscillations
    tried a different make inductor?

    This wouldn't normally be noticed. It's tens of mV rings at 40 or 400
    MHz. It's beyond the frequency ranges of the visible components.

    I guess we'll dump the LTM things and go with old, slow switchers, and
    then try to physically segregate them as much as possible, and add a
    lot of secondary filtering. Create clean and dirty zones on the board,
    draw a boundary line, and filter the power sigs that cross the line.
    That might work better for small 40 MHz nasties than for big 400s.

    But what's resonating? It doesn't seem to be the pcb itself.

    I thought we might have a guard-ring-SRD snap in the schottky diode,
    but any diode does it, and it rings on both switching edges.


    --

    If a man will begin with certainties, he shall end with doubts,
    but if he will be content to begin with doubts he shall end in certainties. Francis Bacon

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to John Larkin on Fri Mar 11 18:22:42 2022
    John Larkin wrote:
    On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje
    <pNaonStpealmtje@yahoo.com> wrote:

    On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin
    <jlarkin@highland_atwork_technology.com> wrote in
    <h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>:

    I used to love the LTM8078 dual switcher module. But it rings hard at
    around 400 MHz at every switch transition. This is called a "Silent
    Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any
    effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>
    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    Is the 10 nF 30 Ohm parallel to the diode a damping network?

    Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1.

    Use a series LC there tuned to 50 kHz to short it?

    The problem isn't at 50 KHz, it's the fast ringing on both switching
    edges.


    That said I do not rememebr those oscillations
    tried a different make inductor?

    This wouldn't normally be noticed. It's tens of mV rings at 40 or 400
    MHz. It's beyond the frequency ranges of the visible components.

    I guess we'll dump the LTM things and go with old, slow switchers, and
    then try to physically segregate them as much as possible, and add a
    lot of secondary filtering. Create clean and dirty zones on the board,
    draw a boundary line, and filter the power sigs that cross the line.
    That might work better for small 40 MHz nasties than for big 400s.

    But what's resonating? It doesn't seem to be the pcb itself.

    I thought we might have a guard-ring-SRD snap in the schottky diode,
    but any diode does it, and it rings on both switching edges.



    I hear you.

    Awhile back we did a small power supply board, in an effort to factor
    out the noisy stuff and put it inside a shield, so that we could
    concentrate on what we care about.

    It used a TI LMR23630AFDDAR (clocked at 2.15 MHz) to make +13 from +24,
    which was then inverted by an AOZ1282 to make -16. The other rails were
    made using linears off those ones or off the +24 directly. (Making -16
    from +24 is a bit of a strain for most integrated buck regulator chips
    that can go faster than 2 MHz.)

    It worked fine until we turned on the AOZ1282, at which point the whole
    board became a mass of VHF uglies. The thing was, everything was some
    high harmonic of the 2.15 MHz clock synchronizing the TI chip, selected
    by microstrip stub resonances in the traces. We had 118 MHz ringing
    here, 183 MHz there, all initially very mysterious. Never did work right.

    We've had good success with the 150 kHz Simple Switchers, e.g. the
    LM2594, using powdered-iron toroids and B340A Schottky catch diodes.
    Our QL01 nanowatt photoreceiver has one of those within a couple of
    inches of a very sensitive 10 megohm TIA with a 1 MHz BW, and the
    switching junk is invisible on the output even using a spectrum analyzer
    with a 10-Hz resolution bandwidth. But even that one has issues with
    ground integrity--if the board doesn't make good contact with the box
    ground, low-level harmonics of 150 kHz start showing up.

    At this point we've decided we don't want to be power supply designers,
    so we use the 2W Murata gizmos with the embedded toroids, inside a
    board-level steel shield, with the whole works inside a brass or
    aluminum box with a laser-cut lid. (Laser cutting has recently become monstrous cheap--we pay about $2 per lid in quantity 10, with four-day turnaound.)

    Those U.FL connectors are super useful in distinguishing between stuff
    that our boards are doing and stuff that comes in over the air. The
    amount of tail-chasing they save is astronomical.

    Cheers

    Phil Hobbs






    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Mar 11 19:06:40 2022
    On Fri, 11 Mar 2022 18:22:42 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje
    <pNaonStpealmtje@yahoo.com> wrote:

    On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin >>> <jlarkin@highland_atwork_technology.com> wrote in
    <h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>:

    I used to love the LTM8078 dual switcher module. But it rings hard at
    around 400 MHz at every switch transition. This is called a "Silent
    Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>> It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any
    effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>>
    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    Is the 10 nF 30 Ohm parallel to the diode a damping network?

    Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1.

    Use a series LC there tuned to 50 kHz to short it?

    The problem isn't at 50 KHz, it's the fast ringing on both switching
    edges.


    That said I do not rememebr those oscillations
    tried a different make inductor?

    This wouldn't normally be noticed. It's tens of mV rings at 40 or 400
    MHz. It's beyond the frequency ranges of the visible components.

    I guess we'll dump the LTM things and go with old, slow switchers, and
    then try to physically segregate them as much as possible, and add a
    lot of secondary filtering. Create clean and dirty zones on the board,
    draw a boundary line, and filter the power sigs that cross the line.
    That might work better for small 40 MHz nasties than for big 400s.

    But what's resonating? It doesn't seem to be the pcb itself.

    I thought we might have a guard-ring-SRD snap in the schottky diode,
    but any diode does it, and it rings on both switching edges.



    I hear you.

    Awhile back we did a small power supply board, in an effort to factor
    out the noisy stuff and put it inside a shield, so that we could
    concentrate on what we care about.

    It used a TI LMR23630AFDDAR (clocked at 2.15 MHz) to make +13 from +24,
    which was then inverted by an AOZ1282 to make -16. The other rails were
    made using linears off those ones or off the +24 directly. (Making -16
    from +24 is a bit of a strain for most integrated buck regulator chips
    that can go faster than 2 MHz.)

    It worked fine until we turned on the AOZ1282, at which point the whole
    board became a mass of VHF uglies. The thing was, everything was some
    high harmonic of the 2.15 MHz clock synchronizing the TI chip, selected
    by microstrip stub resonances in the traces. We had 118 MHz ringing
    here, 183 MHz there, all initially very mysterious. Never did work right.

    We've had good success with the 150 kHz Simple Switchers, e.g. the
    LM2594, using powdered-iron toroids and B340A Schottky catch diodes.
    Our QL01 nanowatt photoreceiver has one of those within a couple of
    inches of a very sensitive 10 megohm TIA with a 1 MHz BW, and the
    switching junk is invisible on the output even using a spectrum analyzer
    with a 10-Hz resolution bandwidth. But even that one has issues with
    ground integrity--if the board doesn't make good contact with the box
    ground, low-level harmonics of 150 kHz start showing up.

    At this point we've decided we don't want to be power supply designers,
    so we use the 2W Murata gizmos with the embedded toroids, inside a >board-level steel shield, with the whole works inside a brass or
    aluminum box with a laser-cut lid. (Laser cutting has recently become >monstrous cheap--we pay about $2 per lid in quantity 10, with four-day >turnaound.)

    Those U.FL connectors are super useful in distinguishing between stuff
    that our boards are doing and stuff that comes in over the air. The
    amount of tail-chasing they save is astronomical.

    Cheers

    Phil Hobbs

    We might make provision for one of those Laird shield boxes, just in
    case.

    The next challange is to soft-start the +24 to +5 switcher. Those old
    parts just grunt at startup. The +24 supply has to deliver 1 amp to
    pull up a 1 amp load. The LM2576 has an enable pin, but it's not a
    soft start.

    A time delay and huge amount of bulk capacitance on +24 is one way to
    do it. I have, I think, seven various goofy ideas for sorta or
    actually soft-starting this beast. Without a Spice model, I'll just
    have to try them.



    --

    I yam what I yam - Popeye

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jan Panteltje@21:1/5 to jlarkin@highland_atwork_technology. on Sat Mar 12 08:04:02 2022
    On a sunny day (Fri, 11 Mar 2022 13:29:42 -0800) it happened John Larkin <jlarkin@highland_atwork_technology.com> wrote in <oren2h57dsrckacrocvdu0u9vhlm7v3dta@4ax.com>:

    On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje
    <pNaonStpealmtje@yahoo.com> wrote:

    On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin >><jlarkin@highland_atwork_technology.com> wrote in >><h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>:

    I used to love the LTM8078 dual switcher module. But it rings hard at >>>around 400 MHz at every switch transition. This is called a "Silent >>>Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>effect on the ringing frequency.
    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>
    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    Is the 10 nF 30 Ohm parallel to the diode a damping network?

    Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1.

    Use a series LC there tuned to 50 kHz to short it?

    The problem isn't at 50 KHz, it's the fast ringing on both switching
    edges.

    Yes of course I did mean that 40 MHz



    That said I do not rememebr those oscillations
    tried a different make inductor?

    This wouldn't normally be noticed. It's tens of mV rings at 40 or 400
    MHz. It's beyond the frequency ranges of the visible components.

    I guess we'll dump the LTM things and go with old, slow switchers, and
    then try to physically segregate them as much as possible, and add a
    lot of secondary filtering. Create clean and dirty zones on the board,
    draw a boundary line, and filter the power sigs that cross the line.
    That might work better for small 40 MHz nasties than for big 400s.

    But what's resonating? It doesn't seem to be the pcb itself.

    It is likely the coil !

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Martin Brown@21:1/5 to John Larkin on Sat Mar 12 09:05:02 2022
    On 11/03/2022 21:12, John Larkin wrote:
    On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings hard at
    around 400 MHz at every switch transition. This is called a "Silent
    Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any
    effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>
    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    In discontinuous current mode, an asynchronous switcher will produce EMI
    at the free resonance of the inductor. If you don't mind the
    efficiency hit at low current, a diode + RC snubber would probably fix it.

    The LTM is a synchronous switcher, and my 2576 is running continuous.

    Looking at the timings on by breadboard, the rings seem to start at
    the big di/dt current transitions in the schottky. But nothing we can
    do changes the ring frequency, so what's resonating?

    They will be immediately after the discontinuity aka Gibb's phenomena on
    a truncated Fourier expansion for a square wave. It may not be a
    resonance as such but a side effect of the slew rate limit of the
    device. It doesn't die away quickly enough to be just that though.

    There is a hard high frequency cutoff in gain and some ringing is pretty
    much what you would expect on a square wave with a truncated Fourier
    expansion. It may be being exaggerated in time and amplitude by some unfortunate choice of component values providing Q > 1 in addition.

    As Phil said some sort of snubber would be the most likely amelioration.
    There will be an efficiency hit though so you have to choose how quiet
    you need it vs what losses you can live with.

    --
    Regards,
    Martin Brown

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jan Panteltje@21:1/5 to All on Sat Mar 12 09:42:04 2022
    It is likely the coil !

    Look at the coil construction, it is much like a 30 MHz or there about air coil I often use.
    Probably at that high MHz frequency the far away core material is ignored by them electrons.
    Leaves turns and circuit capacitance for tuning.
    I use these kind of coils, less spacing between winding and core material:
    http://panteltje.com/pub/LM2596_3.3_as_current_source_test_setup_2_IMG_5225.JPG

    I could be wrong, give it a try?

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Stefan Schidl@21:1/5 to John Larkin on Sat Mar 12 04:55:36 2022
    On Friday, March 11, 2022 at 10:29:57 PM UTC+1, John Larkin wrote:
    But what's resonating? It doesn't seem to be the pcb itself.

    It is likely that the parasitic inductance of the diode/high-side switch (L_par) resonates with the switching node capacitance (C_par). In order that your dampening network works the resistors must be about R=sqrt(L_par/C_par). My gut feeling is, that
    your dampening resistor is too high to work sufficiently. At typical low inductance layouts one lands in the range of 2...10R for the damping resistors these days. If the resistor is chosen too high or too low the effect on the ringing is very small.



    Best regards
    Stefan

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From legg@21:1/5 to jlarkin@highland_atwork_technology. on Sat Mar 12 10:32:51 2022
    On Fri, 11 Mar 2022 11:39:10 -0800, John Larkin <jlarkin@highland_atwork_technology.com> wrote:

    I used to love the LTM8078 dual switcher module. But it rings hard at
    around 400 MHz at every switch transition. This is called a "Silent >Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any
    effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!



    Check noise effect when scope probe/ground lead is removed/replaced/manipulated. Above 20mhZ, it's going
    to be radiated.

    More ground bonds to PC ground backing near IC, on both ground
    plane edges, where cut by power train.

    Move your ceramic decoupling caps closer to the IC body tab.
    Shuffle the polymer/ceramic positions, so both work in tandem.

    Same with schottky and it's snubber. Take output gound out of
    switching current loop.

    SchottKy RC R too big? small? Cap on flying node - R to ground plane.

    Move Noise monitors closer to filtered nodes, or filtered nodes
    closer to noise monitors. Bare leads feeding sheilded coax? I arsk
    yer!

    RL

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to '''newspam'''@nonad.co.uk on Sat Mar 12 07:21:47 2022
    On Sat, 12 Mar 2022 09:05:02 +0000, Martin Brown
    <'''newspam'''@nonad.co.uk> wrote:

    On 11/03/2022 21:12, John Larkin wrote:
    On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings hard at
    around 400 MHz at every switch transition. This is called a "Silent
    Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>> It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any
    effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>>
    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    In discontinuous current mode, an asynchronous switcher will produce EMI >>> at the free resonance of the inductor. If you don't mind the
    efficiency hit at low current, a diode + RC snubber would probably fix it. >>
    The LTM is a synchronous switcher, and my 2576 is running continuous.

    Looking at the timings on by breadboard, the rings seem to start at
    the big di/dt current transitions in the schottky. But nothing we can
    do changes the ring frequency, so what's resonating?

    They will be immediately after the discontinuity aka Gibb's phenomena on
    a truncated Fourier expansion for a square wave. It may not be a
    resonance as such but a side effect of the slew rate limit of the
    device. It doesn't die away quickly enough to be just that though.

    There is a hard high frequency cutoff in gain and some ringing is pretty
    much what you would expect on a square wave with a truncated Fourier >expansion. It may be being exaggerated in time and amplitude by some >unfortunate choice of component values providing Q > 1 in addition.

    As Phil said some sort of snubber would be the most likely amelioration. >There will be an efficiency hit though so you have to choose how quiet
    you need it vs what losses you can live with.

    There is an RC snubber to ground... see my schematic. The R value is
    about optimized, and the overall effect is a very modest reduction in
    the ringing amplitude, no visible effect on the ring frequency or Q.

    I can find only one thing that has any effect on the ringing
    frequency: the +24 input voltage. Higher voltage results in a very
    slight increase in ring frequency.

    It's Saturday, but I might go in and play with it for a couple more
    hours. I need to be in that part of town anyhow. It's better commute
    on Saturday.

    It's probably good enough, with layout improvements and secondary
    filtering, but it's interesting and annoying.

    Next issue is soft-starting this old beast, so the system always comes
    up. The 24v supply will be a wart type thing. We'll have a Cuk
    converter to make +24 into -5, and that chip soft starts. My part, +24
    to +5, doesn't.

    I could let the Cuk start up, sense its output, and then start up my
    LM2576... somehow. The "enable" pin is just on/off, so any soft start
    would probably involve the fb pin. Nuisance.





    --

    I yam what I yam - Popeye

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Rick C@21:1/5 to jla...@highlandsniptechnology.com on Sat Mar 12 09:09:40 2022
    On Saturday, March 12, 2022 at 10:22:01 AM UTC-5, jla...@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 09:05:02 +0000, Martin Brown
    <'''newspam'''@nonad.co.uk> wrote:

    On 11/03/2022 21:12, John Larkin wrote:
    On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs
    <pcdhSpamM...@electrooptical.net> wrote:

    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings hard at >>>> around 400 MHz at every switch transition. This is called a "Silent >>>> Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>> It switches at 50 KHz. And at every switching edge, it rings at about >>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    In discontinuous current mode, an asynchronous switcher will produce EMI >>> at the free resonance of the inductor. If you don't mind the
    efficiency hit at low current, a diode + RC snubber would probably fix it.

    The LTM is a synchronous switcher, and my 2576 is running continuous.

    Looking at the timings on by breadboard, the rings seem to start at
    the big di/dt current transitions in the schottky. But nothing we can
    do changes the ring frequency, so what's resonating?

    They will be immediately after the discontinuity aka Gibb's phenomena on
    a truncated Fourier expansion for a square wave. It may not be a
    resonance as such but a side effect of the slew rate limit of the
    device. It doesn't die away quickly enough to be just that though.

    There is a hard high frequency cutoff in gain and some ringing is pretty >much what you would expect on a square wave with a truncated Fourier >expansion. It may be being exaggerated in time and amplitude by some >unfortunate choice of component values providing Q > 1 in addition.

    As Phil said some sort of snubber would be the most likely amelioration. >There will be an efficiency hit though so you have to choose how quiet
    you need it vs what losses you can live with.
    There is an RC snubber to ground... see my schematic. The R value is
    about optimized, and the overall effect is a very modest reduction in
    the ringing amplitude, no visible effect on the ring frequency or Q.

    I can find only one thing that has any effect on the ringing
    frequency: the +24 input voltage. Higher voltage results in a very
    slight increase in ring frequency.

    It's Saturday, but I might go in and play with it for a couple more
    hours. I need to be in that part of town anyhow. It's better commute
    on Saturday.

    It's probably good enough, with layout improvements and secondary
    filtering, but it's interesting and annoying.

    Next issue is soft-starting this old beast, so the system always comes
    up. The 24v supply will be a wart type thing. We'll have a Cuk
    converter to make +24 into -5, and that chip soft starts. My part, +24
    to +5, doesn't.

    I could let the Cuk start up, sense its output, and then start up my LM2576... somehow. The "enable" pin is just on/off, so any soft start
    would probably involve the fb pin. Nuisance.

    If the oscillations are in the coil self-resonance wouldn't it make sense to put the snubber across the coil? Or put one on each end of the coil? Snubbing one end of the coil to ground isn't going to stop the current or voltage of the ringing across
    the coil.

    --

    Rick C.

    - Get 1,000 miles of free Supercharging
    - Tesla referral code - https://ts.la/richard11209

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to legg on Sat Mar 12 08:18:34 2022
    On Sat, 12 Mar 2022 10:32:51 -0500, legg <legg@nospam.magma.ca> wrote:

    On Fri, 11 Mar 2022 11:39:10 -0800, John Larkin ><jlarkin@highland_atwork_technology.com> wrote:

    I used to love the LTM8078 dual switcher module. But it rings hard at >>around 400 MHz at every switch transition. This is called a "Silent >>Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!



    Check noise effect when scope probe/ground lead is >removed/replaced/manipulated. Above 20mhZ, it's going
    to be radiated.

    The input and output monitors are coax. I'm monitoring the switch node
    with a 10x scope probe. Removing the probe has no effect on the 40 MHz
    ring on the output.


    More ground bonds to PC ground backing near IC, on both ground
    plane edges, where cut by power train.

    The bottom is all ground. Various jumpers/plier grabs/kluges to the
    ground have zero effect on the ring.


    Move your ceramic decoupling caps closer to the IC body tab.
    Shuffle the polymer/ceramic positions, so both work in tandem.

    Same with schottky and it's snubber. Take output gound out of
    switching current loop.

    SchottKy RC R too big? small? Cap on flying node - R to ground plane.

    Different schottkies, or parallel schottkies, have no effect.

    I don't think swapping the RC in the damper would affect 40 MHz.


    Move Noise monitors closer to filtered nodes, or filtered nodes
    closer to noise monitors. Bare leads feeding sheilded coax? I arsk
    yer!

    An inch of wire flat on a ground plane won't have any effect at 40
    MHz. It's only a 500 MHz scope.

    The scope is hi-Z. It won't allow 50r and AC coupling. I might go to
    50r with an external DC block. The cables might be ringing. Or I can
    change cable lengths and see what happens. It would be great if the
    mysterious ringing is the cables, but it feels unlikely.



    RL


    --

    I yam what I yam - Popeye

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to jlarkin@highlandsniptechnology.com on Sat Mar 12 13:05:05 2022
    jlarkin@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 09:05:02 +0000, Martin Brown
    <'''newspam'''@nonad.co.uk> wrote:

    On 11/03/2022 21:12, John Larkin wrote:
    On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>> around 400 MHz at every switch transition. This is called a "Silent
    Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>>> It switches at 50 KHz. And at every switching edge, it rings at about >>>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>>>
    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    In discontinuous current mode, an asynchronous switcher will produce EMI >>>> at the free resonance of the inductor. If you don't mind the
    efficiency hit at low current, a diode + RC snubber would probably fix it. >>>
    The LTM is a synchronous switcher, and my 2576 is running continuous.

    Looking at the timings on by breadboard, the rings seem to start at
    the big di/dt current transitions in the schottky. But nothing we can
    do changes the ring frequency, so what's resonating?

    They will be immediately after the discontinuity aka Gibb's phenomena on
    a truncated Fourier expansion for a square wave. It may not be a
    resonance as such but a side effect of the slew rate limit of the
    device. It doesn't die away quickly enough to be just that though.

    There is a hard high frequency cutoff in gain and some ringing is pretty
    much what you would expect on a square wave with a truncated Fourier
    expansion. It may be being exaggerated in time and amplitude by some
    unfortunate choice of component values providing Q > 1 in addition.

    As Phil said some sort of snubber would be the most likely amelioration.
    There will be an efficiency hit though so you have to choose how quiet
    you need it vs what losses you can live with.

    There is an RC snubber to ground... see my schematic. The R value is
    about optimized, and the overall effect is a very modest reduction in
    the ringing amplitude, no visible effect on the ring frequency or Q.

    I can find only one thing that has any effect on the ringing
    frequency: the +24 input voltage. Higher voltage results in a very
    slight increase in ring frequency.

    It's Saturday, but I might go in and play with it for a couple more
    hours. I need to be in that part of town anyhow. It's better commute
    on Saturday.

    It's probably good enough, with layout improvements and secondary
    filtering, but it's interesting and annoying.

    Next issue is soft-starting this old beast, so the system always comes
    up. The 24v supply will be a wart type thing. We'll have a Cuk
    converter to make +24 into -5, and that chip soft starts. My part, +24
    to +5, doesn't.

    I could let the Cuk start up, sense its output, and then start up my LM2576... somehow. The "enable" pin is just on/off, so any soft start
    would probably involve the fb pin. Nuisance.

    Another approach is to precharge the output cap before enabling the
    switcher.

    Cheers

    Phil Hobbs


    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Martin Brown on Sat Mar 12 13:03:27 2022
    Martin Brown wrote:
    On 11/03/2022 21:12, John Larkin wrote:
    On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings
    hard at around 400 MHz at every switch transition. This is
    called a "Silent Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar
    LM2576. It switches at 50 KHz. And at every switching edge, it
    rings at about 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far
    has any effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>>



    The damper on the 2576 circuit reduces ring amplitude a
    little.


    Maybe all switchers do this!

    In discontinuous current mode, an asynchronous switcher will
    produce EMI at the free resonance of the inductor. If you
    don't mind the efficiency hit at low current, a diode + RC
    snubber would probably fix it.

    The LTM is a synchronous switcher, and my 2576 is running
    continuous.

    Looking at the timings on by breadboard, the rings seem to start at
    the big di/dt current transitions in the schottky. But nothing we
    can do changes the ring frequency, so what's resonating?

    They will be immediately after the discontinuity aka Gibb's
    phenomena on a truncated Fourier expansion for a square wave.

    You don't get Gibbs' ears on just any square wave--you have to use the
    wrong filter. ;)


    It may not be a resonance as such but a side effect of the slew rate
    limit of the device. It doesn't die away quickly enough to be just
    that though.

    There is a hard high frequency cutoff in gain and some ringing is
    pretty much what you would expect on a square wave with a truncated
    Fourier expansion.

    There isn't, though. It's just a MOSFET, two poles at most.

    It may be being exaggerated in time and amplitude by some unfortunate
    choice of component values providing Q > 1 in addition.

    As Phil said some sort of snubber would be the most likely
    amelioration. There will be an efficiency hit though so you have to
    choose how quiet you need it vs what losses you can live with.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to jlarkin@highlandsniptechnology.com on Sat Mar 12 13:06:30 2022
    jlarkin@highlandsniptechnology.com wrote:
    On Fri, 11 Mar 2022 18:22:42 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje
    <pNaonStpealmtje@yahoo.com> wrote:

    On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin >>>> <jlarkin@highland_atwork_technology.com> wrote in
    <h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>:

    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>> around 400 MHz at every switch transition. This is called a "Silent
    Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>>> It switches at 50 KHz. And at every switching edge, it rings at about >>>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>>>
    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    Is the 10 nF 30 Ohm parallel to the diode a damping network?

    Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1.

    Use a series LC there tuned to 50 kHz to short it?

    The problem isn't at 50 KHz, it's the fast ringing on both switching
    edges.


    That said I do not rememebr those oscillations
    tried a different make inductor?

    This wouldn't normally be noticed. It's tens of mV rings at 40 or 400
    MHz. It's beyond the frequency ranges of the visible components.

    I guess we'll dump the LTM things and go with old, slow switchers, and
    then try to physically segregate them as much as possible, and add a
    lot of secondary filtering. Create clean and dirty zones on the board,
    draw a boundary line, and filter the power sigs that cross the line.
    That might work better for small 40 MHz nasties than for big 400s.

    But what's resonating? It doesn't seem to be the pcb itself.

    I thought we might have a guard-ring-SRD snap in the schottky diode,
    but any diode does it, and it rings on both switching edges.



    I hear you.

    Awhile back we did a small power supply board, in an effort to factor
    out the noisy stuff and put it inside a shield, so that we could
    concentrate on what we care about.

    It used a TI LMR23630AFDDAR (clocked at 2.15 MHz) to make +13 from +24,
    which was then inverted by an AOZ1282 to make -16. The other rails were
    made using linears off those ones or off the +24 directly. (Making -16 >>from +24 is a bit of a strain for most integrated buck regulator chips
    that can go faster than 2 MHz.)

    It worked fine until we turned on the AOZ1282, at which point the whole
    board became a mass of VHF uglies. The thing was, everything was some
    high harmonic of the 2.15 MHz clock synchronizing the TI chip, selected
    by microstrip stub resonances in the traces. We had 118 MHz ringing
    here, 183 MHz there, all initially very mysterious. Never did work right. >>
    We've had good success with the 150 kHz Simple Switchers, e.g. the
    LM2594, using powdered-iron toroids and B340A Schottky catch diodes.
    Our QL01 nanowatt photoreceiver has one of those within a couple of
    inches of a very sensitive 10 megohm TIA with a 1 MHz BW, and the
    switching junk is invisible on the output even using a spectrum analyzer
    with a 10-Hz resolution bandwidth. But even that one has issues with
    ground integrity--if the board doesn't make good contact with the box
    ground, low-level harmonics of 150 kHz start showing up.

    At this point we've decided we don't want to be power supply designers,
    so we use the 2W Murata gizmos with the embedded toroids, inside a
    board-level steel shield, with the whole works inside a brass or
    aluminum box with a laser-cut lid. (Laser cutting has recently become
    monstrous cheap--we pay about $2 per lid in quantity 10, with four-day
    turnaound.)

    Those U.FL connectors are super useful in distinguishing between stuff
    that our boards are doing and stuff that comes in over the air. The
    amount of tail-chasing they save is astronomical.

    Cheers

    Phil Hobbs

    We might make provision for one of those Laird shield boxes, just in
    case.

    The next challange is to soft-start the +24 to +5 switcher. Those old
    parts just grunt at startup. The +24 supply has to deliver 1 amp to
    pull up a 1 amp load. The LM2576 has an enable pin, but it's not a
    soft start.

    A time delay and huge amount of bulk capacitance on +24 is one way to
    do it. I have, I think, seven various goofy ideas for sorta or
    actually soft-starting this beast. Without a Spice model, I'll just
    have to try them.

    Not all wall warts start up nicely into BFCs, though. We ship a
    beautiful one with our gizmos, but 50 000 uF will make it misbehave.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Joe Gwinn on Sat Mar 12 13:49:10 2022
    Joe Gwinn wrote:
    On Fri, 11 Mar 2022 18:22:42 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje
    <pNaonStpealmtje@yahoo.com> wrote:

    On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin >>>> <jlarkin@highland_atwork_technology.com> wrote in
    <h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>:

    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>> around 400 MHz at every switch transition. This is called a "Silent
    Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>>> It switches at 50 KHz. And at every switching edge, it rings at about >>>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>>>
    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    Is the 10 nF 30 Ohm parallel to the diode a damping network?

    Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1.

    Use a series LC there tuned to 50 kHz to short it?

    The problem isn't at 50 KHz, it's the fast ringing on both switching
    edges.


    That said I do not rememebr those oscillations
    tried a different make inductor?

    This wouldn't normally be noticed. It's tens of mV rings at 40 or 400
    MHz. It's beyond the frequency ranges of the visible components.

    I guess we'll dump the LTM things and go with old, slow switchers, and
    then try to physically segregate them as much as possible, and add a
    lot of secondary filtering. Create clean and dirty zones on the board,
    draw a boundary line, and filter the power sigs that cross the line.
    That might work better for small 40 MHz nasties than for big 400s.

    But what's resonating? It doesn't seem to be the pcb itself.

    I thought we might have a guard-ring-SRD snap in the schottky diode,
    but any diode does it, and it rings on both switching edges.



    I hear you.

    Awhile back we did a small power supply board, in an effort to factor
    out the noisy stuff and put it inside a shield, so that we could
    concentrate on what we care about.

    It used a TI LMR23630AFDDAR (clocked at 2.15 MHz) to make +13 from +24,
    which was then inverted by an AOZ1282 to make -16. The other rails were
    made using linears off those ones or off the +24 directly. (Making -16 >>from +24 is a bit of a strain for most integrated buck regulator chips
    that can go faster than 2 MHz.)

    It worked fine until we turned on the AOZ1282, at which point the whole
    board became a mass of VHF uglies. The thing was, everything was some
    high harmonic of the 2.15 MHz clock synchronizing the TI chip, selected
    by microstrip stub resonances in the traces. We had 118 MHz ringing
    here, 183 MHz there, all initially very mysterious. Never did work right.

    It can be dicey to feed one switcher directly from another. The power conversion folk do know how to do this, but it requires using a spice
    model encompassing both switchers and the cabling and filter stuff
    between, as well as the loads. LTspice is what they generally use.

    Nor would I be surprised if the switchers were interacting with one
    another such that their switching frequencies adjusted (by injection
    locking) to be in some small-integer rational ratio to one another.


    We've had good success with the 150 kHz Simple Switchers, e.g. the
    LM2594, using powdered-iron toroids and B340A Schottky catch diodes.
    Our QL01 nanowatt photoreceiver has one of those within a couple of
    inches of a very sensitive 10 megohm TIA with a 1 MHz BW, and the
    switching junk is invisible on the output even using a spectrum analyzer
    with a 10-Hz resolution bandwidth. But even that one has issues with
    ground integrity--if the board doesn't make good contact with the box
    ground, low-level harmonics of 150 kHz start showing up.

    If I recall, powered iron toroids have some internal damping, which
    will control ringing. As others have said, I'm thinking that what is bedeviling Larkin may be coil self-resonance.

    Yup. They get pretty toasty at 2 MHz, for sure.
    At this point we've decided we don't want to be power supply designers,
    so we use the 2W Murata gizmos with the embedded toroids, inside a
    board-level steel shield, with the whole works inside a brass or
    aluminum box with a laser-cut lid. (Laser cutting has recently become
    monstrous cheap--we pay about $2 per lid in quantity 10, with four-day
    turnaound.)

    In my experience, what is mostly done these days in power supplies for
    low phase noise electronics is a pair of regulators before the
    sensitive electronics. The first regulator (a switcher) drops the
    voltage to almost the final output voltage (and inverts the polarity
    if needed). The second regulator (analog) brings the voltage down to
    the voltage needed by the sensitive electronics. There are low-pass
    and EMI filters as needed before and after the switcher, and after the
    analog regulator. And, the design is verified by LTspice before
    prototyping.

    We generally use cap multipliers right on the switcher outputs. With
    two poles in the base circuit and one in the collector, you can get ~140
    dB suppression in one stage at SMPS frequencies. Regulators won't get
    into that territory.

    Those U.FL connectors are super useful in distinguishing between stuff
    that our boards are doing and stuff that comes in over the air. The
    amount of tail-chasing they save is astronomical.

    I believe it. I've had the same experience with people trying to
    estimate the temperature of a transistor junction from six inches
    away. (Insert standard joke about drunk looking for car keys under
    the light.) The fix was to insist on a thermocouple glued to the AlN
    spacer between transistor casa and heat sink. Not perfect, but orders
    of magnitude better, cutting tail-chasing by a like ratio.

    Yup. For testing I've been known to fuse the thermocouple into a
    heatsink using one of those big crude $150 transformer-based spot
    welders. Dramatically better thermal contact than using epoxy!

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Joe Gwinn@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Sat Mar 12 13:21:34 2022
    On Fri, 11 Mar 2022 18:22:42 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje
    <pNaonStpealmtje@yahoo.com> wrote:

    On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin >>> <jlarkin@highland_atwork_technology.com> wrote in
    <h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>:

    I used to love the LTM8078 dual switcher module. But it rings hard at
    around 400 MHz at every switch transition. This is called a "Silent
    Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>> It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any
    effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>>
    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    Is the 10 nF 30 Ohm parallel to the diode a damping network?

    Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1.

    Use a series LC there tuned to 50 kHz to short it?

    The problem isn't at 50 KHz, it's the fast ringing on both switching
    edges.


    That said I do not rememebr those oscillations
    tried a different make inductor?

    This wouldn't normally be noticed. It's tens of mV rings at 40 or 400
    MHz. It's beyond the frequency ranges of the visible components.

    I guess we'll dump the LTM things and go with old, slow switchers, and
    then try to physically segregate them as much as possible, and add a
    lot of secondary filtering. Create clean and dirty zones on the board,
    draw a boundary line, and filter the power sigs that cross the line.
    That might work better for small 40 MHz nasties than for big 400s.

    But what's resonating? It doesn't seem to be the pcb itself.

    I thought we might have a guard-ring-SRD snap in the schottky diode,
    but any diode does it, and it rings on both switching edges.



    I hear you.

    Awhile back we did a small power supply board, in an effort to factor
    out the noisy stuff and put it inside a shield, so that we could
    concentrate on what we care about.

    It used a TI LMR23630AFDDAR (clocked at 2.15 MHz) to make +13 from +24,
    which was then inverted by an AOZ1282 to make -16. The other rails were
    made using linears off those ones or off the +24 directly. (Making -16
    from +24 is a bit of a strain for most integrated buck regulator chips
    that can go faster than 2 MHz.)

    It worked fine until we turned on the AOZ1282, at which point the whole
    board became a mass of VHF uglies. The thing was, everything was some
    high harmonic of the 2.15 MHz clock synchronizing the TI chip, selected
    by microstrip stub resonances in the traces. We had 118 MHz ringing
    here, 183 MHz there, all initially very mysterious. Never did work right.

    It can be dicey to feed one switcher directly from another. The power conversion folk do know how to do this, but it requires using a spice
    model encompassing both switchers and the cabling and filter stuff
    between, as well as the loads. LTspice is what they generally use.

    Nor would I be surprised if the switchers were interacting with one
    another such that their switching frequencies adjusted (by injection
    locking) to be in some small-integer rational ratio to one another.


    We've had good success with the 150 kHz Simple Switchers, e.g. the
    LM2594, using powdered-iron toroids and B340A Schottky catch diodes.
    Our QL01 nanowatt photoreceiver has one of those within a couple of
    inches of a very sensitive 10 megohm TIA with a 1 MHz BW, and the
    switching junk is invisible on the output even using a spectrum analyzer
    with a 10-Hz resolution bandwidth. But even that one has issues with
    ground integrity--if the board doesn't make good contact with the box
    ground, low-level harmonics of 150 kHz start showing up.

    If I recall, powered iron toroids have some internal damping, which
    will control ringing. As others have said, I'm thinking that what is bedeviling Larkin may be coil self-resonance.


    At this point we've decided we don't want to be power supply designers,
    so we use the 2W Murata gizmos with the embedded toroids, inside a >board-level steel shield, with the whole works inside a brass or
    aluminum box with a laser-cut lid. (Laser cutting has recently become >monstrous cheap--we pay about $2 per lid in quantity 10, with four-day >turnaound.)

    In my experience, what is mostly done these days in power supplies for
    low phase noise electronics is a pair of regulators before the
    sensitive electronics. The first regulator (a switcher) drops the
    voltage to almost the final output voltage (and inverts the polarity
    if needed). The second regulator (analog) brings the voltage down to
    the voltage needed by the sensitive electronics. There are low-pass
    and EMI filters as needed before and after the switcher, and after the
    analog regulator. And, the design is verified by LTspice before
    prototyping.


    Those U.FL connectors are super useful in distinguishing between stuff
    that our boards are doing and stuff that comes in over the air. The
    amount of tail-chasing they save is astronomical.

    I believe it. I've had the same experience with people trying to
    estimate the temperature of a transistor junction from six inches
    away. (Insert standard joke about drunk looking for car keys under
    the light.) The fix was to insist on a thermocouple glued to the AlN
    spacer between transistor casa and heat sink. Not perfect, but orders
    of magnitude better, cutting tail-chasing by a like ratio.


    Joe Gwinn

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Joe Gwinn on Sat Mar 12 15:02:21 2022
    Joe Gwinn wrote:
    On Sat, 12 Mar 2022 13:49:10 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    Joe Gwinn wrote:
    On Fri, 11 Mar 2022 18:22:42 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje
    <pNaonStpealmtje@yahoo.com> wrote:

    On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin >>>>>> <jlarkin@highland_atwork_technology.com> wrote in
    <h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>:

    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>>>> around 400 MHz at every switch transition. This is called a "Silent >>>>>>> Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>>>>> It switches at 50 KHz. And at every switching edge, it rings at about >>>>>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    Is the 10 nF 30 Ohm parallel to the diode a damping network?

    Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1.

    Use a series LC there tuned to 50 kHz to short it?

    The problem isn't at 50 KHz, it's the fast ringing on both switching >>>>> edges.


    That said I do not rememebr those oscillations
    tried a different make inductor?

    This wouldn't normally be noticed. It's tens of mV rings at 40 or 400 >>>>> MHz. It's beyond the frequency ranges of the visible components.

    I guess we'll dump the LTM things and go with old, slow switchers, and >>>>> then try to physically segregate them as much as possible, and add a >>>>> lot of secondary filtering. Create clean and dirty zones on the board, >>>>> draw a boundary line, and filter the power sigs that cross the line. >>>>> That might work better for small 40 MHz nasties than for big 400s.

    But what's resonating? It doesn't seem to be the pcb itself.

    I thought we might have a guard-ring-SRD snap in the schottky diode, >>>>> but any diode does it, and it rings on both switching edges.



    I hear you.

    Awhile back we did a small power supply board, in an effort to factor
    out the noisy stuff and put it inside a shield, so that we could
    concentrate on what we care about.

    It used a TI LMR23630AFDDAR (clocked at 2.15 MHz) to make +13 from +24, >>>> which was then inverted by an AOZ1282 to make -16. The other rails were >>>> made using linears off those ones or off the +24 directly. (Making -16 >>> >from +24 is a bit of a strain for most integrated buck regulator chips >>>> that can go faster than 2 MHz.)

    It worked fine until we turned on the AOZ1282, at which point the whole >>>> board became a mass of VHF uglies. The thing was, everything was some >>>> high harmonic of the 2.15 MHz clock synchronizing the TI chip, selected >>>> by microstrip stub resonances in the traces. We had 118 MHz ringing
    here, 183 MHz there, all initially very mysterious. Never did work right. >>>
    It can be dicey to feed one switcher directly from another. The power
    conversion folk do know how to do this, but it requires using a spice
    model encompassing both switchers and the cabling and filter stuff
    between, as well as the loads. LTspice is what they generally use.

    Nor would I be surprised if the switchers were interacting with one
    another such that their switching frequencies adjusted (by injection
    locking) to be in some small-integer rational ratio to one another.


    We've had good success with the 150 kHz Simple Switchers, e.g. the
    LM2594, using powdered-iron toroids and B340A Schottky catch diodes.
    Our QL01 nanowatt photoreceiver has one of those within a couple of
    inches of a very sensitive 10 megohm TIA with a 1 MHz BW, and the
    switching junk is invisible on the output even using a spectrum analyzer >>>> with a 10-Hz resolution bandwidth. But even that one has issues with
    ground integrity--if the board doesn't make good contact with the box
    ground, low-level harmonics of 150 kHz start showing up.

    If I recall, powered iron toroids have some internal damping, which
    will control ringing. As others have said, I'm thinking that what is
    bedeviling Larkin may be coil self-resonance.

    Yup. They get pretty toasty at 2 MHz, for sure.

    At this point we've decided we don't want to be power supply designers, >>>> so we use the 2W Murata gizmos with the embedded toroids, inside a
    board-level steel shield, with the whole works inside a brass or
    aluminum box with a laser-cut lid. (Laser cutting has recently become >>>> monstrous cheap--we pay about $2 per lid in quantity 10, with four-day >>>> turnaound.)

    In my experience, what is mostly done these days in power supplies for
    low phase noise electronics is a pair of regulators before the
    sensitive electronics. The first regulator (a switcher) drops the
    voltage to almost the final output voltage (and inverts the polarity
    if needed). The second regulator (analog) brings the voltage down to
    the voltage needed by the sensitive electronics. There are low-pass
    and EMI filters as needed before and after the switcher, and after the
    analog regulator. And, the design is verified by LTspice before
    prototyping.

    We generally use cap multipliers right on the switcher outputs. With
    two poles in the base circuit and one in the collector, you can get ~140
    dB suppression in one stage at SMPS frequencies. Regulators won't get
    into that territory.

    I don't recall people using cap multipliers. I'm sure that the power
    supply folk know of such things, so there must be a reason. I will
    ask around when I can.

    It's hard to achieve 140 dB in one stage (well, circuit board), due to
    sneak leakage paths et al, so injection locking may be able to work
    despite a 140 dB theoretical path loss. About 85 dB is more like it.

    The theoretical loss is even larger. It's very difficult to make
    advanced discrete front ends without them, because single-ended
    amplifier circuitry doesn't have much in the way of supply rejection.

    With a switcher whose output ripple is 100 mV, 140 dB gets you down to
    10 nV. That's easily visible on a spectrum analyzer, especially if you
    apply it to one end of the photodiode whose other end goes to your
    sub-nanovolt TIA.

    If the suppression were only 85 dB, I'd need two of them in cascade.
    (I've done that on occasion--four poles, two transistors.)

    Cheers

    Phil Hobbs


    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Joe Gwinn@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Sat Mar 12 14:55:13 2022
    On Sat, 12 Mar 2022 13:49:10 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    Joe Gwinn wrote:
    On Fri, 11 Mar 2022 18:22:42 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje
    <pNaonStpealmtje@yahoo.com> wrote:

    On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin >>>>> <jlarkin@highland_atwork_technology.com> wrote in
    <h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>:

    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>>> around 400 MHz at every switch transition. This is called a "Silent >>>>>> Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>>>> It switches at 50 KHz. And at every switching edge, it rings at about >>>>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    Is the 10 nF 30 Ohm parallel to the diode a damping network?

    Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1.

    Use a series LC there tuned to 50 kHz to short it?

    The problem isn't at 50 KHz, it's the fast ringing on both switching
    edges.


    That said I do not rememebr those oscillations
    tried a different make inductor?

    This wouldn't normally be noticed. It's tens of mV rings at 40 or 400
    MHz. It's beyond the frequency ranges of the visible components.

    I guess we'll dump the LTM things and go with old, slow switchers, and >>>> then try to physically segregate them as much as possible, and add a
    lot of secondary filtering. Create clean and dirty zones on the board, >>>> draw a boundary line, and filter the power sigs that cross the line.
    That might work better for small 40 MHz nasties than for big 400s.

    But what's resonating? It doesn't seem to be the pcb itself.

    I thought we might have a guard-ring-SRD snap in the schottky diode,
    but any diode does it, and it rings on both switching edges.



    I hear you.

    Awhile back we did a small power supply board, in an effort to factor
    out the noisy stuff and put it inside a shield, so that we could
    concentrate on what we care about.

    It used a TI LMR23630AFDDAR (clocked at 2.15 MHz) to make +13 from +24,
    which was then inverted by an AOZ1282 to make -16. The other rails were >>> made using linears off those ones or off the +24 directly. (Making -16 >>>from +24 is a bit of a strain for most integrated buck regulator chips
    that can go faster than 2 MHz.)

    It worked fine until we turned on the AOZ1282, at which point the whole
    board became a mass of VHF uglies. The thing was, everything was some
    high harmonic of the 2.15 MHz clock synchronizing the TI chip, selected
    by microstrip stub resonances in the traces. We had 118 MHz ringing
    here, 183 MHz there, all initially very mysterious. Never did work right. >>
    It can be dicey to feed one switcher directly from another. The power
    conversion folk do know how to do this, but it requires using a spice
    model encompassing both switchers and the cabling and filter stuff
    between, as well as the loads. LTspice is what they generally use.

    Nor would I be surprised if the switchers were interacting with one
    another such that their switching frequencies adjusted (by injection
    locking) to be in some small-integer rational ratio to one another.


    We've had good success with the 150 kHz Simple Switchers, e.g. the
    LM2594, using powdered-iron toroids and B340A Schottky catch diodes.
    Our QL01 nanowatt photoreceiver has one of those within a couple of
    inches of a very sensitive 10 megohm TIA with a 1 MHz BW, and the
    switching junk is invisible on the output even using a spectrum analyzer >>> with a 10-Hz resolution bandwidth. But even that one has issues with
    ground integrity--if the board doesn't make good contact with the box
    ground, low-level harmonics of 150 kHz start showing up.

    If I recall, powered iron toroids have some internal damping, which
    will control ringing. As others have said, I'm thinking that what is
    bedeviling Larkin may be coil self-resonance.

    Yup. They get pretty toasty at 2 MHz, for sure.

    At this point we've decided we don't want to be power supply designers,
    so we use the 2W Murata gizmos with the embedded toroids, inside a
    board-level steel shield, with the whole works inside a brass or
    aluminum box with a laser-cut lid. (Laser cutting has recently become
    monstrous cheap--we pay about $2 per lid in quantity 10, with four-day
    turnaound.)

    In my experience, what is mostly done these days in power supplies for
    low phase noise electronics is a pair of regulators before the
    sensitive electronics. The first regulator (a switcher) drops the
    voltage to almost the final output voltage (and inverts the polarity
    if needed). The second regulator (analog) brings the voltage down to
    the voltage needed by the sensitive electronics. There are low-pass
    and EMI filters as needed before and after the switcher, and after the
    analog regulator. And, the design is verified by LTspice before
    prototyping.

    We generally use cap multipliers right on the switcher outputs. With
    two poles in the base circuit and one in the collector, you can get ~140
    dB suppression in one stage at SMPS frequencies. Regulators won't get
    into that territory.

    I don't recall people using cap multipliers. I'm sure that the power
    supply folk know of such things, so there must be a reason. I will
    ask around when I can.

    It's hard to achieve 140 dB in one stage (well, circuit board), due to
    sneak leakage paths et al, so injection locking may be able to work
    despite a 140 dB theoretical path loss. About 85 dB is more like it.


    Those U.FL connectors are super useful in distinguishing between stuff
    that our boards are doing and stuff that comes in over the air. The
    amount of tail-chasing they save is astronomical.

    I believe it. I've had the same experience with people trying to
    estimate the temperature of a transistor junction from six inches
    away. (Insert standard joke about drunk looking for car keys under
    the light.) The fix was to insist on a thermocouple glued to the AlN
    spacer between transistor casa and heat sink. Not perfect, but orders
    of magnitude better, cutting tail-chasing by a like ratio.

    Yup. For testing I've been known to fuse the thermocouple into a
    heatsink using one of those big crude $150 transformer-based spot
    welders. Dramatically better thermal contact than using epoxy!

    That would certainly do it, as would capacitor-discharge welding of TC
    wires to said heat sink. But couldn't do that without destroying the
    circuitry being debugged. What was used was silver-loaded epoxy.

    Joe Gwinn

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From legg@21:1/5 to All on Sat Mar 12 16:09:26 2022
    On Sat, 12 Mar 2022 08:18:34 -0800, jlarkin@highlandsniptechnology.com
    wrote:

    On Sat, 12 Mar 2022 10:32:51 -0500, legg <legg@nospam.magma.ca> wrote:

    On Fri, 11 Mar 2022 11:39:10 -0800, John Larkin >><jlarkin@highland_atwork_technology.com> wrote:

    I used to love the LTM8078 dual switcher module. But it rings hard at >>>around 400 MHz at every switch transition. This is called a "Silent >>>Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about
    40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>effect on the ringing frequency.
    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>
    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!



    Check noise effect when scope probe/ground lead is >>removed/replaced/manipulated. Above 20mhZ, it's going
    to be radiated.

    The input and output monitors are coax. I'm monitoring the switch node
    with a 10x scope probe. Removing the probe has no effect on the 40 MHz
    ring on the output.


    More ground bonds to PC ground backing near IC, on both ground
    plane edges, where cut by power train.

    The bottom is all ground. Various jumpers/plier grabs/kluges to the
    ground have zero effect on the ring.


    Move your ceramic decoupling caps closer to the IC body tab.
    Shuffle the polymer/ceramic positions, so both work in tandem.

    Same with schottky and it's snubber. Take output gound out of
    switching current loop.

    SchottKy RC R too big? small? Cap on flying node - R to ground plane.

    Different schottkies, or parallel schottkies, have no effect.

    I don't think swapping the RC in the damper would affect 40 MHz.


    Move Noise monitors closer to filtered nodes, or filtered nodes
    closer to noise monitors. Bare leads feeding sheilded coax? I arsk
    yer!

    An inch of wire flat on a ground plane won't have any effect at 40
    MHz. It's only a 500 MHz scope.

    The scope is hi-Z. It won't allow 50r and AC coupling. I might go to
    50r with an external DC block. The cables might be ringing. Or I can
    change cable lengths and see what happens. It would be great if the >mysterious ringing is the cables, but it feels unlikely.


    The photo doesn't show much effective ground plane stitching.
    Loop length from the SBD, through the board, to IC contacts is
    pretty dicey, where SBD turn-off current is expected to flow.

    Check a reactance chart, looking at the 40MHz line for familiar
    layout and component values. You started with a 400MHz ringing
    and stepped backwards to see 40MHz in earlier work. You have to
    ask yourself, as well, just how much it matters in your application.
    It helps if there's an actual problem that needs solving.

    Probing has its own issues. Can the probe produce the coax waveform?
    What do the ground plane points look like on the scope probe?
    Do the coax outputs shift when the scope probe shifts or the
    scope probe (plus ground probe) is removed?

    Does the coax output shift as you finger certain components?
    Choke bodies can be screened and grounded, if they carry a
    lot of noise for re-radiation.

    Traces that connect to the measurement point, without local
    series impedance, can also act as a pick-up to pump current
    into local non-ideal decouplers. Sense lines are the most
    frequently ignored.

    I scanned an old bundle of paper that sometimes saves time
    wasted googling crap. It's a big file just because that's
    what the scanner pumped out, so it won't be mounted for long.
    Page 3 (?) is useful for checking ground plane and wide trace
    L/W ratios for L.

    http://ve3ute.ca/query/Trace_resistance_inductance.pdf

    RL

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to legg on Sat Mar 12 13:51:33 2022
    On Sat, 12 Mar 2022 16:09:26 -0500, legg <legg@nospam.magma.ca> wrote:

    On Sat, 12 Mar 2022 08:18:34 -0800, jlarkin@highlandsniptechnology.com
    wrote:

    On Sat, 12 Mar 2022 10:32:51 -0500, legg <legg@nospam.magma.ca> wrote:

    On Fri, 11 Mar 2022 11:39:10 -0800, John Larkin >>><jlarkin@highland_atwork_technology.com> wrote:

    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>around 400 MHz at every switch transition. This is called a "Silent >>>>Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>>It switches at 50 KHz. And at every switching edge, it rings at about >>>>40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>effect on the ringing frequency.
    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>>
    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!



    Check noise effect when scope probe/ground lead is >>>removed/replaced/manipulated. Above 20mhZ, it's going
    to be radiated.

    The input and output monitors are coax. I'm monitoring the switch node
    with a 10x scope probe. Removing the probe has no effect on the 40 MHz
    ring on the output.


    More ground bonds to PC ground backing near IC, on both ground
    plane edges, where cut by power train.

    The bottom is all ground. Various jumpers/plier grabs/kluges to the
    ground have zero effect on the ring.


    Move your ceramic decoupling caps closer to the IC body tab.
    Shuffle the polymer/ceramic positions, so both work in tandem.

    Same with schottky and it's snubber. Take output gound out of
    switching current loop.

    SchottKy RC R too big? small? Cap on flying node - R to ground plane.

    Different schottkies, or parallel schottkies, have no effect.

    I don't think swapping the RC in the damper would affect 40 MHz.


    Move Noise monitors closer to filtered nodes, or filtered nodes
    closer to noise monitors. Bare leads feeding sheilded coax? I arsk
    yer!

    An inch of wire flat on a ground plane won't have any effect at 40
    MHz. It's only a 500 MHz scope.

    The scope is hi-Z. It won't allow 50r and AC coupling. I might go to
    50r with an external DC block. The cables might be ringing. Or I can
    change cable lengths and see what happens. It would be great if the >>mysterious ringing is the cables, but it feels unlikely.


    The photo doesn't show much effective ground plane stitching.
    Loop length from the SBD, through the board, to IC contacts is
    pretty dicey, where SBD turn-off current is expected to flow.

    Check a reactance chart, looking at the 40MHz line for familiar
    layout and component values. You started with a 400MHz ringing
    and stepped backwards to see 40MHz in earlier work. You have to
    ask yourself, as well, just how much it matters in your application.
    It helps if there's an actual problem that needs solving.

    Probing has its own issues. Can the probe produce the coax waveform?
    What do the ground plane points look like on the scope probe?
    Do the coax outputs shift when the scope probe shifts or the
    scope probe (plus ground probe) is removed?

    Does the coax output shift as you finger certain components?
    Choke bodies can be screened and grounded, if they carry a
    lot of noise for re-radiation.

    Traces that connect to the measurement point, without local
    series impedance, can also act as a pick-up to pump current
    into local non-ideal decouplers. Sense lines are the most
    frequently ignored.

    I scanned an old bundle of paper that sometimes saves time
    wasted googling crap. It's a big file just because that's
    what the scanner pumped out, so it won't be mounted for long.
    Page 3 (?) is useful for checking ground plane and wide trace
    L/W ratios for L.

    http://ve3ute.ca/query/Trace_resistance_inductance.pdf

    RL





    The 40 MHz ringing is in fact the coax up to the scope. If I add a DC
    block and set the scope to 50 ohms, the ring goes away. What I see
    then is a pretty nasty impulse with a bit of 170 MHz ring; that's what
    was shocking the coax.

    https://www.dropbox.com/s/n6qcuzr752c7yqo/Z532_noise_50r.jpg?raw=1

    It's only about 15 mV p-p. Probably the current turn-on into the catch
    diode makes this noise. It's small enough that I shouldn't complain.

    Of course the DC block wrecks the low frequency response. I should
    hack a giant blocking cap and a source terminator onto my board, and
    run the scope hi-z again.

    Electronics is fun. You get so many puzzles to solve. Poirot had it
    easy in comparison.


    The 400 MHz ring from the LTM8078 is very real. We can see that
    everywhere in our box.



    --

    I yam what I yam - Popeye

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Sat Mar 12 13:30:52 2022
    On Sat, 12 Mar 2022 13:05:05 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    jlarkin@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 09:05:02 +0000, Martin Brown
    <'''newspam'''@nonad.co.uk> wrote:

    On 11/03/2022 21:12, John Larkin wrote:
    On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>>> around 400 MHz at every switch transition. This is called a "Silent >>>>>> Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>>>> It switches at 50 KHz. And at every switching edge, it rings at about >>>>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    In discontinuous current mode, an asynchronous switcher will produce EMI >>>>> at the free resonance of the inductor. If you don't mind the
    efficiency hit at low current, a diode + RC snubber would probably fix it.

    The LTM is a synchronous switcher, and my 2576 is running continuous.

    Looking at the timings on by breadboard, the rings seem to start at
    the big di/dt current transitions in the schottky. But nothing we can
    do changes the ring frequency, so what's resonating?

    They will be immediately after the discontinuity aka Gibb's phenomena on >>> a truncated Fourier expansion for a square wave. It may not be a
    resonance as such but a side effect of the slew rate limit of the
    device. It doesn't die away quickly enough to be just that though.

    There is a hard high frequency cutoff in gain and some ringing is pretty >>> much what you would expect on a square wave with a truncated Fourier
    expansion. It may be being exaggerated in time and amplitude by some
    unfortunate choice of component values providing Q > 1 in addition.

    As Phil said some sort of snubber would be the most likely amelioration. >>> There will be an efficiency hit though so you have to choose how quiet
    you need it vs what losses you can live with.

    There is an RC snubber to ground... see my schematic. The R value is
    about optimized, and the overall effect is a very modest reduction in
    the ringing amplitude, no visible effect on the ring frequency or Q.

    I can find only one thing that has any effect on the ringing
    frequency: the +24 input voltage. Higher voltage results in a very
    slight increase in ring frequency.

    It's Saturday, but I might go in and play with it for a couple more
    hours. I need to be in that part of town anyhow. It's better commute
    on Saturday.

    It's probably good enough, with layout improvements and secondary
    filtering, but it's interesting and annoying.

    Next issue is soft-starting this old beast, so the system always comes
    up. The 24v supply will be a wart type thing. We'll have a Cuk
    converter to make +24 into -5, and that chip soft starts. My part, +24
    to +5, doesn't.

    I could let the Cuk start up, sense its output, and then start up my
    LM2576... somehow. The "enable" pin is just on/off, so any soft start
    would probably involve the fb pin. Nuisance.

    Another approach is to precharge the output cap before enabling the
    switcher.


    That would be just as bad as letting the thing just grunt.

    The laptop-type supply is rated 24v and 65 watts. If it's shorted, it
    makes a 100 ms 9 amp pulse about once a second. So maybe I can ignore
    the switcher startup, on the theory that the supply can brute-force
    the load up to +5, and then the switcher will start to switch.

    Laptop type supplies must be designed to pull up nasty loads.



    --

    I yam what I yam - Popeye

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  • From Phil Hobbs@21:1/5 to jlarkin@highlandsniptechnology.com on Sat Mar 12 17:17:21 2022
    jlarkin@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 13:05:05 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    jlarkin@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 09:05:02 +0000, Martin Brown
    <'''newspam'''@nonad.co.uk> wrote:

    On 11/03/2022 21:12, John Larkin wrote:
    On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>>>> around 400 MHz at every switch transition. This is called a "Silent >>>>>>> Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>>>>> It switches at 50 KHz. And at every switching edge, it rings at about >>>>>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    In discontinuous current mode, an asynchronous switcher will produce EMI >>>>>> at the free resonance of the inductor. If you don't mind the
    efficiency hit at low current, a diode + RC snubber would probably fix it.

    The LTM is a synchronous switcher, and my 2576 is running continuous. >>>>>
    Looking at the timings on by breadboard, the rings seem to start at
    the big di/dt current transitions in the schottky. But nothing we can >>>>> do changes the ring frequency, so what's resonating?

    They will be immediately after the discontinuity aka Gibb's phenomena on >>>> a truncated Fourier expansion for a square wave. It may not be a
    resonance as such but a side effect of the slew rate limit of the
    device. It doesn't die away quickly enough to be just that though.

    There is a hard high frequency cutoff in gain and some ringing is pretty >>>> much what you would expect on a square wave with a truncated Fourier
    expansion. It may be being exaggerated in time and amplitude by some
    unfortunate choice of component values providing Q > 1 in addition.

    As Phil said some sort of snubber would be the most likely amelioration. >>>> There will be an efficiency hit though so you have to choose how quiet >>>> you need it vs what losses you can live with.

    There is an RC snubber to ground... see my schematic. The R value is
    about optimized, and the overall effect is a very modest reduction in
    the ringing amplitude, no visible effect on the ring frequency or Q.

    I can find only one thing that has any effect on the ringing
    frequency: the +24 input voltage. Higher voltage results in a very
    slight increase in ring frequency.

    It's Saturday, but I might go in and play with it for a couple more
    hours. I need to be in that part of town anyhow. It's better commute
    on Saturday.

    It's probably good enough, with layout improvements and secondary
    filtering, but it's interesting and annoying.

    Next issue is soft-starting this old beast, so the system always comes
    up. The 24v supply will be a wart type thing. We'll have a Cuk
    converter to make +24 into -5, and that chip soft starts. My part, +24
    to +5, doesn't.

    I could let the Cuk start up, sense its output, and then start up my
    LM2576... somehow. The "enable" pin is just on/off, so any soft start
    would probably involve the fb pin. Nuisance.

    Another approach is to precharge the output cap before enabling the
    switcher.


    That would be just as bad as letting the thing just grunt.

    You don't have to do it like a wildman. ;)

    Something like a 78L05 with a MOSFET on its output would charge it up
    quickly and then go away.


    The laptop-type supply is rated 24v and 65 watts. If it's shorted, it
    makes a 100 ms 9 amp pulse about once a second. So maybe I can ignore
    the switcher startup, on the theory that the supply can brute-force
    the load up to +5, and then the switcher will start to switch.

    Laptop type supplies must be designed to pull up nasty loads.

    I've built a fair number of POC systems powered by random old laptop bricks.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Rick C@21:1/5 to Joe Gwinn on Sat Mar 12 15:47:09 2022
    On Saturday, March 12, 2022 at 2:55:30 PM UTC-5, Joe Gwinn wrote:
    On Sat, 12 Mar 2022 13:49:10 -0500, Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote:

    Joe Gwinn wrote:
    On Fri, 11 Mar 2022 18:22:42 -0500, Phil Hobbs
    <pcdhSpamM...@electrooptical.net> wrote:

    John Larkin wrote:
    On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje
    <pNaonSt...@yahoo.com> wrote:

    On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin
    <jlarkin@highland_atwork_technology.com> wrote in
    <h58n2h1ssfbd3enfc...@4ax.com>:

    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>>> around 400 MHz at every switch transition. This is called a "Silent >>>>>> Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about >>>>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little. >>>>>>

    Maybe all switchers do this!

    Is the 10 nF 30 Ohm parallel to the diode a damping network?

    Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1.

    Use a series LC there tuned to 50 kHz to short it?

    The problem isn't at 50 KHz, it's the fast ringing on both switching >>>> edges.


    That said I do not rememebr those oscillations
    tried a different make inductor?

    This wouldn't normally be noticed. It's tens of mV rings at 40 or 400 >>>> MHz. It's beyond the frequency ranges of the visible components.

    I guess we'll dump the LTM things and go with old, slow switchers, and >>>> then try to physically segregate them as much as possible, and add a >>>> lot of secondary filtering. Create clean and dirty zones on the board, >>>> draw a boundary line, and filter the power sigs that cross the line. >>>> That might work better for small 40 MHz nasties than for big 400s.

    But what's resonating? It doesn't seem to be the pcb itself.

    I thought we might have a guard-ring-SRD snap in the schottky diode, >>>> but any diode does it, and it rings on both switching edges.



    I hear you.

    Awhile back we did a small power supply board, in an effort to factor >>> out the noisy stuff and put it inside a shield, so that we could
    concentrate on what we care about.

    It used a TI LMR23630AFDDAR (clocked at 2.15 MHz) to make +13 from +24, >>> which was then inverted by an AOZ1282 to make -16. The other rails were >>> made using linears off those ones or off the +24 directly. (Making -16 >>>from +24 is a bit of a strain for most integrated buck regulator chips >>> that can go faster than 2 MHz.)

    It worked fine until we turned on the AOZ1282, at which point the whole >>> board became a mass of VHF uglies. The thing was, everything was some >>> high harmonic of the 2.15 MHz clock synchronizing the TI chip, selected >>> by microstrip stub resonances in the traces. We had 118 MHz ringing
    here, 183 MHz there, all initially very mysterious. Never did work right.

    It can be dicey to feed one switcher directly from another. The power
    conversion folk do know how to do this, but it requires using a spice
    model encompassing both switchers and the cabling and filter stuff
    between, as well as the loads. LTspice is what they generally use.

    Nor would I be surprised if the switchers were interacting with one
    another such that their switching frequencies adjusted (by injection
    locking) to be in some small-integer rational ratio to one another.


    We've had good success with the 150 kHz Simple Switchers, e.g. the
    LM2594, using powdered-iron toroids and B340A Schottky catch diodes.
    Our QL01 nanowatt photoreceiver has one of those within a couple of
    inches of a very sensitive 10 megohm TIA with a 1 MHz BW, and the
    switching junk is invisible on the output even using a spectrum analyzer >>> with a 10-Hz resolution bandwidth. But even that one has issues with
    ground integrity--if the board doesn't make good contact with the box >>> ground, low-level harmonics of 150 kHz start showing up.

    If I recall, powered iron toroids have some internal damping, which
    will control ringing. As others have said, I'm thinking that what is
    bedeviling Larkin may be coil self-resonance.

    Yup. They get pretty toasty at 2 MHz, for sure.

    At this point we've decided we don't want to be power supply designers, >>> so we use the 2W Murata gizmos with the embedded toroids, inside a
    board-level steel shield, with the whole works inside a brass or
    aluminum box with a laser-cut lid. (Laser cutting has recently become >>> monstrous cheap--we pay about $2 per lid in quantity 10, with four-day >>> turnaound.)

    In my experience, what is mostly done these days in power supplies for
    low phase noise electronics is a pair of regulators before the
    sensitive electronics. The first regulator (a switcher) drops the
    voltage to almost the final output voltage (and inverts the polarity
    if needed). The second regulator (analog) brings the voltage down to
    the voltage needed by the sensitive electronics. There are low-pass
    and EMI filters as needed before and after the switcher, and after the
    analog regulator. And, the design is verified by LTspice before
    prototyping.

    We generally use cap multipliers right on the switcher outputs. With
    two poles in the base circuit and one in the collector, you can get ~140 >dB suppression in one stage at SMPS frequencies. Regulators won't get
    into that territory.
    I don't recall people using cap multipliers. I'm sure that the power
    supply folk know of such things, so there must be a reason. I will
    ask around when I can.

    I finally figured out why cap multipliers are useful compared to using an op amp and a reference. The op amp frequency response is lower than a single transistor. The cap multiplier has limitations that an op amp and voltage reference don't have, but
    the transistor can be faster. Both designs require a drop out voltage, so can dissipate significant heat.


    It's hard to achieve 140 dB in one stage (well, circuit board), due to
    sneak leakage paths et al, so injection locking may be able to work
    despite a 140 dB theoretical path loss. About 85 dB is more like it.
    Those U.FL connectors are super useful in distinguishing between stuff >>> that our boards are doing and stuff that comes in over the air. The
    amount of tail-chasing they save is astronomical.

    I believe it. I've had the same experience with people trying to
    estimate the temperature of a transistor junction from six inches
    away. (Insert standard joke about drunk looking for car keys under
    the light.) The fix was to insist on a thermocouple glued to the AlN
    spacer between transistor casa and heat sink. Not perfect, but orders
    of magnitude better, cutting tail-chasing by a like ratio.

    Yup. For testing I've been known to fuse the thermocouple into a
    heatsink using one of those big crude $150 transformer-based spot
    welders. Dramatically better thermal contact than using epoxy!
    That would certainly do it, as would capacitor-discharge welding of TC
    wires to said heat sink. But couldn't do that without destroying the circuitry being debugged. What was used was silver-loaded epoxy.

    When people talk about such dramatic improvements in one aspect of a design, I have to wonder how much difference it makes in performance. The overclockers use all manner of thermal paste when the thermal resistance of the few micron thick layer of
    paste has a lower thermal resistance than the metal of the heat sink because it is a much longer path. If you are going to optimize, why try to optimize the part that has the tiniest impact on the result?

    --

    Rick C.

    + Get 1,000 miles of free Supercharging
    + Tesla referral code - https://ts.la/richard11209

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  • From sea moss@21:1/5 to All on Sat Mar 12 15:37:09 2022
    A ferrite bead in series with the buck inductor might be worth trying. If the SMPS spike is using the inductor's winding capacitance as a conduction path, then the ferrite bead should definitely make a difference in that 10-100MHz range.

    Have you ever tried these "amobead" parts? Might try one in series with the freewheeling diode.

    https://www.toshiba-tmat.co.jp/pdf/en/product/3-1_am_parts_absse.pdf

    --- SoupGate-Win32 v1.05
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  • From Phil Hobbs@21:1/5 to jlarkin@highlandsniptechnology.com on Sat Mar 12 19:49:05 2022
    jlarkin@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 17:17:21 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    jlarkin@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 13:05:05 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    jlarkin@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 09:05:02 +0000, Martin Brown
    <'''newspam'''@nonad.co.uk> wrote:

    On 11/03/2022 21:12, John Larkin wrote:
    On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>>>>>> around 400 MHz at every switch transition. This is called a "Silent >>>>>>>>> Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
    It switches at 50 KHz. And at every switching edge, it rings at about >>>>>>>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>>>>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little. >>>>>>>>>

    Maybe all switchers do this!

    In discontinuous current mode, an asynchronous switcher will produce EMI
    at the free resonance of the inductor. If you don't mind the >>>>>>>> efficiency hit at low current, a diode + RC snubber would probably fix it.

    The LTM is a synchronous switcher, and my 2576 is running continuous. >>>>>>>
    Looking at the timings on by breadboard, the rings seem to start at >>>>>>> the big di/dt current transitions in the schottky. But nothing we can >>>>>>> do changes the ring frequency, so what's resonating?

    They will be immediately after the discontinuity aka Gibb's phenomena on >>>>>> a truncated Fourier expansion for a square wave. It may not be a
    resonance as such but a side effect of the slew rate limit of the
    device. It doesn't die away quickly enough to be just that though. >>>>>>
    There is a hard high frequency cutoff in gain and some ringing is pretty >>>>>> much what you would expect on a square wave with a truncated Fourier >>>>>> expansion. It may be being exaggerated in time and amplitude by some >>>>>> unfortunate choice of component values providing Q > 1 in addition. >>>>>>
    As Phil said some sort of snubber would be the most likely amelioration. >>>>>> There will be an efficiency hit though so you have to choose how quiet >>>>>> you need it vs what losses you can live with.

    There is an RC snubber to ground... see my schematic. The R value is >>>>> about optimized, and the overall effect is a very modest reduction in >>>>> the ringing amplitude, no visible effect on the ring frequency or Q. >>>>>
    I can find only one thing that has any effect on the ringing
    frequency: the +24 input voltage. Higher voltage results in a very
    slight increase in ring frequency.

    It's Saturday, but I might go in and play with it for a couple more
    hours. I need to be in that part of town anyhow. It's better commute >>>>> on Saturday.

    It's probably good enough, with layout improvements and secondary
    filtering, but it's interesting and annoying.

    Next issue is soft-starting this old beast, so the system always comes >>>>> up. The 24v supply will be a wart type thing. We'll have a Cuk
    converter to make +24 into -5, and that chip soft starts. My part, +24 >>>>> to +5, doesn't.

    I could let the Cuk start up, sense its output, and then start up my >>>>> LM2576... somehow. The "enable" pin is just on/off, so any soft start >>>>> would probably involve the fb pin. Nuisance.

    Another approach is to precharge the output cap before enabling the
    switcher.


    That would be just as bad as letting the thing just grunt.

    You don't have to do it like a wildman. ;)

    Something like a 78L05 with a MOSFET on its output would charge it up
    quickly and then go away.

    There's still the load. Disconnecting the load and charging the cap
    won't help much. As soon as the load is connected, the output cap will collapse, the switcher will go to 100% duty cycle, the input current
    will equal the load current, and there we are.


    Don't think so. Simple switchers do everything cycle-by-cycle, so if it doesn't collapse during normal operation, it shouldn't do it on the
    first cycle either.

    Besides that, the 7805 and the mosfet and the timing would be a
    nuisance.

    Sure. It's just a possible alternative.

    If the 24 supply can provide 1.5 amps, the max 5v load current, it
    will just work. I think. The switcher *is* the linear regulator during startup!

    Sort of.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Sat Mar 12 16:38:42 2022
    On Sat, 12 Mar 2022 17:17:21 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    jlarkin@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 13:05:05 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    jlarkin@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 09:05:02 +0000, Martin Brown
    <'''newspam'''@nonad.co.uk> wrote:

    On 11/03/2022 21:12, John Larkin wrote:
    On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>>>>> around 400 MHz at every switch transition. This is called a "Silent >>>>>>>> Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>>>>>> It switches at 50 KHz. And at every switching edge, it rings at about >>>>>>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>>>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little. >>>>>>>>

    Maybe all switchers do this!

    In discontinuous current mode, an asynchronous switcher will produce EMI
    at the free resonance of the inductor. If you don't mind the
    efficiency hit at low current, a diode + RC snubber would probably fix it.

    The LTM is a synchronous switcher, and my 2576 is running continuous. >>>>>>
    Looking at the timings on by breadboard, the rings seem to start at >>>>>> the big di/dt current transitions in the schottky. But nothing we can >>>>>> do changes the ring frequency, so what's resonating?

    They will be immediately after the discontinuity aka Gibb's phenomena on >>>>> a truncated Fourier expansion for a square wave. It may not be a
    resonance as such but a side effect of the slew rate limit of the
    device. It doesn't die away quickly enough to be just that though.

    There is a hard high frequency cutoff in gain and some ringing is pretty >>>>> much what you would expect on a square wave with a truncated Fourier >>>>> expansion. It may be being exaggerated in time and amplitude by some >>>>> unfortunate choice of component values providing Q > 1 in addition.

    As Phil said some sort of snubber would be the most likely amelioration. >>>>> There will be an efficiency hit though so you have to choose how quiet >>>>> you need it vs what losses you can live with.

    There is an RC snubber to ground... see my schematic. The R value is
    about optimized, and the overall effect is a very modest reduction in
    the ringing amplitude, no visible effect on the ring frequency or Q.

    I can find only one thing that has any effect on the ringing
    frequency: the +24 input voltage. Higher voltage results in a very
    slight increase in ring frequency.

    It's Saturday, but I might go in and play with it for a couple more
    hours. I need to be in that part of town anyhow. It's better commute
    on Saturday.

    It's probably good enough, with layout improvements and secondary
    filtering, but it's interesting and annoying.

    Next issue is soft-starting this old beast, so the system always comes >>>> up. The 24v supply will be a wart type thing. We'll have a Cuk
    converter to make +24 into -5, and that chip soft starts. My part, +24 >>>> to +5, doesn't.

    I could let the Cuk start up, sense its output, and then start up my
    LM2576... somehow. The "enable" pin is just on/off, so any soft start
    would probably involve the fb pin. Nuisance.

    Another approach is to precharge the output cap before enabling the
    switcher.


    That would be just as bad as letting the thing just grunt.

    You don't have to do it like a wildman. ;)

    Something like a 78L05 with a MOSFET on its output would charge it up
    quickly and then go away.

    There's still the load. Disconnecting the load and charging the cap
    won't help much. As soon as the load is connected, the output cap will collapse, the switcher will go to 100% duty cycle, the input current
    will equal the load current, and there we are.

    Besides that, the 7805 and the mosfet and the timing would be a
    nuisance.

    If the 24 supply can provide 1.5 amps, the max 5v load current, it
    will just work. I think. The switcher *is* the linear regulator during
    startup!



    The laptop-type supply is rated 24v and 65 watts. If it's shorted, it
    makes a 100 ms 9 amp pulse about once a second. So maybe I can ignore
    the switcher startup, on the theory that the supply can brute-force
    the load up to +5, and then the switcher will start to switch.

    Laptop type supplies must be designed to pull up nasty loads.

    I've built a fair number of POC systems powered by random old laptop bricks.

    Right. My concerns are probably silly, given that things like this
    usually work.



    --

    I yam what I yam - Popeye

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  • From Anthony William Sloman@21:1/5 to jla...@highlandsniptechnology.com on Sat Mar 12 17:48:03 2022
    On Sunday, March 13, 2022 at 12:17:23 PM UTC+11, jla...@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 15:37:09 -0800 (PST), sea moss
    <danlu...@gmail.com> wrote:

    A ferrite bead in series with the buck inductor might be worth trying. If the SMPS spike is using the inductor's winding capacitance as a conduction path, then the ferrite bead should definitely make a difference in that 10-100MHz range.

    Have you ever tried these "amobead" parts? Might try one in series with the freewheeling diode.

    https://www.toshiba-tmat.co.jp/pdf/en/product/3-1_am_parts_absse.pdf

    I think we want the switch node to have minimum activity, so low diode impedance might be best. I tried a fb in series with the switcher output pin and it was horrible.

    You do have to provide an alternative (capacitative) path to ground for the high frequency current that the ferrite bead is supposed to block. Without that the ferrite bead just produces a larger voltage spike at the output of the switcher, which isn't
    helpful.

    As I discovered today, the 40 MHz ring was actually the coax to the scope. Oops. Dogged persistance is a workable substitute for extreme intelligence.

    Somebody smart enough to spell "persistence" correctly might have found the problem faster.

    This might work for startup:

    https://www.dropbox.com/s/bhfr9yod5261t8e/T501_Sw_4.jpg?raw=1

    You do need capacitors to ground before the inductors.

    The laptop supply grunts as much as necessary to get the +5D (D=dirty) load up. When +5 and +24 are stable, the MAX809 waits a bit and enables the Cuk converter, which soft starts.

    Whatever.

    --
    Bill Sloman, Sydney

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  • From Anthony William Sloman@21:1/5 to sea moss on Sat Mar 12 17:34:24 2022
    On Sunday, March 13, 2022 at 10:37:17 AM UTC+11, sea moss wrote:
    A ferrite bead in series with the buck inductor might be worth trying. If the SMPS spike is using the inductor's winding capacitance as a conduction path, then the ferrite bead should definitely make a difference in that 10-100MHz range.

    Have you ever tried these "amobead" parts? Might try one in series with the freewheeling diode.

    https://www.toshiba-tmat.co.jp/pdf/en/product/3-1_am_parts_absse.pdf

    Beads are nonwound components, and have appreciably lower parallel capacitance. The Toshiba link also covers wound "spike killers" which have a couple of turns of wire on a toroid, which typically means about 1pF of parallel capacitance, although their
    text doesn't seem to mentions this..

    LTSpice lists a great many W├╝rth Elektronik GmbH & Co. ferrite chip parts - not all of them by any means.

    https://www.we-online.com/catalog/en/pbs/emc_components/ferrites_for_pcb_assembly

    It does make it easy to play around in a simulation.

    --
    Bill Sloman, Sydney

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to danluster81@gmail.com on Sat Mar 12 17:17:09 2022
    On Sat, 12 Mar 2022 15:37:09 -0800 (PST), sea moss
    <danluster81@gmail.com> wrote:

    A ferrite bead in series with the buck inductor might be worth trying. If the SMPS spike is using the inductor's winding capacitance as a conduction path, then the ferrite bead should definitely make a difference in that 10-100MHz range.

    Have you ever tried these "amobead" parts? Might try one in series with the freewheeling diode.

    https://www.toshiba-tmat.co.jp/pdf/en/product/3-1_am_parts_absse.pdf


    I think we want the switch node to have minimum activity, so low diode impedance might be best. I tried a fb in series with the switcher
    output pin and it was horrible. As I discovered today, the 40 MHz ring
    was actually the coax to the scope. Oops. Dogged persistance is a
    workable substitute for extreme intelligence.

    This might work for startup:

    https://www.dropbox.com/s/bhfr9yod5261t8e/T501_Sw_4.jpg?raw=1

    The laptop supply grunts as much as necessary to get the +5D (D=dirty)
    load up. When +5 and +24 are stable, the MAX809 waits a bit and
    enables the Cuk converter, which soft starts.



    --

    I yam what I yam - Popeye

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  • From legg@21:1/5 to All on Sun Mar 13 15:02:20 2022
    On Sat, 12 Mar 2022 13:51:33 -0800, jlarkin@highlandsniptechnology.com
    wrote:

    <snip>

    The 40 MHz ringing is in fact the coax up to the scope. If I add a DC
    block and set the scope to 50 ohms, the ring goes away. What I see
    then is a pretty nasty impulse with a bit of 170 MHz ring; that's what
    was shocking the coax.

    https://www.dropbox.com/s/n6qcuzr752c7yqo/Z532_noise_50r.jpg?raw=1

    It's only about 15 mV p-p. Probably the current turn-on into the catch
    diode makes this noise. It's small enough that I shouldn't complain.

    Of course the DC block wrecks the low frequency response. I should
    hack a giant blocking cap and a source terminator onto my board, and
    run the scope hi-z again.

    Electronics is fun. You get so many puzzles to solve. Poirot had it
    easy in comparison.


    The 400 MHz ring from the LTM8078 is very real. We can see that
    everywhere in our box.

    There's also a lower frequency concideration where the two coax
    pick-offs exit the board at opposite ends, then connect to the
    same (or connected) monitoring instrument(s). The two coax lines
    form a loop, that will generate antiphase readings, as common-mode
    shield current is seen as dif mode by the monitor.

    So, compare the two coax to see if the 'bump' is visible in both,
    inverted.

    RL

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  • From LM@21:1/5 to All on Mon Mar 14 01:24:15 2022
    On Sat, 12 Mar 2022 13:30:52 -0800, jlarkin@highlandsniptechnology.com
    wrote:

    On Sat, 12 Mar 2022 13:05:05 -0500, Phil Hobbs ><pcdhSpamMeSenseless@electrooptical.net> wrote:

    jlarkin@highlandsniptechnology.com wrote:
    On Sat, 12 Mar 2022 09:05:02 +0000, Martin Brown
    <'''newspam'''@nonad.co.uk> wrote:

    On 11/03/2022 21:12, John Larkin wrote:
    On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    John Larkin wrote:
    I used to love the LTM8078 dual switcher module. But it rings hard at >>>>>>> around 400 MHz at every switch transition. This is called a "Silent >>>>>>> Switcher!"

    I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>>>>> It switches at 50 KHz. And at every switching edge, it rings at about >>>>>>> 40 MHz.

    We tried all sorts of stuff on both switchers. Nothing so far has any >>>>>>> effect on the ringing frequency.

    https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

    The damper on the 2576 circuit reduces ring amplitude a little.


    Maybe all switchers do this!

    In discontinuous current mode, an asynchronous switcher will produce EMI >>>>>> at the free resonance of the inductor. If you don't mind the
    efficiency hit at low current, a diode + RC snubber would probably fix it.

    The LTM is a synchronous switcher, and my 2576 is running continuous. >>>>>
    Looking at the timings on by breadboard, the rings seem to start at
    the big di/dt current transitions in the schottky. But nothing we can >>>>> do changes the ring frequency, so what's resonating?

    They will be immediately after the discontinuity aka Gibb's phenomena on >>>> a truncated Fourier expansion for a square wave. It may not be a
    resonance as such but a side effect of the slew rate limit of the
    device. It doesn't die away quickly enough to be just that though.

    There is a hard high frequency cutoff in gain and some ringing is pretty >>>> much what you would expect on a square wave with a truncated Fourier
    expansion. It may be being exaggerated in time and amplitude by some
    unfortunate choice of component values providing Q > 1 in addition.

    As Phil said some sort of snubber would be the most likely amelioration. >>>> There will be an efficiency hit though so you have to choose how quiet >>>> you need it vs what losses you can live with.

    There is an RC snubber to ground... see my schematic. The R value is
    about optimized, and the overall effect is a very modest reduction in
    the ringing amplitude, no visible effect on the ring frequency or Q.

    I can find only one thing that has any effect on the ringing
    frequency: the +24 input voltage. Higher voltage results in a very
    slight increase in ring frequency.

    It's Saturday, but I might go in and play with it for a couple more
    hours. I need to be in that part of town anyhow. It's better commute
    on Saturday.

    It's probably good enough, with layout improvements and secondary
    filtering, but it's interesting and annoying.

    Next issue is soft-starting this old beast, so the system always comes
    up. The 24v supply will be a wart type thing. We'll have a Cuk
    converter to make +24 into -5, and that chip soft starts. My part, +24
    to +5, doesn't.

    I could let the Cuk start up, sense its output, and then start up my
    LM2576... somehow. The "enable" pin is just on/off, so any soft start
    would probably involve the fb pin. Nuisance.

    Another approach is to precharge the output cap before enabling the >>switcher.


    That would be just as bad as letting the thing just grunt.

    The laptop-type supply is rated 24v and 65 watts. If it's shorted, it
    makes a 100 ms 9 amp pulse about once a second. So maybe I can ignore
    the switcher startup, on the theory that the supply can brute-force
    the load up to +5, and then the switcher will start to switch.

    Laptop type supplies must be designed to pull up nasty loads.
    440MHz radios needed shields, preventing EMI was hard even then.
    I wonder how large is your coil diode system. At 400Mhz even short
    wires have impedance.

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