• Low-leakage PNP or PMOS

    From Piotr Wyderski@21:1/5 to All on Sat Jan 29 12:48:47 2022
    Hi,

    knowing that you often take parts way beyond their specs, I would like
    to use your wisdom.

    I need a high-side controllable switch capable of charging 5pF to 400V
    and maintaining most of that charge for a good fraction of a second. Not important if it is a PMOS or a PNP transistor. Could you please
    recommend me a part known for a particularly low leakage current? Or
    should I use any transistor and a diode in series with the collector? A diode-connected transistor perhaps?

    T_MAX is, say, 50 degrees Celcius and there will be no self-heating.
    The lower the leakage the better.

    Best regards, Piotr

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to bombald@protonmail.com on Sat Jan 29 04:21:37 2022
    On Sat, 29 Jan 2022 12:48:47 +0100, Piotr Wyderski
    <bombald@protonmail.com> wrote:

    Hi,

    knowing that you often take parts way beyond their specs, I would like
    to use your wisdom.

    I need a high-side controllable switch capable of charging 5pF to 400V
    and maintaining most of that charge for a good fraction of a second. Not >important if it is a PMOS or a PNP transistor. Could you please
    recommend me a part known for a particularly low leakage current? Or
    should I use any transistor and a diode in series with the collector? A >diode-connected transistor perhaps?

    T_MAX is, say, 50 degrees Celcius and there will be no self-heating.
    The lower the leakage the better.

    Best regards, Piotr

    Most highside switches will have more than 5 pF of capacitance
    themselves.

    How fast does it have to charge that cap? My first choice would be a
    relay!

    Two sections of a BAV23 in series make a 500v diode with below 0.5 pF. MMBD5004S would be a bit better, 1KV and about 0.25 pF. But you'd need
    a pulldown after the big switch. Gets ugly fast.

    You could possibly bootstrap the leakage. Maybe not.

    Why not just leave the switch on for a second? In other words, what
    are you actually trying to do?







    --

    I yam what I yam - Popeye

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  • From Anthony William Sloman@21:1/5 to Piotr Wyderski on Sat Jan 29 05:58:01 2022
    On Saturday, January 29, 2022 at 10:49:05 PM UTC+11, Piotr Wyderski wrote:
    Hi,

    knowing that you often take parts way beyond their specs, I would like
    to use your wisdom.

    I need a high-side controllable switch capable of charging 5pF to 400V
    and maintaining most of that charge for a good fraction of a second. Not important if it is a PMOS or a PNP transistor. Could you please
    recommend me a part known for a particularly low leakage current? Or
    should I use any transistor and a diode in series with the collector? A diode-connected transistor perhaps?

    T_MAX is, say, 50 degrees Celcius and there will be no self-heating.
    The lower the leakage the better.

    Back in 1979 I used some insulated gate FETs which relied on silicon nitride rather than silicon oxide for the gate insulator. They had very low gate leakage and could be turned off quite hard. Google doesn't show up anything helpful.

    The data sheet numbers were limited by the current that could be measured in short time on the production line, They did a lot better in real life.

    --
    Bill Sloman, Sydney

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  • From Piglet@21:1/5 to Piotr Wyderski on Sat Jan 29 16:26:19 2022
    On 29/01/2022 11:48, Piotr Wyderski wrote:
    Hi,

    knowing that you often take parts way beyond their specs, I would like
    to use your wisdom.

    I need a high-side controllable switch capable of charging 5pF to 400V
    and maintaining most of that charge for a good fraction of a second. Not important if it is a PMOS or a PNP transistor. Could you please
    recommend me a part known for a particularly low leakage current? Or
    should I use any transistor and a diode in series with the collector? A diode-connected transistor perhaps?

    T_MAX is, say, 50 degrees Celcius and there will be no self-heating.
    The lower the leakage the better.

        Best regards, Piotr

    Assuming that 5pF and second timescale is not a typo then you are asking
    for nano ampere leakage at 400V reverse bias at 50degC - let alone the
    charge injection or switch capacitance. That is very hard. Is there any
    way you can bootstrap?

    piglet

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to All on Sat Jan 29 08:46:56 2022
    On Sat, 29 Jan 2022 16:26:19 +0000, Piglet <erichpwagner@hotmail.com>
    wrote:

    On 29/01/2022 11:48, Piotr Wyderski wrote:
    Hi,

    knowing that you often take parts way beyond their specs, I would like
    to use your wisdom.

    I need a high-side controllable switch capable of charging 5pF to 400V
    and maintaining most of that charge for a good fraction of a second. Not
    important if it is a PMOS or a PNP transistor. Could you please
    recommend me a part known for a particularly low leakage current? Or
    should I use any transistor and a diode in series with the collector? A
    diode-connected transistor perhaps?

    T_MAX is, say, 50 degrees Celcius and there will be no self-heating.
    The lower the leakage the better.

    Best regards, Piotr

    Assuming that 5pF and second timescale is not a typo then you are asking
    for nano ampere leakage at 400V reverse bias at 50degC -

    Picoamp.

    The problem is under-specified.



    --

    I yam what I yam - Popeye

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  • From whit3rd@21:1/5 to Piotr Wyderski on Sat Jan 29 12:02:56 2022
    On Saturday, January 29, 2022 at 3:49:05 AM UTC-8, Piotr Wyderski wrote:
    Hi,

    knowing that you often take parts way beyond their specs, I would like
    to use your wisdom.

    I need a high-side controllable switch capable of charging 5pF to 400V
    and maintaining most of that charge for a good fraction of a second.

    If you can get a canonical photocell (vacuum type) with a UVLED
    cathode excitation, the OFF resistance of a dark photocell is more than good enough.
    Semiconductors are tested, in mass-production fashion, to loose leakage specifications,
    it's HARD to test nanoamps/picoamps. With tubes, though, it's guaranteed by design.

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to bombald@protonmail.com on Sat Jan 29 13:15:00 2022
    On Sat, 29 Jan 2022 21:21:58 +0100, Piotr Wyderski
    <bombald@protonmail.com> wrote:

    jlarkin@highlandsniptechnology.com wrote:

    How fast does it have to charge that cap? My first choice would be a
    relay!

    In below 1us, ~1kHz in the worst case. And you are totally right, the >parasitic capacitance kills this simple idea. Now it is obvious, wasn't
    at the beginning.

    Picoamps of leakage will be a problem too, if you care about
    discharging 5 pF. A GM tube is very sensitive to voltage so you can't
    tolerate much droop.


    Why not just leave the switch on for a second? In other words, what
    are you actually trying to do?

    Pulse charge a GM tube to experiment with the time-to-first-count
    approach. Basically, you quickly charge this GM cap and measure how long
    it takes to record the discharge pulse triggered by a particle. Then you >apply statistics.

    I have actually succeeded in Ltspice with a single pulse boost
    converter. I deliver a controlled quantity of energy and the GM tube
    charges from 0 to 400V in 330ns. The discharge pulse is sort of safe,
    because there is only so much energy in the parasitic and explicit >capacitances. Dunno how it would work in a real case, still waiting for
    the tube.

    Best regards, Piotr

    You could charge the tube from regulated 400DC through a resistor,
    which current limits nicely and has low capacitance. Self-quenching.
    And simple.

    A resistor and a series inductor is interesting but probably not
    practical.

    You can get statistics from the discharge frequency. That's the
    traditional way.

    Each shot is the start time for the next time-to-first-count. Given a
    random pulse, "start time" is anything you want to call it!





    --

    I yam what I yam - Popeye

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  • From Piotr Wyderski@21:1/5 to jlarkin@highlandsniptechnology.com on Sat Jan 29 21:21:58 2022
    jlarkin@highlandsniptechnology.com wrote:

    How fast does it have to charge that cap? My first choice would be a
    relay!

    In below 1us, ~1kHz in the worst case. And you are totally right, the
    parasitic capacitance kills this simple idea. Now it is obvious, wasn't
    at the beginning.

    Why not just leave the switch on for a second? In other words, what
    are you actually trying to do?

    Pulse charge a GM tube to experiment with the time-to-first-count
    approach. Basically, you quickly charge this GM cap and measure how long
    it takes to record the discharge pulse triggered by a particle. Then you
    apply statistics.

    I have actually succeeded in Ltspice with a single pulse boost
    converter. I deliver a controlled quantity of energy and the GM tube
    charges from 0 to 400V in 330ns. The discharge pulse is sort of safe,
    because there is only so much energy in the parasitic and explicit capacitances. Dunno how it would work in a real case, still waiting for
    the tube.

    Best regards, Piotr

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  • From Piotr Wyderski@21:1/5 to jlarkin@highlandsniptechnology.com on Sat Jan 29 21:48:00 2022
    jlarkin@highlandsniptechnology.com wrote:

    Picoamp.

    The problem is under-specified.

    Indeed. Now I can clearly see it is not about low leakage, it is all
    about rapid charging and maintaining that charge. But, to my surprise,
    it does not imply low leakage at all, at least not in the pA range. A
    series 100M resistor to V_SUPPLY will take care of any leakage.

    Please consider the case closed, even if in an unexpected way. Thank you
    all for your input, I appreciate it very much. I'll post a Spice sim
    when I'm happy with its performance.

    Best regards, Piotr

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to bombald@protonmail.com on Sat Jan 29 14:38:30 2022
    On Sat, 29 Jan 2022 22:29:02 +0100, Piotr Wyderski
    <bombald@protonmail.com> wrote:

    jlarkin@highlandsniptechnology.com wrote:

    You could charge the tube from regulated 400DC through a resistor,
    which current limits nicely and has low capacitance. Self-quenching.
    And simple.

    This is the traditional way, good for the low dose cases.

    You can get statistics from the discharge frequency. That's the
    traditional way.

    Each shot is the start time for the next time-to-first-count. Given a
    random pulse, "start time" is anything you want to call it!

    It works as long as the tube is not saturated. At high dose rates you
    stop getting pulses, let alone clear pulses. So the idea is to quench
    the tube hard by turning off the supply and periodically energize it
    rapidly. Rumor has it that it allows for 2 more orders of magnitude and
    I wanted to check it for fun. It might turn out to be simple enough to >integrate it into a DIY meter.

    OTOH, Phil Hobbs was right. The more I want the more it looks like a >scintillation probe. Not giving up yet, though.

    Best regards, Piotr

    You could use a half H-bridge to power and quench the tube fast. Like
    an IR2213 and a couple of small mosfets and a bit of timing logic.

    GM tubes are go or nogo, so can't do spectroscopy. Scintillators and
    PMTs are more fun.




    --

    I yam what I yam - Popeye

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  • From Phil Hobbs@21:1/5 to Piotr Wyderski on Mon Jan 31 18:33:58 2022
    Piotr Wyderski wrote:
    Hi,

    knowing that you often take parts way beyond their specs, I would like
    to use your wisdom.

    I need a high-side controllable switch capable of charging 5pF to 400V
    and maintaining most of that charge for a good fraction of a second. Not important if it is a PMOS or a PNP transistor. Could you please
    recommend me a part known for a particularly low leakage current? Or
    should I use any transistor and a diode in series with the collector? A diode-connected transistor perhaps?

    T_MAX is, say, 50 degrees Celcius and there will be no self-heating.
    The lower the leakage the better.

        Best regards, Piotr

    Late to the party here, but for one-offs I'd probably use a tube for
    that sort of thing. (They don't come in P-channel, of course.)

    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

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