Low-leakage PNP or PMOS...

[Piotr Wyderski]

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

 

[server]

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

 

[Anthony William Sloman]

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

 

[Piglet]

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

 

[server]

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

 

[whit3rd]

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.

 

[Piotr Wyderski]

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

 

[Piotr Wyderski]

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

 

[server]

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

 

[Piotr Wyderski]

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

 

[server]

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

 

[Phil Hobbs]

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