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
Low-leakage PNP or PMOS
Started by ●January 29, 2022
Reply by ●January 29, 20222022-01-29
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, PiotrMost 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
Reply by ●January 29, 20222022-01-29
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
Reply by ●January 29, 20222022-01-29
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, PiotrAssuming 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
Reply by ●January 29, 20222022-01-29
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
Reply by ●January 29, 20222022-01-29
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.
Reply by ●January 29, 20222022-01-29
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
Reply by ●January 29, 20222022-01-29
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
Reply by ●January 29, 20222022-01-29
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, PiotrYou 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
Reply by ●January 29, 20222022-01-29
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
