Shopping for low C PFET

George Herold · 2018-08-17T16:09:59+00:00

Hi All, well one last swing at my HV reset spad circuit. (circuit scribble or spice file on request.) I need a low input C, P-fet with decent Vds (~40V or greater.)* (It needs to turn off fast (

Aug 17, 2018 21 Replies

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George Herold 7 years ago

Yeah I think that's Mario's (see paper's upstream) first low side quench and reset circuit. He pulses in some charge through a cap. It's a diode and he takes advantage of that. (sometimes my model of a diode is a switch, and that stops me from thinking in reverse. :^) Then my power supply has to swallow the reverse over-voltage?

Umm, I don't know/ recall it.

I need (maybe) a delayed reset and blanking pulse. I've got 74hc14s, diodes, R's and C's,

I was going to ask for links with circuits, but it's simple enough, (I think) I can use the RC delay on one side and the diode on the other. That still leaves a lot of permutations.

George H.

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bill.sloman 7 years ago

A 74HC14 isn't all that quick. There are faster flavours of CMOS around and ECLinPS is even quicker.

The faster stuff doesn't cope with large voltage swings, so you have to cascode the output into a quick transistor with a decent collector-to-emitter breakdown voltage. I dug up something suitable in a earlier thread on this subject.

The literature was full of them.

Keeping the actual avalanche period as short as possible seems to be a good idea.

The charge carrier generation that creates and sustains the avalanche seems to dump energy into the channel, which shows up as after-pulsing, and the less of that, the better.

Bill Sloman, Sydney

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