Re: Diode recovery pulse generator


Tim Williams wrote:
Ok, so if I understand the procedure properly, you are to charge a diode
with some forward current, then reverse the terminals, putting it in reverse
recovery, and when you have gobs of current flowing, and the charge carriers
suddenly run out, and it says oh *** and makes a huge dI/dt and switches
off. Or something like that.

Well here's my circuit to test it.
http://webpages.charter.net/dawill/Images/Diode%20Recovery%20Pulse%20Generator.gif
Note the circuit is optimized for turn-on only (the 2N4401 output, without
FET load, has a risetime of about 8ns, comparable to my signal generator,
coincidentially), so repeat rate is pretty crappy (~200kHz).

With FET, Tr is about 50ns. Er.. I forget if that's before or after Rg.
Drain risetime is pretty spanking, of course.

So, when the FET slams on, current in the two turns of hookup wire quickly
rises, and the diode goes reverse... after some time, it plinks and the
inductor discharges as a negative-going flyback pulse, after which the
voltage falls further as the MOSFET saturates, then turns off and everything
relaxes until the next bit of excitement.

But the thing is, I went through pretty much all my diodes and the best I've
seen is a pulse around 40ns across at the base (about 20V tall with supply
as shown). I've got the best results from high speed damper diodes (1.5kV,
<200ns trr, etc.), and the worst from power rectifiers (1.5kV, trr ~1us).
Schottkies of course just ring (gimme a break, it's lashed up on a
protoboard), with no RR to speak up the FET just slams on.

Where's the 1ns *** everyone else seems to be getting? Step recovery
diodes?

When I worked at Cambridge Instruments we used step-recovery diodes to
generate 100psec snap-off steps to test our 0.5nsec long electron beam
pulses in the stroboscopic electron beam testers.

We bought the step-recovery diodes from Hewlett-Packard via U.K.
distributors, and drove them with an H-P pulse generator. I had a plan
to make a pulse generator board with a couple of BFR93 (NPN) and BFT93
(PNP) wideband transistors driving a BFR96 output transistor, but never
got around to it. The BFR93 and the BFT93 are still available from
Farnell for about a $1.50 each. The BFR96 seems to have been superseded
by things like the BFQ19 ($2.50)and the BFG97 ($1.0) and Farnell now
carries the 8 GHz BFG135 ($4.5) which might be interesting. I always
had to put a "base-stopper" resistor in series with every base -
something between 22R and 33R to stop the parts oscillating at a couple
of GHz.

I'd been using 5GHz bndwidth transistors for some time - see

Ghiggino, K.P., Phillips, D., and Sloman, A.W. "Nanosecond pulse
stretcher", Journal of Physics E: Scientific Instruments, 12, 686-687
(1979).

which owes a lot to good advice from colleagues at EMI Central
Research.

For sources of step-recovery diodes see

http://cp.literature.agilent.com/litweb/pdf/5966-4998E.pdf

--
Bill Sloman, Nijmegen

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