Re: Diode recovery pulse generator

On Mon, 29 May 2006 21:29:04 -0500, "Tim Williams"
<tmoranwms@xxxxxxxxxxx> 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 shit 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 shit everyone else seems to be getting? Step recovery
diodes?

Tim


Yup. SRDs are designed to do this. The doping profile has to be
hyperbolic or something like that so that all the reverse charge gets
used up at once. Plus, it needs to have low junction capacitance.

Getting below 100 ps isn't hard with a decent SRD. Below 40 is hard.

There is a dilemma: fast-switching SRDs store a small amount of
charge, and for a healthy snap we want a lot of reverse current
flowing at the instant it snaps. If we ramp up the reverse current too
slowly, we'll deplete all the available charge while the current is
still low, so the snap will be wimpy. The faster the SRD, the less
stored charge, so the faster your fet must turn on and the smaller the
series inductor must be. To get a 100 ps edge, the drive current
risetime would need to be well below 10 ns maybe. Your circuit might
not be fast enough to snap the speedier SRDs.

A couple of tricks:

Some varicap diodes apparently make decent SRDs.

Larger junctions, like rectifier diodes, don't snap well, but get
better if, instead of DC forward bias, they are forward biased for a
short time, 100 ns maybe, before being reverse driven. This is the
Grehkov "drift step recovery" effect. In one of our pulse generators,
we forward bias the diode +48 volts (no typo!) for about 80 ns before
we slam it with a -400 reverse drive.

Some transistor b-c junctions will snap.


Ma/Com makes probably the only SRDs that are stocked by distributors,
although they are fairly slow...

http://www.macom.com/DataSheets/MA44700%20Series.pdf

Most other people seem to make them to order, and don't usually stock
packaged parts.

Of course, you need a fast scope, preferably a sampler, to resolve 100
ps edges, and a pure 50 ohm system or else a resistive divider probe,
to avoid the capacitive loading of a scope probe. And a very tight
layout, of course.

Email me your address and I'll send you a few of the Ma/Com parts to
play with.

John


.

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