On Thu, 6 May 2004 17:43:10 +0200, "Fred Bartoli"
r_AndThisToo wrote:


"John Larkin" a écrit dans
le message news: ...
On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!


If the drain load is resonated and you want some control over the resonant
frequency (i.e. you have external to the mosfet tuning Cs) I don't see much
discontinuities in inductors current. Of course when all the stuff is well
"wired" and that's an entirely different matter, isn't it Paul ? ;-)

John, can you suggest some refs that nicely snap ?


I think that most mosfet substrate diodes are now designed to have
soft recovery, so that they won't snap and make a horrible dv/dt. I
blew up a lot of early Motorola mosfets in an h-bridge motor driver...
had to switch to darlingtons, and only figured it out later.

Anything with a p-i-n structure has a chance of being a snap diode. I
think it needs a hyperbolic doping profile or something to work well.
1N4005-7 types work, but usually only after a brief forward bias, not
DC. Somebody told me that many varicaps snap, but I haven't verified
that. Specifically-designed PIN diodes (the kind used in RF switches
and attenuators) don't snap, as they are doped to have very long
recovery times.

We tested over 60 different TO-220 power diodes to find the best
high-voltage drift step-recovery part. Then we found something else,
much better and more repeatable, but that's still a secret.

John