Dave Platt wrote:



Maybe a short inverse spike condition during powerup, which
reverse-biases the base and makes it avalanche?

Analyzing the bias supply for the base, I can't really SEE how it could get
a reverse voltage spike. I certainly thought about this on power-up.
OHHH, wait! Maybe it could be on power DOWN! There is a bias network on
the base with a capacitor to ground, and the supply to the emitters is
connected to a reference supply that might dump VERY quickly when power is
cut off. That could maybe put about -6 V bias on the base when the emitter
supply drains back toward zero. I could probably stick a diode there to
limit the reverse bias.

Or, is there any chance that when the one-shot fires, the transitions
are slow enough that the transistor passes outside of its SOA while
turning on or off?
Well, maybe. But, it is only 23 mA, and differential pairs are usually
pretty benign when they transition. We just got this module out of a pile
of stuff, it is the only one we have, and we have no history on it. It
might have sat in some setup for 30 years powered on. Or, it might have
experienced some kind of transient failure of the power supply that powered
it in the past. This one-shot can't go over 100 ns, and the transition
should be REALLY sharp, just a couple of ns.

So, the only way to know if it will be reliable is to test it a bit.

Thanks,

Jon