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Re: [TCML] New SISG failure mode
In a message dated 6/13/08 8:25:54 P.M. Eastern Daylight Time,
>Have you found any heating of the Sidac's since our last discussion?
Haven't run the coil since.
>I decided to leave them "unsunk" but staggered the Sidac fins for more air
>gap between them.
I staggered the SIDAC tabs for more voltage standoff between them. I
know it's only a 300V theoretical difference between the tabs, but they're all
floating at several kV and passing lots of RF. Guess it can't hurt...
If the current is high enough, one single initial pulse from the first
bang could kill any given IGBT (or any semi). Heatsinking wouldn't matter at a
high enough current level, because it still takes a finite time to conduct
the heat out from the die. That's the "brick wall" I worry about. I think it's
more likely that the IGBT I recently found that died from a secondary
breakdown was probably suffering from steady-state overheating. If it was on the
ragged edge of the max pulsed current I probably would've killed more than just
one. OTOH, as more sections burn out in the SISG, instantaneous current
should go down because the cap will charge to progressively lower voltages before
firing. But the firing rate will go up, increasing the steady-state heating.
I guess you could test it by firing a single bang, and checking if any
SISG modules died. Terry tested them to an extreme, but I don't think we've
found that precise level yet.
The temp coefficient might help a little with steady-state heating for
the IGBTs, but I think we need to force-ventilate if we're pushing the
Super-247s in Pig use. My last run was with .185 uF primary cap with my pig cranked
to 17kV, and I was drawing about 30 amps steady state. About 8kVA, and the
heatsinks got hot rapidly.
The other thing is keeping the voltage carefully at a point where the
BPS doesn't run away. A fine line, and I've been intentionally pushing it to
see how bad I can hurt things...
Center for the Advanced Study of Ballistic Improbabilities
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