Re: IGBTs are pretty fast

"Terry Given" <my_name@xxxxxxxx> wrote in message
news:1139357225.272181@xxxxxxxxxx
ye gods

Sounding just a little stark there Terry...

one single intermittent connection can destroy your IGBTs.

Well, it hasn't yet, and I've had a few instances, so I don't know what to
say...

I have, evidently, blown (open, not shorted) the PNP gate drive/follower
transistor (which is still only a 2N4403, not the ZTX I purchased for the
purpose). This results in a slow, constant turn-off time, since the current
mirror is in effect through what's left of the Vbe (I guess the collector
blew in this case). And, of course, the desat shoots, turning off at least
one half of the bridge within 3µs.

I have had one instance where something happened to the gate and it stayed
high, resulting in the power transformer groaning *as if the bridge had
shorted*. In reality, the battleship sized transistors were just owning it.
;-)

Not very healthy... but if it happened with the old wiring I would probably
be down another $40. The tighter bridge wiring works much better.

such breadboards are great for prototyping, I've made gatedrives on them >
myself. And once the circuit operates correctly, re-build it on a piece
of copper-clad PCB. that way bits wont move, impedances are far more
controlled, stray inductance reduces (perhaps dramatically), and wires
wont fall off.

Inductance, sure, but I'm still not seeing how the breadboard is going to
screw things up. Honestly, I've used longer air runs between components
when messing around with the output of my 1ns pulse generator, and the
pulses still plink around well enough (20% rule in effect) whatever I have
hooked up.

when it all works nicely, you can then build a little box around the
circuit using more Cu-clad PCB, and solder up all the edges. that helps
keep all the nasty fields out (and/or in), as well as clipped off leads
etc.

Hmm... fields...(yeah, "hummmm indeed", ha! ha ha!). Well the thing is,
it's pretty much as bad as it's going to get, *as is*. But I'm not getting
any trouble, even with the induction coil less than a foot away. So if I
pack it up on a PCB, I'll have less to worry about, which means...I'll have
nothing to worry about?

All the spurious signals occur on edges, and the gate drive and all circuits
already know what they are doing on the edges, plus I have power supply
bypasses scattered about, so I don't really worry about it.

make sure all the HV stuff is well secured (eg screwed to a large
plank), with a shield overtop (1mm lexan is good stuff). that will help
prevent blowups, contain the carnage and reduce the shock hazard. If its
on the floor, dont slip and fall on the bus-bars while its live.

That's not a bad idea. I almost peed my pants last time I had some MOSFETs
go off like shotguns.

The strange thing is, though, the last about 20 transistors I've
burned...didn't. Fuckers won't even tell me who died!

I guess that means I'm getting good at this solid state thing, but I'd much
rather they just ooze the smoke so I don't have to lift and probe each damn
lead to find the dead one(s)...

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


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