Re: IGBTs are pretty fast

Tim Williams wrote:
"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...

how can you be sure? intermittent connections tend to work except when you are looking.


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.


an open-circuit could do that :)

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.
;-)

heres a clue - "something" shouldnt happen once the circuit works.


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.


try running the bridge at full power, then bashing the side of your gatedrive mockup with the handle of a large screwdriver. wear safety goggles.

the interconnects in prototype boards start out as cheap and nasty. Unlike fine wine, they do not improve with age. A diode-like interconnect once cost me, another engineer and a tech 3 days once - we narrowed the fault down to a resistive divider that didnt work linearly. replacing the protoboard fixed the problem; the old one got Widlarised.

inductance causes three problems:

firstly, the loops radiate H fields, making EMC compliance harder.

Secondly, they pick up H fields, and can convert them to gatedrive signals.

Thirdly they increase the output impedance of your gatedriver - trace the loop from gate thru Rg, npn or pnp (turn-on or turn-off loops), supply rail, cap, 0V, emitter - the loop is in series with the gate.


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?


didnt your IGBT bridge blow up? doesnt that count as "trouble" ?!

now I've seen the gatedrive construction, I'd list mechanical problems at the top of the "why my igbts died" list.

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.

solve the mechanical issues first. while you do that, you might as well build it on a ground plane - it will take no longer than soldering together a rats nest, while minimising susceptibility to stray fields.



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

its cheaper to not break them in the first place.

power electronics is more about how you do things than what you do. the circuitry is often the easy part.

Cheers
Terry
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