Re: IGBTs are pretty fast

Tim Williams wrote:
"Terry Given" <my_name@xxxxxxxx> wrote in message
news:1139430680.684090@xxxxxxxxxx

how can you be sure? intermittent connections tend to work except when
you are looking.
^^^
not


Er, so when I'm looking, they tend to be intermittent, thus, I would know
about them?


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.


Screwdrivers aren't part of the design equation, you're changing the
conditions! ;-)

what, too chicken to try it? why not, you seem to like your gatedrive construction....




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.


Yabbut, moot point as the loops aren't particularly wide and the only signal
they recieve is, at most, in the milivolt range -- a pulse could cause a
comparator to switch early, but only when it's about to switch anyway.


how do you know that?

it is often quite easy to cause gatedrivers to switch several times on any given edge. that can be a great way of making switching losses much higher than you expected. I have tracked down several such problems in the past, that exhibited themselves as "random' failures during soak testing.

after enough time spent tracking down these sorts of faults, one learns to avoid them in the first place.


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.


I have 0.1uF ceramics at the transistors, so the output loop on the board is
under an inch diameter. There's more before the lead turns to twisted pair!


ignore the twisted pair. that inch or so just added a hundred nH or so to Zg.


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


The circuit, before and after the faliure, tested fine. It was the bridge's
fault, as near as I can tell.


you blew up the igbts without breaking the gatedrivers? thats a good trick, normally the collector shorts across to the gate, and trashes the gatedrive output stage (or more).


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


Afraid I have to disagree on this one. Heh, the IGBTs blew too fast for any
mechanical fault to have a reasonable propability to show up. ;)


you obviously dont understand my point.

one single intermittent connection in a gatedrive/igbt assembly can *destroy* the power electronics. your construction technique (OK, I was going to say "hairy-assed mess") is just *begging* for such an event to occur. Hell, it may have done so already (and IMO probably has).

whats worse, once you fix everything, the intermittent connection might not be obvious.

the only practical solution is to build the damn thing properly.

plus murphys law applies directly here.



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.


As in those RF lashups? Uhh...no.

For GHz circuits I would take the time and tediousness for it, but for
pete's sake Terry my edges are two and a half orders of magnitude slower,
and even as nearby as things are, the inverse square law is with me on stray
fields.

fancy pushing on some of the proto-board wires while its running? no? why not?

AFAT inverse square law goes, look at the impedances too - if its a nice high-Z circuit node, it might not need much to push it around. and with enough wiring inductance, even a low-Z node can look high-Z to a nice fast edge.

hell, once 50fF of capacitance buggered up a perfectly good circuit I'd designed. yep, 0.05pF. 5V square wave with 10ns edges, 50fF to the -ve input of an opamp, with Z = 10k or so.

thats a moot point though - if you need to improve the mechanical construction, you might as well do it in as "RF" a manner as possible.


why not think about it from a risk management perspective? them IGBTs aint cheap, it should behoove one to try not to break them.



I can do point-to-point wiring on perfboard, or a step up from that, the
perfboard RS sells that has individual copper pads. This doesn't lend
itself to ground plane technique very easily.

it does if you sit it on top of a ground plane.

I don't see PCB happening any
time soon since I don't have PCB design software, resist, etchant, or any
reasonable way whatsoever to drill the holes lined up properly.

Tim

I dont do those things either, I build circuits on top of a piece of copper-clad board. some chips end up upside down, others dont. go read the two books edited by Jim Williams, and/or some of the analog devices & linear tech app notes on how to build a decent prototype.

with a bit of practice, its no slower than using proto-boards.

Peroration:

there are a wide variety of problems that can, and do, beset power electronics. physical construction is usually at the top of the list (my circuit looks OK, why does it blow up) - both from an EMC and a reliability perspective.

If you deliberately avoid the really dumb mistakes (a wire fell off, a clipped lead got into the hardware, I forgot to fasten the IGBT to the heatsink, giant loops everywhere, DIY thru-plated pcbs etc) that leaves you free to focus on the real problems.


Cheers
Terry
.

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