Re: IGBTs are pretty fast

Tim Williams wrote:
"Terry Given" <my_name@xxxxxxxx> wrote in message
news:1138839797.613799@xxxxxxxxxx
<snip>

... and the gate lead and Rg's sitting on top of it - IOW a "ground
plane" around the IGBT gate connections/parts.

it doesnt matter how well twisted or ground-planed the rest of the
gatedrive is, this loop buggers it up.


Alright, so I "at least squish[ed] the gate resistors in".
http://webpages.charter.net/dawill/Images/IGBT3.jpg
There's a strip of cardboard there to try to ensure they don't short out.
The bottom left (high side) resistor runs across the transistor, placing it
closer to whatever loops are in the transistor, and the collector (B+ rail)
current waveform. This should at least be similar to the emitter current,
no? The alternative is to route it down and around, which opens up a 1/4"
loop, which is "bad".

The upper right (low side) transistor has the same problem and solution,
except there's only the transistor to "ground plane" it.


Secondly, the inductance "softens" the gate drive response to an edge.
For Rg = 10R, 100nH has the same impedance at 16MHz - a rise time of
around 20-30ns. for negligible contribution, rise times need to be less
than 200-300ns, which they are NOT. So when some evil dV/dt happens (say >

every edge), current flows into the gate, raising (or lowering) Vge....

Yabbut that's what the negative bias (-5V OFF state) is for, as I recall,
isn't it? (That and emitter L flyback.)

Current can fall at 50ns (IGBT t_off), but gate voltage falls in around
200ns. It easily passes the linear region in 50ns though.


can U show us a pic of the gatedrive PCB....


It isn't.
http://webpages.charter.net/dawill/Images/GateDrive.jpg

ye gods

It's all mounted on an aluminum backplane, but I don't see a ground for it.
Can always mount a screw for one, I suppose. The circuit seems to be
working fine at the moment.

Tim

one single intermittent connection can destroy your IGBTs.

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.

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.

also dont forget the dangers of small components (eg nuts, washers), tools etc.

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.

Cheers
Terry
.

Relevant Pages

  • Re: IGBTs are pretty fast
    ... plane" around the IGBT gate connections/parts. ... this loop buggers it up. ... The bottom left resistor runs across the transistor, ... every edge), current flows into the gate, raising Vge.... ...
    (sci.electronics.design)
  • Re: Chaos in FF metastability
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