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RE: OL-DRSSTC 7 - It's Alive!

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Jason,

At 05:51 PM 10/10/2005, you wrote: 
Hi Terry,

> Were you using gate drive resistors and all??  The IRG4PG50WD
> can take 750 amps!!  But if the gate voltage is being driven
> high by "shoot through" (Miller capacitance from drain to
> gate is driving the gate voltage too high and blowing out
> that gate capacitor), they will blow real fast...  Maybe
> clamp the gates and slow the dV/dT...

I tried gate resistors from 0 to 50 Ohms and used 25V TVS between gate
and emitter. I have used the IRG4PF50WD IGBTs up to 550 amps in an OLTC
and take your word that they will do 750 amps. What I was trying to say
before is that I don't think they can hard switch currents of that
level. My problem only seemed to occur when I was hard switching current
much over 200 amps (peak 320 amps)which is what was happening due to
driver delays and the 130Khz operating freq.

I see. The "peak power" in that case might be enough to super heat the die over about say 200C for an instant. That will explode the die. When it is not switching, there is no switching loss in the equation. But if the die has say 320 amps 350VDC for 1uS, the peak power is over 100kW(!!) and 0.1J.

Figure 6 in the data sheet puts the pulsed power thermal resistance at 0.007 c/w absolute minimum. Assuming we can go from 25C to 175C and survive, the pulse power is 150C / 0.007 = 21kW. Thus, anytime the IGBT sees that, destruction is almost assured!! There is no fudge or safety factor in that number and the destruction is "instant".

Assuming the voltage drops by 1/2 and the current is 1/2 at the worst power peak in the switching and the bus voltage is say 320V...

1/2 x 320 x 1/2 x Ipeak = 21000 Ipeak = 262 amps. Pretty close to your 320 amps!!

Without switching, at about 750 amps with 25VDC on the gate, the current is so high that the IGBT starts to fall out of saturation and the voltage across it start to dramatically rise. More gate voltage does not help at that point. 21000 / 750 = 28 volts. So above 750 amps, the IGBT will turn into a resistor, and then a fire cracker real fast once Vce gets to about 28V.



The gate spikes I was seeing were going negative for about 50 ns, but I
never saw them exceed the gate voltage rating. Could those fast spikes
tear the gate apart without exceeding the voltage rating ?

Could they have been scope probe glitches?? I see odd little glitches like that all the time. Moving the probes around changes them all over the place. Sometimes it is best just to switch the scope bandwidth down to 20MHz and be happy :o) Miller switching glitches go positive not negative. The gates are pretty good 3.3nF capacitors at that point so a 50nS glitch across them would need a lot of current.

V=1/C INT i Dt  15 = 1/(3.3e-9) I 50e-9    I = 1.0 amp

A 1 amp gate current will not damage the IGBT. They are using 10mil gate bond wires and all that. I think it is a peak power thing instead.


Maybe I should set up a test rig and try to prove either way if hard
switching 200+ amps = smoke. That still may leave doubt as to the exact
mechanism of failure thgouh.

I think maybe fine tuning the numbers in the peak power thing above will answer the question without any more smoke ;-)

In my case, I am hoping the gate drive will get faster as the peak current drives the gate CTs harder preventing a peak power bang problem. That is certainly a critical thing to watch!

http://hot-streamer.com/temp/OL-DRSSTC-2005-10-10-001.gif

Cheers,

        Terry



Jason.

>
> I don't know for sure, but your problem sounds like it needs
> TVS right across the gates and gate drive resistors to slow
> the switching down a little...
>
> Cheers,
>
>          Terry