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Mysterious FET destruction on high-power H bridge

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cur8xgo:

--- Quote from: rschlaikjer on June 21, 2019, 07:39:35 pm ---
--- Quote ---Were you doing the "backdriving" thing during these plots?
--- End quote ---

Yes, during these tests we are doing the same oscillatory driving behaviour.


--- Quote ---Or you could just set the timebase to 1uS and trigger on voltages higher than Vds...
--- End quote ---

Tried this - with the 40A supply and a timebase of 1uS; the scope did not trigger on Vgs with a trigger level of 15V, and did not trigger for Vds with trigger level of 25V.

--- End quote ---

Hmm. That may be enough for me anyways..to say its not Vds avalanche or Vgs being exceeded. Of course..its not with the 100A supply.

Have you considered temporarily swapping in some much beefier fully avalanche rated fets, at least for the 100A test? That way if they survive, you can be sure you are observing whatever killed the wimpier fets.

Need to divide and conquer here..get some kind of evidence that it is or is not one type of failure or another. Narrow it and narrow it until its trapped

rschlaikjer:

--- Quote ---Have you considered temporarily swapping in some much beefier fully avalanche rated fets, at least for the 100A test? That way if they survive, you can be sure you are observing whatever killed the wimpier fets.
--- End quote ---

Yeah, we can do that - what exactly do you mean by "fully avalanche rated"?

For example, the two fields in the datasheet for this TK90S06N1L with 'avalanche' in them are the single-pulse energy and the single-pulse current of the avalanche event: https://www.digikey.com/product-detail/en/toshiba-semiconductor-and-storage/TK90S06N1LLQ/TK90S06N1LLQCT-ND/4815247

The avalanche energy is lower than that of the MCU80N06 we've been using, but while the MCU80N06 doesn't actually specify an avalanche current, this Toshiba part does specify an Ias of 90A - would this count as 'fully rated' for this application? Or is there something else I should be looking for.

If this seems like something that would fulfill the avalanche ruggedness requirement, I can order a couple of them now to test out on Monday.

cur8xgo:

--- Quote from: rschlaikjer on June 21, 2019, 08:07:24 pm ---
--- Quote ---Have you considered temporarily swapping in some much beefier fully avalanche rated fets, at least for the 100A test? That way if they survive, you can be sure you are observing whatever killed the wimpier fets.
--- End quote ---

Yeah, we can do that - what exactly do you mean by "fully avalanche rated"?

For example, the two fields in the datasheet for this TK90S06N1L with 'avalanche' in them are the single-pulse energy and the single-pulse current of the avalanche event: https://www.digikey.com/product-detail/en/toshiba-semiconductor-and-storage/TK90S06N1LLQ/TK90S06N1LLQCT-ND/4815247

The avalanche energy is lower than that of the MCU80N06 we've been using, but while the MCU80N06 doesn't actually specify an avalanche current, this Toshiba part does specify an Ias of 90A - would this count as 'fully rated' for this application? Or is there something else I should be looking for.

If this seems like something that would fulfill the avalanche ruggedness requirement, I can order a couple of them now to test out on Monday.

--- End quote ---

Check this out:

https://www.vishay.com/docs/90160/an1005.pdf

You might end up with the impression I got, that is that just because there is an entry in a datasheet that specifies some avalanche something, doesn't mean its the same as a fet which really goes out of its way to show you how avalanche proof it is..for instance:

https://www.infineon.com/dgdl/irfp7430pbf.pdf?fileId=5546d462533600a40153562ca5682025

Right on the front of the datasheet:

" Improved Gate, Avalanche and Dynamic dV/dt Ruggedness"

 Fully Characterized Capacitance and Avalanche SOA"

And that along with AN1005 (which IRF also references) gives you big confidence they are fully okay with you being thermally limited in avalanche and using it in avalanche. Also the page of the datasheet where it talks about its avalanche specs has more data than just a single number.

This versus a single avalanche energy value from MCC which, who knows how they anticipate usage in avalanche. Lots of ways to put numbers in a datasheet that don't translate to what you would expect.

Anyways this is just a test, I think you should be able to do what you want with the MCC fets you have once you isolate the issue. Better this problem pops up now then later, you can make this really robust before it hits the street.

cur8xgo:
BTW I just like talking about avalanche. The problem you are having could be completely unrelated. Don't want to get too narrow visioned

rschlaikjer:
Alright, picked up some of those IRFP7430PBF‎, as well as some IRF3205STRLPBF‎, which also claim to be 'fully avalanche rated' directly in the datasheet. On Monday I'll bastardize these parts onto the board and see how they do. Thanks again for all the advice!

And yes, it's possible it's not avalanche - the current limiting has been mentioned by quite a few people now. I'll see if I can also expedite a spin of the board with the proper plumbing on the current sensor. My design intuition for this is to have two comparators connected to the sense line from the hall sensor (which at 0A is at vcc/2), and have one comparator go low when the voltage goes over 90% vcc and the other when it goes under 10%. Then just AND those together and AND that result with each of the A/B control signals to inhibit drive when the current gets high. Any obvious problems people can see with that approach?

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