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IGBT transistor failure mode - slower closing?
daqq:
Hi guys,
I'm dealing with a device where an IGBT transistor connects a capacitor bank to a pulse transformer, which then feeds a resistive load. The transistor is opened for about 2us, then closed, the resulting waveform is square-ish. The normal input voltage is about 300VDC, the figures displayed are from a 40V test voltage, but the issue is the same for both voltages. The current @300V is around 500Amps. The transistor is FZ900R12KE4 ( https://www.infineon.com/dgdl/Infineon-FZ900R12KE4-DS-v02_03-EN.pdf?fileId=db3a30431f848401011fb7c00c74594e ), a 1200V IGBT, being driven by the 2ED300C17 driver module ( https://www.infineon.com/dgdl/Infineon-2ED300C17_S_ST-DS-v04_03-en.pdf?fileId=db3a304412b407950112b429b99741da ). There's 2 Ohms in series with the Gate. When the device is working properly, the waveform looks like this:
There are several units, all of them have waveforms like this - a fast rise, a fast fall. However, one stands out:
There seems to be a much longer falling edge and the waveform is of a very ugly shape - not square-ish at all. The load is the same. I have measured the gate voltage waveform and it is absolutely identical to a known good device.
Now, I'm trying to isolate the problem - I'm fairly confident that this is something that showed up recently and that the device was not doing this before - we probably would have caught it during initial testing as waveform shape is checked. That said, it could have happened. The transistor looks the same (no physical damage that I can see).
My question is, is there a mode of failure of IGBT transistors that would result in a slow fall time, but keep the other parameters (gate charging, rise time etc.) the same?
There's also a few snubbers and other things, I'm checking them as well, but I can't find anything wrong with the device.
Thanks,
David
krish2487:
Try placing a UFR diode anti parallel to the 2 ohm series gate resistor. From what I can see, I can only agree with you.. The IGBTS are turning off too slow..
Placing a diode anti parallel with allo you to control the on time while still having a quick off time..
However, EMI might become a potential issue with such a fast turn off..
On closer reading... I think you might have an issue with the gate driver... Can you just swap the drivers between the good / ugly boards to see if the problem remains on the same setup or not?? It might give some insight to locating the problem..
daqq:
OK, mystery solved, it was a faulty diode in the RCD snubber. It failed and shorted out... this'll be a real headache... :( Now to find out why the damn thing failed, it was a DSEI2X101-12A ( http://ixapps.ixys.com/DataSheet/DSEI2x101-12A.pdf ).
It probably wasn't overvoltage or current since the resistors are perfectly fine and would have died probably... this'll be fun.
capt bullshot:
One rough guess:
The repetitive 500A peak current into the diode creates too much power dissipation when the pulse repetition rate is too high.
Not taking the turn on transient of the diode into account, just forward voltage over peak current, the 500A roughly creates 700W peak power into the diode. So I'd recommend to calculate the Joules per pulse and check the power dissipation capabilities.
daqq:
At a pulse repetition rate of 250pps and the average power is negligible. There are devices running, the temperature rise on the diode is around 2-3 deg C or so, nothing particularly amazing.
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