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Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
I never bought into the "partially failed" thing. You can usually measure the device pretty well with a meter. Check resistances and voltage drops with a known good device. If it matches within a few % then i re-use it. Of course, i shouldnt claim to have experienced every type of failure, so just take this as my opinion and not a fact... but its how *I* do things.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I just tested some CM600HA-24H IGBT bricks the other week. I started with five that I thought were good. Four tested as I'd expect with a DMM, but the fifth one showed a low gate resistance. (When testing IGBTs or MOSFETs, any gate resistance less than "Overrange" on your meter is cause for concern.) However, it would still switch 30V fine, although it drew noticeable gate current when I turned it on.
So I went on to measure the breakdown voltage of all the bricks carefully, by applying small amounts of HV current to the collector through a MOT, diode, and series resistor. I carefully increased the voltage to the MOT using a variac, while looking at the collector voltage on a scope with HV probe. The gate was shorted to the emitter. You might find this alarming, but I've done it before with no apparent ill effects. I believe that with large IGBTs, they don't even notice a few mA of breakdown.
The four "good" ones all started to conduct and clamp the voltage around 1250-1300V which was cool. The fifth one leaked so badly that it couldn't get above 500V at any variac setting, and the series resistor smoked. High frequency oscillations could be seen on the collector.
So yes, it does seem possible for semiconductors to partly die. That IGBT brick could still switch low voltages, but didn't seem to function properly at its rated voltage any more. It was pulled from some piece of industrial equipment, so it was maybe damaged in a fault. It could have been the classic scenario that Marko hinted at: an inverter blows out, all six IGBTs get replaced, and any that didn't actually need scraped off the ceiling end up on eBay...
Or maybe it would have continued to work fine like that: the leakage could have been insignificant in the context of the machine it was in, even if it was enough to smoke a sugarcube resistor. I guess I'll try applying 600V DC to it and see if it blows up :P
It was not one of the bricks from the OLTC2: they both tested fine
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
That's interesting.. I did have mosfets with such 'leaky gates' but never 'usable' like that (fully 100% dead). I don't think you'l ever probe a bad device as good if you are used.
Gate should hold it's charge when touched with 9V battery, and mosfet should show it's ON resistance. If something happened to the mosfet I think you'l notice it...
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"Half dead" semiconductors
