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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
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EN: The SOA doesn't show high currents for low voltages because they're not possible. The "resistance" of the device (yes I know it's non-linear and not really a resistance) means that you need nearly 10V across it to force 150A through it.
Richie Burnett and I did a lot of research on this back when DRSSTCs were cool We knew that overdriving HAD to hurt reliability of the devices. If there were no drawbacks to it, the manufacturer could just up the gate voltage rating, sell the device as a higher current one, and make more money.
I found a textbook that said that overdriving the gate shortens the life of the device, by making impurities migrate and short the gate out. It was a book about MOSFETs but I guess it applies to IGBTs too. This won't be an issue in hobbyist applications, but for industrial ones, like a SMPS or motor drive that needs to run 24/7 for 10 years without a hitch, it makes a big difference. This book recommended using the absolute minimum gate voltage, even down to 8V.
I haven't found much hard evidence on the effects of running too much current through the thing. There was one paper that looked at stress testing the Motorola IGBT dice that are used in some bricks, and they found that beyond a certain current they started to give up quicker and quicker. They used a lot of technical words to explain this, but as far as I could tell it was a thermal effect: parts of the die were getting hot enough that the metallization layer changed its structure in some bad way that I can't remember.
The bottom line is: IGBT makers know a lot of stuff we don't. They probably have computer models of everything that happens in the device (that they keep secret) and using these models, they design the device to be reliable enough for the above mentioned motor drive that runs for 10 years, because that's where the market is. Then they check that design by testing a sample of the parts.
So it's probably the same deal as what Terry Fritz found with MMC capacitors: overdriving them hurts the reliability, but it's still more than good enough for us.
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'Overdriving' IGBT gate
We knew that overdriving HAD to hurt reliability of the devices. If there were no drawbacks to it, the manufacturer could just up the gate voltage rating, sell the device as a higher current one, and make more money.