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Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
This thread is getting interesting :)
One thing that is related to this temperature issue is different failure modes. So far i have seen two different failure modes in drsstc applications (both measure dead short over C-E)
1. Only one or rarely two parallel dies are shorted, very little visible damage and usually rest of the brick is still fine (cut bonding wires and put it back together -> works)
2. Everything is obliterated, usually IGBT case is also ruptured.
Violent failures are usually result of large scale overvoltage or overcurrent but how about these silent failures? Maybe thermal cycling stress -> igbt chip delamination and localized overheating -> pop?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I think if the bonding wires survive, the failure is "silent". If they fuse, a power arc forms which fries the insides and the expanding gasses can rupture the case.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
I think that the silent deaths are exactly what's been just pointed out. Uneven incuctance causes one die to recieve a higher transient than the rest. No over current or blown bonding wires, just an over voltage failure.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, but the point is that semiconductors always fail short circuit, and we like to use them with big energy storage cap banks connected across them. No matter how the module is stressed, one die will always fail before the others, and then the DC bus will discharge through it and its bond wires. A H-bridge has two devices in series across the DC bus, but if one fails, the other will find itself driving a short circuit and will fail very soon after.
I've seen big IGBT brick and SCR datasheets that quote a "case rupture I2t". Presumably the idea is to use a fuse with a smaller I2t than this, so the part won't explode violently, blow the equipment to pieces and maybe injure some personnel.
But presumably there's some lower value of I2t where the die that fails won't explode violently enough to nuke all of the others.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
Steve Conner wrote ...
I've seen big IGBT brick and SCR datasheets that quote a "case rupture I2t". Presumably the idea is to use a fuse with a smaller I2t than this, so the part won't explode violently, blow the equipment to pieces and maybe injure some personnel.
But presumably there's some lower value of I2t where the die that fails won't explode violently enough to nuke all of the others.
now if only that fuse was a breaker that would solve the over-current problem in coiling.
and while we're at it maybe a tvs that when a transient appears an internal sensor cuts power to the dies, and diverts it... but then the igbt would cease being a brick, and become a computerized switching device XD
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I have some solid-state coils that are getting on for 10 years old. They still have the original IGBTs. I've never blown a single one.
I think the only overcurrent problem in coiling is people who turn the current limiter up too far or run without one. If you have a current limiter, a driver that does zero current switching, and a nice low inductance bus layout, there's your computerised switching device right there.
When you get to brick sized IGBTs, it's hard to find TVS-type devices big enough to catch the massive spikes that you can generate. In my OLTC work I tried and gave up, I used an active clamping scheme that fired the IGBT permanently on and shut down the power supply.
That was needed because the OLTC used a single switch supplied with constant current through a DC bus inductor. A DRSSTC uses a voltage source inverter. If your bus inductance is low enough, the diodes built into the IGBTs will catch voltage spikes and dump them back into the DC bus capacitors, so no TVS are needed.
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igbt speed
that would solve the over-current problem in coiling. 