mosfets keep failing
- From: "exxos" <exxos@xxxxxxxxxx>
- Date: Wed, 14 Dec 2005 13:03:27 -0000
Hi all,
simple half bridge setup, mosfet power rail running at 70VDC (200V max
mosfet spec), loaded with approx 50ma (about 20amps mosfet max spec)...
frequency of approx 100khz, running into a dummy load resistor (hence the
50ma draw).
Problem is it works fine for about 20 seconds then the lowside mosfet fails
with S-D short. I have had the high side fail also, but seems to be the low
side failing at the moment. Mosfet gate voltage 12V. All IC driven, highside
with charge pump.
Everything works fine, then blows the fuse after 20 seconds. The current
remains constant at 50ma, then for no reason at all it jumps to 3.5amps and
blows the fuse. Mosfet always fails S-D short. The gate never seems to be
effected.
I can't imagine any reason for this whatsoever. The only thing I can think
of is maybe the gate has a massive overshoot of like 50V or something and is
blowing the mosfet gate, though I would have thought the gate would short to
Source or Drain at least, but it never does.
If the mosfets were overloading, due to deadtime problems or something, then
they would get pretty hot and normally fail totally short, in extreme case
explode. Though I am nowhere near the spec limits of the mosfet this time.
I "don't have the time" to take any scope readings before the mosfets fail
either, I can only think that the gate has a massive overshoot and is
killing the mosfet, though I do not understand why it would fail S-D short
each time. Also can't see how it could spike that high from a direct drive
chip on 12V. Even with some ringing or overshoot it would still have to get
up to like 20V to blow the gate, ive never seen anything *that* bad before.
Can anyone suggest why my mosfets always fail S-D short after about 20
seconds ?
Cheers,
Chris
.
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