Re: mosfets keep failing
- From: "siliconmike" <siliconmike@xxxxxxxxx>
- Date: 15 Dec 2005 08:29:15 -0800
Your explanation suggests that somewhere you have an inductive load and
probably on top of the drain.
Not considering any wiring errors or wrong soldering or hairline
shorts, could it be the output inductance of your power supply??
Just a wild guess..
Mike
exxos wrote:
> 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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