Re: mosfets keep failing
- From: cbarn24050@xxxxxxx
- Date: 18 Dec 2005 05:33:31 -0800
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
Overvoltage caused by the mark/space ratio not being 1:1. Overcurrent
due to not enough dead time on drive waveforms. Get yourself a storage
scope and capure the waveforms, or power it up on a low duty cyle.
.
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