[Fwd: LF: Testing FETs]

[Andre Kesteloot]

James Moritz wrote:

> At 20:58 27/10/2002 +0000, you wrote:
> >I have a faulty Decca PA module and I suspect that one or more of the
> >IRF250s have blown.
>
> Dear Peter, LF Group,
>
> I have in the past found power MOSFETs can partially fail, where they
> appear to be working when simple go/no-go tests are applied, but quickly
> get hot and fail when put back in the circuit. so it is certainly a good
> idea to do the tests Alan and Stewart have suggested. In particular, pretty
> well no gate current should flow, so if you connect a clip lead to the gate
> to bias it on, then remove the lead without touching the gate, it should
> stay biased on for several seconds at least, probably minutes or more.. The
> "on" resistance of an IRF250 is 0.085ohms maximum (with 10V Vgs bias), so
> too small to measure with a multimeter - better to put a known current, say
> 1A, through it and measure the voltage drop between source and drain leads
> (should be below 85mV).
>
> Also check all the other components in the PA circuit - all the diodes and
> zeners, the driver transistor etc. When one component goes, it tends to
> take others with it. When you put it back together, with no "HT" supply
> check there are reasonably clean 136kHz square waves driving all 4 gates at
> about 15 - 20V pk-pk, symetrical around 0V. Start off running the module on
> a low "HT" voltage, say 5V (but don't reduce the driver supply voltage!!!)
> - the current drain should be down in proportion to the reduction in supply
> voltage with a class D amplifier. When you are sure it is working OK at low
> voltage, gradually increase to the full supply voltage, checking that
> things don't get hot. Starting off with a low supply greatly reduces the
> possibilities for blowing things up again.
>
> Cheers, Jim Moritz
> 73 de M0BMU