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
|
