Mosfet noise - Clicking and Screetching
 

I have a custom power supply that has an input rating of 120vac and an=20
ouput rating of 24vdc @ 6amps (apx 140watts working). The main switching=20
MOSFET is making an audible noise... clicking when first turned on and=20
then it screetches and voltage drops to 0 then clicking and slowly back=20
up to 24=B1 vdc then screetching and back down to 0. What could be causing=
=20
this? - could this be a bad rectifier in the AC/DC wave form conversion=20
or do you think it could be a bad capacitor that is leaving the circuit=20
out of ballance... I have tested everything and it seems to check out.=20
All resistors are within spec. MOSFET has been replaced (no change). The=20
capacitors under 100uf test out ok (I have no way to test the 2 caps=20
that are 200v 330uf).


Thanks,
kj

Sounds like the unit is trying to drive into a dead short

Paul

In article ,
says...
Sounds like the unit is trying to drive into a dead short

Paul



Paul, Thanks for the reply...

I have a few days to figure this one out for now, once it is warmed up
it seems stable - Maintaining 24v solid... and the noise reduces. Would
you reccomend looking at any particular component?

kj

The clue is warming up

do you have pre driver transistors for the MOSFETS?

the other way to find out is to let it warm up and hit it with freezer
spray, various components, probably driver transistors and caps


Paul

loedown wrote:

The clue is warming up

do you have pre driver transistors for the MOSFETS?

There is a small 8 pin IC that looks like it is driving the gate on the
MOSFET... would heat cause this IC to work? - I will try the freeze test as
soon as I can get the board back out - we were forced to put it back in use
temporarly...

I would imagine diode test on the IC when removed should let me know if
there is a short or too much resistance...

the other way to find out is to let it warm up and hit it with freezer
spray, various components, probably driver transistors and caps


Paul

Thanks for the tip on the spray freeze... I was thinking about using canned
air to freeze the components...

-kj

The IC could be very much the culprit

Paul