Mosfet noise - Clicking and Screetching

K

krazzyj

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


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

Paul
 
In article <4150b137$1@duster.adelaide.on.net>, zero242@internode.on.net
says...
Sounds like the unit is trying to drive into a dead short

Paul
Click to expand...

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

-kj
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top