How to test N-channnel Mostfet marked 'SFF7N60'?

[Willen]

How to test N-channnel Mostfet marked 'SFF7N60'? (PC SMPS Problem)

It is used on SMPS of my PC. And SMPS has been damaged. Everytime when I turn ON, the fuse of SMPS is burn out. I guessed the mosfet has problem (not fix) so want to test its condition.

Also if you have any idea about my problem please tell me something. I guessed that components before transformer is 'Power stage' and plays role to burn fuse instantly when I turned ON. On this SMPS there are mostly only three active components before transformer-
1) C5027 transistor and it's fine.
2) SFF7N60 N channel Mosfet
3) A 8 din DIP IC 'UC3843A'

so guessed that problem (fuse burn everytime) is on 2nd and 3rd components, isn't it? (Manufacturer is 'perfect ltd.' from China I think)

 

[mvs sarma]

instead of landing on 7N60, check for shorted diodes and replace them at the power input. after that check for any possible short across the rectified 310V or so. if the mosfet is short, a resistor on its emitter burns off and opens the circuit.

 

[dr pepper]

A very simple check for the fet is meter pins 2 and 3, it will read a dead short if the fet is dead, but this is only a basic test and may not show anything up.
Also check the above items on mvs's list, and then check the transformer primary, last time I had this and the time before the transformer was shot, apllying mains for anything more than a few uS toasts the primary.
The 3843 is a smpsu controller and should be easily obtainable, changing it as a matter of course is prudent.

 

[Willen]

if the mosfet is short, a resistor on its emitter burns off and opens the circuit.

...and if burnt resistor opens the circuit once, it won't burn off the fuse again when I turn ON the SMPS I think. So almost that mosfet has no problem.

I will test each diodes on power section (before transformer). But how to be sure if primary of transformer has been shorted? (continuity test will not work here).

Also don't have any ideas to test SMPSU IC.

 

[Willen]

Wow yes! I found a shorted dioded. I replaced it and fuse burn problem has been solved but SMPS is not working, I burnt my hand instead (due to storage current of high voltage capacitor).

Another- do you have an idea to turn ON the SMPS of PC without motherboard?

 

[dr pepper]

Connect green to black on the motherboard connector.