Switch Mode Power Supply repair -Mosfets?

[april]

I have a 20 amp supply that I want to salvage . It blew up and two of the Mosfets really blew out
They are D209L 602F?T?C? The last letter is one of the three I think.
Its a fast switching transistor
Can I replace them with IRFB17N60 Mosfets

 

[dougy83]

Probably not. MOSFETs have different drive characteristics to NPN transistors. The MOSFET will want ~10V gate voltage to be properly on, while the NPN will want ~1A base current (and only ~1V).

Assuming the drive voltage will actually go to 10V (with the NPN removed), you'll probably need a strong pull-down resistor to turn the MOSFET off when the output is no longer being driven. If the drive voltage can exceed 30V, then you'll want some protection in the form of e.g. a 24V zener.

If you can provide a schematic then you can get a more definitive answer.

 

[april]

Thanks very much I don't have a schematic its a repair but there is a lot of difference between the two is'nt there. I'd better go for the transistors

 

[dougy83]

Yes, it's best to use the original spec'd transistors. You can use similar ones that have better ratings as well.

There may be other problems in the supply as well; according to https://en.wikipedia.org/wiki/Switched-mode_power_supply#Failure_modes, the power transistors failing can be the result of the bulk capacitors degrading. There's also a section on testing and repair of SMPS in https://www.electro-tech-online.com/custompdfs/2013/05/slva085.pdf ; it doesn't describe your supply, but the info may be helpful.

Basically the input rectifier, capacitors, switching transistors, output diodes and output capacitors may be faulty as well; so you can either test them all [preferably out of circuit] (capacitors: ESR, value, leakage resistance; diodes: forward and reverse voltage drop), or pray that it's just the transistors that are bung. Praying is not guaranteed to work unfortunately.

Best of luck, don't fry yourself doing it.