Substituting MOSFET on PC Motherboard

[sparksflying]

My computer stopped working yesterday so I tested the PSU, then plugged in things one at a time. Turns out there was a short across the 12V 4pin motherboard connector. I hooked it up to the bench supply and pumped 5A into the shorted input, one of the MOSFETs was burning hot. I found a Drain-Source short on it after desoldering.

The original part is a NEC 2SK3918 http://datasheet.eeworld.com.cn/pdf/NEC/57164_2SK3918-ZK.pdf

I had a similar N-Channel MOSFET in my cabinet, the IRLR8729 http://www.irf.com/product-info/datasheets/data/irlr8729pbf.pdf

Luckily it was the same package and footprint so I soldered it in, reassembled computer and it worked! ....but only for 24 hours...It shutdown again, desoldered the replaced part and it was also Drain-Source shorted. I replaced it again, this time with a glamorous wire heatsink just in case overheating was the problem. http://i61.tinypic.com/e8a3yo.jpg

Are these parts compatible? The specs looked quite similar. What do you have to match when finding replacement MOSFETs?

[T3sl4co1l]

Likely the same thing that cooked the first one, cooked the second...

Tim

[theatrus]

These FETs are the switchers for the DC/DC converters used for the CPU and chipset power rails (efficient stepdown to <= 1V at 100W in aggregate, there are usually several channels). Based on there being two FETs and no diodes, this is like a synchronous converter - check that the second FET is in spec, and check for issues with the inductor.

[NiHaoMike]

Check for failing caps with an ESR meter. Also disable any (not so) smart fan settings or set them to more reasonable settings. They tend to default to "lazy".