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Excellent write-up on FET replacements in a PWM application! I am pleased to hear that it resolved the problem. Eventually, under normal operation, if the TO-220 mounting screw loosens, heat will kill the part. There are two other possibilities that will "shorten" the life of a power MOSFET.
1: The gate drive circuit becomes slow, that means that the On/Off pin of the MOSFET is not being driven hard/fast enough. This causes the MOSFET to spend excessive amounts of time in it's "linear" region, this generates massive heat and results in failure of the MOSFET. Contrary to popular belief, a MOSFET is a linear device... not an on/off relay... unless you drive the gate fast enough.
2: If there is a separate exterior "catch diode" on the output of the MOSFET that prevents the negative inductive kickback voltage from the fan motor, the MOSFET will die due to excessive voltage spikes on its output. This is more of a gradual death that blows tiny holes in the insulation barriers of the gate... failure eventually shows up as a hard short or a wide open on the output.
Once again, great write-up... just adding excessive info.
jorrell
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92 245 250K miles, IPD'd to the hilt, 06 XC70, 00 Eclipse custom Turbo setup...currently taking names and kicking reputations!
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