Re: I'm popping MOSFETS....linear derating factor involved?

On Mon, 20 Mar 2006 11:13:49 +1200, Terry Given <my_name@xxxxxxxx>
wrote:


I'm in the 2nd hour of a thermal cycling test which brings the FETs up
to a heat sink temperature of 83C and back down to 30C every four
minutes. I believe that this is more strenuous than merely holding
the FET at a high temperature, though I did that for a couple of hours
first.

oh yes. large (and rapid) dTj stresses the die attach, which can (and
does) degrade the connection. this increases the thermal resistance,
eventually leading to a runaway failure mechanism.



We've been shipping NMR gradient drivers for over 10 years. Various
units have from two TO-220s (3 amp versions) up to 32 TO-247s (100
amp, 17 KW peak out.) All run pulsed at 5-10% duty cycle and all push
the junction temps 50-80C per pulse, roughly one pulse per second.
They run a lot, since the machines are busy and expensive.

So far, we're not seeing any evidence of a fet wearout mechanism.

John

.