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

On Sat, 18 Mar 2006 19:01:33 GMT, John <jmuchow@xxxxxxxxxxxxxxxxxxxxx>
wrote:

Without the spreader the temp of the heat sink (with AAvid's UltraStick compound)
directly next to the FETs was 87C-88C.

With the copper heat spreader (and Aavid's UltraStick compound), the temps were 82C-84C. About 5C
lower.

I tried the same test using Chemtronics thermal grease, well mixed in
the tube before spreading. I also spread out the grease very thinly
on all surfaces and did a test assembly/disassembly first to make sure
that all surfaces were flat and that the grease was being pressed down
on everywhere....it was.

With the copper heat spreader and silicone grease, the temps were
85C-86C.

This is about 2C-3C higher than the temps I got using the same setup
and Aavid's UltraStick compound, which mirrors early tests results
when I first started using the stuff. On paper, its specs are better
than silicone grease. Grease is a heck of a lot easier to apply and
clean up though.

With proper controls in your iterations, changes to the mounting
method or to the thermal impedance between the semiconductor bodies
and the heatsink will not alter the temperature of the heatsink. Only
the temperature of the semiconductor's mounting surface and the
semiconductor itself will vary.

The thermal impedance from the heatsink mounting surface to free air
is not being changed.

Measure the temperature of the semiconductor's tabs and the
temperature of the heatsink. The deltaT, for the same power, will
reflect improvements in the mounting method and interface.

Changes in heatsink temperature indicate a poor control of possible
variables present.

RL
.

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