"whit3rd" wrote in message
ups.com...
On Sep 11, 8:29 am, "N Cook" wrote:
I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.

The previous post, that said it was the transistor temperature
that would be variable, was correct; remember, the whole
heat output is dumped into the heatsink, and that isn't affected
by the thermal resistance of your mica. Heatsink-to-air temperature
difference is whatever it takes to drain that heat.

It is possible to fit a second heatsink onto the TOP of the
transistor,
if you really need better thermal transfer (using both bottom and top
surfaces of the metal plate).



The interface between the device and the heatsink is probably the most critical
piece in the puzzle, and is the piece that a builder/repairer has most control
over. The device temperature is what you're trying to control. You have no
control over the internal thermal impedances of the device... they are
determined by the manufacturing process and materials.
Given a heatsink has adequate capacity for dissipating the heat (heatsink to air
thermal impedance), then that only leaves the device to heatsink interface to
manipulate. In comparison between greased mica and silpad insulators, the
mica/grease combination has the better thermal characteristics.
Mica can be problematic though, mainly because there seems to be no quality
control on the manufacturing process. The thickness of a mica slab can vary
from 0.010 to 0.050 or more. Mica can withstand about 1000 volts per mil, so it
really doesn't need to be so thick. You can usually split a mica slab several
times and still be safe in using it. The thermal impedance goes down somewhat
with thinner slabs, too.

Overall, mica/grease can exhibit almost half the thermal impedance of a silpad.
Given the same device power dissipation, lower thermal impedances mean lower
device temperature.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra