Quote:
Originally Posted by flores
Even if you make the heatspeader out of copper, the base plate will still be aluminum, right? If that's true I'd say that aluminum is fine for the heatspreader. The system has to lose the heat through the base plate anyway..
Or is that an all to simple conclusion?
|
That's too simple of a conclusion.
Aluminum has about half the thermal conductivity as copper. To transfer the same heat with temperature drop we need twice the cross section area. Easy enough: just buy twice as much aluminum, which will still cost less than the copper.
But the situation is much different when we want to move the heat away from a point source rather than down a constant cross-section. Or more specifically, from a small metal patch on the back of the MOSFET to the heatsink. It becomes a 3D problem. We can't just stuff more material in the contact patch.
A solution is to use the best thermal conductor (copper) to spread the heat out from the point source, and then a lower cost bulk material (aluminum) to transfer the heat.
I haven't done an analysis on the Cougar thermal design, just a quick estimate. I believe the most thermally restrictive element is the thermal pad on the MOSFET. This paper-thin material electrically isolates the MOSFET back from the heat spreader. But even the best thermal pads are far less thermally conductive than copper or aluminum, and they are right where the heat is concentrated.
An obvious thing to do is have the MOSFETs directly contact a thin copper sheet that acts as a first-level heat spreader. But you quickly find out why Paul chose the approach he did. You have to figure out where to put the electrical isolation. The thermal pad material is expensive, and the better you do widening the heat flow path, the more you have to buy. And the mounting hardware really wants to electrically bridge your isolation.
We are designing around using "live" heat spreaders that simultaneously act as bus bars. This lets us avoid the first-level thermal pads. But we still have to get rid of the heat from the bus bar, and there isn't a cheap solution. We can't have live heat sink fins outside the box, so it's $30+ of insulating thermal pad, expensive anodizing, or oil cooling with plastic fittings.