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I agree... most people seem to think the washer is the heatsink but it is not. It is just the mechanical means to force the flats of the FETs against the PCB, which is the real heatsink. Thermal compound is harmless (as long as it is not electrically conductive!) but also pointless. The improvement in thermal conductivity is negligible in comparison to the thermal resistance from the BS170 junction inside the lump of plastic, and the exterior surface of the said lump of plastic.
Marc PA1DMG did think well though, I noticed the heatsink has rounded grooves to match the body of the BS170s and get good thermal contact with them. So it looks like a nice mod! Though as Marc even himself suggests... perhaps not necessary :-D
I still think anyone pushing a QCX-mini past 5W is going a bit wild...
73 Hans G0UPL
On Mon, Feb 8, 2021 at 4:11 PM Bob M. <wa1mik@...> wrote:
Very nice "cooling" article. You probably should put some thermal compound between the FETs and the large solder pad on the circuit board, as that's the primary heat sink for those transistors. The washer just holds the FETs in good contact with the board and has minimal contact area due to the rounded backs of the FETs.