Power Transistor Heat Transfer Materials

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PJD
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Power Transistor Heat Transfer Materials

The e-max controller uses a thin rubber-like sheet between FET's and the heat sink, heat sink compound is also used - on just one side I think...

Question - is the rubber sheet for heat transfer or some other function as well? When reattaching the FETS, should it be used with heat sink compound also? Should the compound be put on both sides?

rgx
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it's for insulation AND heat transfer

The sheet has to do both, voltage insulation and transfer heat. They used to be made of "mica", some kind of stone that can be grinded down to a thin sheet. The voltage insulation is needed because the casing is connected to live parts of the transistor, and these must be insulated from each other and from the heat sink.

The one you found is a modern variant, apparently they make them from silikon rubber nowadays. In the old days you put paste on both sides (not always needed, depending on how much heat you need to transfer). A webshop of a local electronics parts supplier indicates that paste is not needed when using the rubber sheets. On the other hand - it can't hurt. The layer of paste should be very thin in any case.

A typical sheet can insulate 6kV voltage, and has a thermal resistance of 0.4 degC/W.

PJD
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I see...

This is interesting in that as I mentioned in my last post, on the "E-max controller mods" topic, some kind of assembly error apparently caused a short between one of the motor phases and negative. This destroyed a couple FETs. The source lead and associated solder was completely vaporized.

The metal side of the FET that seats on the heat sink is connected to the drain side of the junction, so if any small piece of metal debris gets on this sheet punctures this silicone sheet at a pair of FETs, the controller is shorted!

I also didn't grind the reliefs in the controller case lid for quite enough for the larger capacitors, which resulted in pressure on the PC board and FETs upon assembly - then again, the grinding may have been a source of metal debris. I'm learning slowly, electronic work is delicate stuff.

New IRFP2907's are on order... need to get new silicone sheet material as well...

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