Electronics > Manufacturing & Assembly

Screwing offline Flyback FET and diode to heatsink?

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Faringdon:
Hi,
We have a TO220 FPAK diode and FET, side by side, (~2cm apart) which need pressuring to a flat heatsink behind them both. So we have a strip of FR4 which is across their tabs and screws into the diode tab, and through the heatsink, to a nut at the back of the heatsink. There is also a heatsink hole either side of the FET, and  a screw goes through each of these, with again a nut at the back of each, and this ensures the single FR4 strip pressures both diode and FET against the heatsink, via the three screws and three holes.
Problem is the transformer and capacitor sit right in front of the FET and diode, and the heatsink “rooftops” overt the fet and diode,  so we cant screw the screw…we can only nut-spin the nut at the back of the heatsink…but in time old fashion, spinning the nut simply spins the screw with it and so  it doesn’t tighten. So we need a screw-with-abrasive washer which will bite into the FR4, so that when we nut-spin  the nuts, the screw doesn’t also turn.

Do you know of something for this?

The first bit of the assembly is that fet and diode have to be soldered first to the PCB, so that they are the right height for the screw holes at the back of the heatsink…..then the FR4/screw assembly is put on them, and some gunge so it doesn’t fall off them….then the rest of the SMPS is assembled, then we screw  to the heatsink as described.

tszaboo:
How is your car coming along? Do we have to make every decision in the design for you as well?

Gyro:

--- Quote from: tszaboo on October 02, 2023, 07:13:06 pm ---How is your car coming along? Do we have to make every decision in the design for you as well?

--- End quote ---

It doesn't bode well for the steering rack staying attached!

floobydust:

--- Quote from: Faringdon on October 02, 2023, 07:06:35 pm ---[...] we have a strip of FR4 which is across their tabs and screws into the diode tab, and through the heatsink [...]

--- End quote ---

Post a picture. Using FR-4 for structural sounds a bit cockamamie? It would be an interesting EV with FR-4 bodywork.
TO-220F I have seen spring clips used, best to have no screws.

langwadt:

--- Quote from: floobydust on October 02, 2023, 08:00:57 pm ---
--- Quote from: Faringdon on October 02, 2023, 07:06:35 pm ---[...] we have a strip of FR4 which is across their tabs and screws into the diode tab, and through the heatsink [...]

--- End quote ---

Post a picture. Using FR-4 for structural sounds a bit cockamamie? It would be an interesting EV with FR-4 bodywork.

--- End quote ---

"FR" stands for "flame retardant" so probably a good precaution

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