Screwing offline Flyback FET and diode to  heatsink? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: October 02, 2023, 07:06:35 pm
content: 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.

author: tszaboo
author_rank: Super Contributor
timestamp: October 02, 2023, 07:13:06 pm
content: How is your car coming along? Do we have to make every decision in the design for you as well?

author: Gyro
author_rank: Super Contributor
timestamp: October 02, 2023, 07:20:26 pm
content: Quote from: tszaboo on October 02, 2023, 07:13:06 pmIt doesn't bode well for the steering rack staying attached!

author: floobydust
author_rank: Super Contributor
timestamp: October 02, 2023, 08:00:57 pm
content: Quote from: Faringdon on October 02, 2023, 07:06:35 pmPost 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.

author: langwadt
author_rank: Super Contributor
timestamp: October 02, 2023, 08:06:50 pm
content: Quote from: floobydust on October 02, 2023, 08:00:57 pm"FR" stands for "flame retardant" so probably a good precaution

author: Gyro
author_rank: Super Contributor
timestamp: October 02, 2023, 08:09:47 pm
content: FR4 is a bad choice for clamping (and maybe carbodyworkchassis!). It relaxes with heat and time when under mechanical stress. I guess longevity won't be an issue in this case!

author: jpanhalt
author_rank: Super Contributor
timestamp: October 02, 2023, 08:12:20 pm
content: Is this for my £5,000 EV delivered?

author: langwadt
author_rank: Super Contributor
timestamp: October 02, 2023, 08:20:12 pm
content: Quote from: Gyro on October 02, 2023, 08:09:47 pmmodern version of the Trabant,https://en.wikipedia.org/wiki/Duroplastor just dye it black and say it is carbon fiber

author: nali
author_rank: Frequent Contributor
timestamp: October 02, 2023, 08:27:45 pm
content: Heh considering OP's expertise in Design for Manufacturing; from bent bits of metal to wedging things onto heatsinks to using fishing line, I can't wait to see this car!

author: SeanB
author_rank: Super Contributor
timestamp: October 02, 2023, 10:13:02 pm
content: Make it out of PCB material 3.2mm thick, and put a plated via, and a pad, where that screw goes. Then solder the screw to the pad, and you have a self locking plate. Or just use a M4 rivnut in the hole, unset, and screw into it from the other side, the rivnut being pressed into the hole. Or just have what every cheap PC has, a small piece of angled steel stamping, with a hole punched and threaded in the middle, to pull down. You can glue the FR4 piece to it, or just use isotop packages, or have round silicone heat transfer pads slid over the devices.