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Registered Member #618
Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628
I know alot of you guys place 2 or more fets or heatsink mountable semiconductors to the same heatsink. What I'd like to know is, what and where do you guys get the insulators for these parts? I'd like to know since I'm starting work on Skoris SSTC and was going to use an old P2 heatsink to mount the power fets and transistors, whoever looking over the datasheets and the schematic, it would be shorting out alot of spots.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Personally I like to NOT use insulators if possible, I prefer to have heatsinks directly connected to the drain of mosfets or collector of bjts, of course this means using multiple heatsinks, but the benefit is MUCH better heat transfer from transistor to heatsink. Heatsink compound helps even further with heat transfer. e.g. for a TO220 transistor the thermal resistance from junction to heatsink is about 1 celcius per watt a typical insulator will double this figure, which means half the power can be dissipated. (or the device temperature rise will be double)
If you use mica then also use 'heatsink compound' (typically tin-oxide in silicon grease) don't bother with expensive heatsink compounds if you use an insulating pad. (silver-loaded etc.) Most commercial products now use 'sil-pads' which have a higher thermal resistance but don't require heatsink compound. These grey looking sheets can be recycled from many consumer devices.
Two small heatsinks without insulators works MUCH better than one double-size heatsink with insulators.
If you do use separate heatsinks without insulation consider the voltages on the heatsinks regarding safety.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, Sil-Pads are pretty good, and most good electronics distributors sell them. They are also known as heatsink pads or greaseless insulators, I think, but a quick look in the Mouser or Digikey catalogue should turn some up.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Is there any wrong about using grease with sil-pads? I thought that if there are still some gaps in there, it's better that they are filled with grease than with air.
Registered Member #514
Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295
Lots of eletronic junk I've taken apart use grease between the sil-pads and heatsinks.
If the heatsinks surface is smooth enough adding grease between the pad and the heatsink won't make a difference. I still like to use it tough. I like to think it helps to fill any small "gaps" between the pad and sink. Makes replacing dead parts a lot messier, tough.
So, using grease with sil-pads shouldn't be a problem, and might even help a little (as long as you don't go overboard and use like 10g of grease )
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Heatsinks and electrical insulation.
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