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What the best way to mount HIGH voltage semiconductors to grounded coolers?
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001:
What the best way to mount HIGH voltage semiconductors to grounded coolers (radiators)?
For example TO-247 transistor at 600VDC?
NiHaoMike:
Use insulating thermal pads.
Ian.M:
There are several things you have to get just right for long term reliability.

Burr free and flat heatsink surface.

Spring clip across the package so you don't have to worry about creepage distance from the metal pad in the package base to the mounting bolt or sleeve the bolt with an insulator.  The clip provides spring pressure on the center of the package that does not vary significantly as its thermally cycled.

Mica insulating sheet, which if a good grade of natural Mica is typically more reliable long term than a silpad.

Light smear of heatsink compound on both sides of the mica sheet.

If the Mica thickness is appropriate, the above is good for over a kilovolt for many years.  An appropriately rated glass-fibre reinforced silpad (no heatsink compound needed) is better for automated assembly and will be good  for a few years, but I wouldn't bet on its reliability a decade later as  silpads can fail by deforming under pressure as they age then pin-holing at or near a package corner.



Gyro:
For higher voltages (probably 1kV+) you can get solid Aluminium Oxide washers. They have surprisingly good thermal performance but you need to have some tension control on the screw (crinkle washers etc) to avoid the risk of cracking them.

eg. https://uk.rs-online.com/web/c/?sra=oss&r=t&searchTerm=aluminium+oxide+washer

kony:
Alumina is shit compared to aluminium nitride ceramics - that has comparable heat conductivity to the alloy heatsink is extruded from. So if you are running out of the thermal resistance budget, this is a very good fix.
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