Electronics > Projects, Designs, and Technical Stuff
What the best way to mount HIGH voltage semiconductors to grounded coolers?
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Wolfram:
Those kapton foils are nothing but trouble in high voltage applications, especially in cases with high dv/dt. I optimistically used some to isolate some SiC MOSFETs and diodes in a 750 V buck converter, and they failed after a short time in operation. Further testing gave consistent failures of this material. The manufacturer of this material (Aavid in this case) blamed the surface finish on our heatsinks, but they don't actually publish any information on the recommended surface finish for kapton foil insulators. Swapping them with alumina pads solved the problem completely.
floobydust:
I would look at what the automotive industry is using.
Anyone know what Tesla Motors used here in older Model S?  I think it's a modified TO-247 IGBT package.  I couldn't find pics of the other side.

jbb:
+1 for spring clips. Having a hole in you insulator through which you put a metal screw is really bad for creepage & clearance. Also some TO-220 devices very thin metal tabs which deform (causing increased thermal resistance) when the screws are tightened.

A Sil-Pad style tends to add about 1 deg C / W of thermal resistance which must be taken into account.

Regarding the Tesla inverter; those look like a modified TO-247. By not having the hole you can get more silicon into the same package. IXYS make some standard parts like that, but at Tesla scale they could be custom devices.
wraper:

--- Quote from: Ian.M on November 17, 2018, 02:18:32 pm ---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.

--- End quote ---
Silpad should be more reliable because it's thicker and won't crack. However mica + thermal grease have better thermal conductivity.
Ian.M:
Mica  only cracks if grossly abused, and is very stable under pressure.  Its also virtually immune to ageing as its millions of years old to start with!   If you start with a good clear piece without any signs of delamination or cracking, and don't mangle it during installation, or have any burrs on the heatsink or device surfaces, its very reliable and fully reusable.

Silpads however tend to degrade with ageing and high temperatures, and if they've been in use for any significant length of time should *always* be replaced if disturbed.   Their only advantage is convenience on the assembly line for low cost products, as their resilience makes the package base and heatsink flatness, and initial clamping pressure less critical, and if you specify self-adhesive ones, they can make automated assembly much easier.
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