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What the best way to mount HIGH voltage semiconductors to grounded coolers?
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wraper:

--- Quote from: Ian.M on November 18, 2018, 03:53:10 am ---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.

--- End quote ---
If all of those conditions are met, which often are not. FWIW I've seen plenty of MICA insulators shoot through.
Ian.M:
Certainly, it only takes a small surface defect or bit of debris for it to suffer crush damage during assembly.  Also there is the problem of obtaining high enough grade mica sheet for HV work - you absolutely cant tolerate significant non-mica inclusions, and I'd be very wary of built-up (composite) mica sheets, so in large enough pieces to provide sufficient creepage distance at the edges,  its likely to be expensive compared to other solutions and any attempt  made to reduce costs either or materials or by using less skilled labour has a high risk of introducing defects that can  cause it to breakdown under electrical or mechanical stress, either rapidly or much later when the heatsink compound has deteriorated enough for it to cease to entirely fill the defect.
SeanB:
Having taken apart a few power blocks, they all use a large aluminia insulator, with the power devices soldered to the ceramic on silver plated pads, and the bottom side soldered to a large copper heat spreader. Good for 2kV of isolation, and very reliable.

You probably will want to get large alumina plates ( at least a lot larger than the devices for creepage clearance), and use a spring clip or a top clamp with Belleville washers to provide the correct spring clamping to the devices.  Then appropriate thermal compound and the manufacturer of the devices recommended clamping force per device. Note with the aluminia isolator you do need to have a machined burr free heatsink mounting surface, as extruded is not going to work, it has to be machined to be as close tolerance as the ground aluminia surface to prevent cracking. Remember that the clamp as well needs to be as rigid as the heatsink, so as to apply even pressure, so you will likely either have a bar across the top ( probably with a kapton tape to provide higher isolation) and a clamp assembly both ends, or a longer bar with multiple devices being clamped by it. When doing multiple devices with a single clamp make sure they are all from the same lot number, as they might have slight differences in the thickness of the top epoxy, and tolerances add up.

https://en.wikipedia.org/wiki/Belleville_washer
Basic info, use with a nylock nut to provide the anti loosen requirement, the washer stack provides the even clamping force with changes in operating temperature.

http://www.bellevillesprings.com/belleville-washers.html
UK company, I have bought from them before, good service even international, and very friendly as well. Good quality items.

Worst thing are power SCR pucks, they have a special heatsink assembly that you must use to get the lifetime, and generally no warranty if you do not use it either or use incorrectly. Those things do not let magic smoke out, they instead turn into shrapnel powder with a loud bang, then blow the expensive FF semiconductor fuse.

coppercone2:

--- Quote from: NiHaoMike on November 17, 2018, 07:38:25 pm ---Something else that can be done, if the thermal conductivity of a thermal pad is not good enough, is to mount the transistors or whatever on a block of aluminum or copper, then use a larger thermal pad to couple that to the heatsink.

--- End quote ---

I really like this idea because the peak pressure on the dielectirc material is much smaller since it has much more surface area to distribute it especially if its something like a TO-220 screw. If its a heat sink clip or a press bar it might not make as big a difference but it should still be better because you can use nice grinded or lapped parts that really make a good fit rather then trying to get equal stress from a very cheap part.  I think I will do this from now on.
wraper:

--- Quote from: NiHaoMike on November 17, 2018, 07:38:25 pm ---Something else that can be done, if the thermal conductivity of a thermal pad is not good enough, is to mount the transistors or whatever on a block of aluminum or copper, then use a larger thermal pad to couple that to the heatsink.

--- End quote ---
Example of this in old Soviet amplifier.



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