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Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
Today I just bought 8 mica insulators for TO-247 sized parts, but only now after I've gotten home I've wondered about two things:
1. What maximum voltage can mica tolerate? In a fullbridge running at 240VDC *or higher* will any microscopic flaw cause breakdown and short to heatsink? 2. What thermal conductivity should I expect? Will the mica cause the semiconductors to likely overheat, should I still try to get sil-pads for TO-247 packages?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I would rely on mica rather than (typical) sil-pad insulators for high voltage insulation and from memory, mica with heatsink compound has better thermal conductivity than a sil-pad.
Registered Member #347
Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106
Mica done properly will be about 1 C/W on a TO-220, maybe half or one third that on a TO-247. Mica and thermal grease is quite messy and isn't commonly used anymore.
The modern equivalent is Kapton sheet with a phase change material on both sides that melts when the device heats up. Loctite and Bergquist make this, among others probably. Loctite calls it Isostrate, and Bergquist makes a nice material Hi-Flow 300P. This is spec'd to do 0.95C/W on a TO-220, with a breakdown voltage of 5kV. The only downside is that you can't re-use it, you need to put in a new piece in whenever you remove and replace a device that has gotten hot and melted phase change material.
As for isolating mounted devices, at the company I work for we never use the hole for mounting to grounded heatsinks. The screw is too close to the metal tab to meet insulation requirements, so we always clamp the devices to the heatsink instead, using a spring clamp.
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
I saw the phase change/kapton pads and thought 'expensive!' since a SSTC would need occasional changes. Good to know the opinion of those who work in the industry, thanks guys.
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Mica vs Silicone-pads?
