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NNTP-Posting-Date: Sat, 06 Apr 2024 15:40:45 +0000
From: John Larkin <jl@997PotHill.com>
Newsgroups: sci.electronics.design
Subject: Re: silicone grease
Date: Sat, 06 Apr 2024 08:39:04 -0700
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On Sat, 6 Apr 2024 11:53:25 -0000 (UTC), Jasen Betts
<usenet@revmaps.no-ip.org> wrote:

>On 2024-04-01, john larkin <jl@650pot.com> wrote:
>> On Mon, 01 Apr 2024 09:12:38 -0700, John Larkin <jl@997PotHill.com>
>> wrote:
>>
>> The Bergquist pads I was looking at are theortetically 1.8 W/mK
>> material, but the actual TO-220 theta suggests about half that in real
>> life.
>>
>> T-global makes pads that are rated 5 W/mK.
>
>Honeywell does 8.5 with PTM7950 etc.
>
>It's a phase change material

I need reliable electrical insulation, so phase-change or graphite
won't do. That's another reason not to use mica... it's fragile.

All the gap-pads that we have tested have been awful, numbers like 5
degc/watt for our TO-220. None come within a factor of three of the
theta that we calculate based on their specified material thermal
conductivity. Bummer.

What really works is an AlN insulator with grease, like 0.35 K/W for
the TO-220. The silicon itself is 0.74 j-c. I guess we'll do that,
messy but effective.

At 40 watts/fet, Tj = 80 (= cooler temp) * 40 * (0.74 + 0.35) = 124c

We can't anodize the copper cooler. Hard anodize and phase change
could be another option.