Re: silicone grease

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.