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From: email@example.com (Gibbo)
Date: 27 Dec 2002 03:02:07 GMT
Organization: AOL, http://www.aol.co.uk
Subject: Re: TO-220 insulation
> I have to have two TO-220 transistors mounted on the same heatsink for a
>pretty high-power job, and they will be dissipating quite a bit of heat.
>They must be insulated from each other. If I use mica insulators and washers
>for the screws, will I lose a lot of thermal conductivity? I'm also using a
>high quality thermal paste between the mica sheets and the transistors. How
>bad of a thermal conductor is mica? Thanks for the help!
TO-220s mounted with a screw through the mounting tab are pretty bad. The
pressure goes where it's needed least. *much* better thermal transfer is made
using a fixing method of the type that presses the *body* of the transistor (as
opposed to the [thermally] relatively remote tab) against the heatsink.
Our tests showed that the fully isolated (usually [but not always] suffix "F")
components were *almost* as good as standard TO-220 components in this regard
with "normal" assembly procedures but that with *careful* assembly, better
results could actually be achieved with the isolated components as no mica
washer was required and there was only one layer of compound.
Those silicon impregnated "sil strips" are crap. Forget those. You might aswell
use a piece of wood. Use the thinest mica washer you can find, and *much* more
importantly (but very rarely mentioned) is use the *minimum* amount of thermal
compound you can possibly reply upon.
The trick with thermal compound is *not* to surround everything with lots of
the stuff or make sure there is loads of it between the transistor and
heatsink. The real trick is to make sure that there is the absolute minimum
compound required to exlude all the air between the transistor and the
heatsink. Logic *and* measurements back this up.
Of course the other viewpoint is that if you really are so concerned about
thermal transfer problems perhaps the design needs looking at because in *most*
applications there really is no real need for wasted power these days (I sense
a few flames coming on as a result of that bit).
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