[Amps] 'heat spreader' sizing ??

[Dan Levin]

From: Dan Sawyer <dansawyer at earthlink.net>
> Subject: [Amps] 'heat spreader' sizing ??
> The project is to mount a 1KW, 2 transistor, board on a heat sink. The transistor centers are about 2 inches. The question is:
> What should the dimensions of the heat spreader be? I have seen dimensions of 1/4 x 2 x 4 up to 1/2 x 4 x 7;
> that represents quite a range. The questions are:
>
> 1. What thickness is required. Is 1/4 sufficient?
>
> 2. What is the minimum effective distance from the edge of the devices? 1/2 inch? 1 inch?

Dan, to answer these questions, you have to do some math.  The answer
depends on a variety of factors, including the thermal resistance of
the junction between the transistors and the heat spreader, the
efficiency of the transistors, the heat dissipation capability of the
heat sink, the duty cycle of the amplifier, the ambient air
temperature of your cooling air, etc.

I wrote a bit of a white paper on this topic, aimed at hams building
amps who do not have background in thermal engineering.  I never
finished it, but you may find it useful reading. You can find it at
http://www.k6if.com/ssamp/cooling.html .  It includes some quick and
dirty math that you can do to get close to the answer.  As someone
else suggested, thermal modeling software is the way to get really
close (hi hi).

                        ***dan, K6IF