Will Matney wrote:
> Steve,
>
> That's what I always thought, that the device was supposed to be in contact
> with a flat surface (flat as much as possible, or machined flat). Once
> anything
> is introduced between the device and the metal, the heat transfer starts to
> be
> interupted. For RF power transistors, I would think one would want the very
> minimum amount of compound between the device and the heatsink. Anodizing
> is about the same process that's done to the foil on an electrolytic
> capacitor
> except a dye is introduced. The anodizing itself is what creates the
> insulative
> layer, not the dye per say.
Many of the transistors I use have the mounting flange as the ground
connection. Anodising is a no-no.
It may add to it, I don't know. The way I was always
> taught was black absorbs heat, and anything bright like white or silver
> reflects it.
It works both ways round - I like Tom's analogy to an antenna. If the
heatsink is hotter than its surroundings, black helps it lose the heat
better than silver - although the difference is minimal compared with
the effect of air moving over the surface.
In terms of mass, that helps spread the heat around and move it away
from the device, but it doesn't actually lose any heat. NEC made a TV
transmitter where the transistors (dissipating 80-100W, I think) had no
heatsinks at all, just jets of compressed air directly onto the
underside of the flange. It worked fine, except when the air failed and
devices blew in seconds. A large chunk of metal buys you time if the
airflow fails.
Steve
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