Paul,
> Over the years I've heard many people say that adequately cooling a
> transistor is difficult because it is so small.
That's only half of the picture. The other half is that it needs to be
kept very much cooler than a tube!
> It seems to me the dissipated
> power per area is roughly the same when looking at the devices minus
> their coolers.
In some cases, comparing the more compact tubes with the larger
transistors, that could actually be true. But there is one VERY BIG
difference: The maximum allowable dissipated power is rated at a core
temperature of typically 200 to 250 degrees Celsius for the tube, and at
only 25 degrees Celsius for the transistor! This is what makes cooling
transistors harder.
You can rather easily keep a ceramic tube's core below the maximum rated
temperature, while the tube is producing its full rated heat, by blowing
a modest amount of air through a modest heat sink (which usually comes
as an integral part of the tube). At a top room temperature of maybe 30
degrees on a hot summer day, you still have roughly 200 degrees of
difference. At that temperature difference, even a small amount of air
will remove a lot of heat.
But you absolutely cannot keep a transistor at 25 degrees while it is
dissipating its full rated power, unless you are working outdoors in the
arctic in mid winter, or you use cryogenic techniques. So the
dissipation rating of transistors ALWAYS has to be drastically de-rated,
to account for the actual temperature at which you can hold it. In
practice, this might look like a nominal "1200 Watt" transistor (at 25
degrees Celsius) being derated to 600 watts, which allows it to operate
safely at roughly 90 degrees at its mounting surface, and then you need
to remove that amount of heat while having only 60 degrees of
temperature difference between the transistor and the air. That will
require a much larger heatsink than for a tube dissipating 500 watts,
and a much larger airflow too.
This is not a "terrible" problem, but one must understand it and design
the thermal side of an amplifier as carefully as the rest, or the
transistor will likely overheat and fail. I have seen many ham
homebrewers just guessing the proper size of a heatsink, and burning out
their transistors because it was much too small, or because they mounted
the transistors on insulators that could never transfer heat well
enough. I have seen this happening even with many homebrew power
supplies using 2N3055 transistors, which have a really low power density
compared to many RF transistors!
And that's why correctly designed solid state amplifiers often look like
one VERY BIG HEATSINK with a tiny little bit of circuitry attached to it!
And the compound the problem, the highest power transistors actually
have a higher power per contact area than a tube's core. That's where we
run into the limitations of thermal conductivity of metals, and only the
best materials will do. This is why high power transistors usually need
a heat spreader made of copper, before the heat can flow through a
larger cross section of a cheaper material, such as aluminum. Even then
you might still end up with 30 degrees of thermal gradient between the
transistor's mounting surface and the main part of the heatsink!
> What would be interesting is if a transistor
> manufacturer took a page from the tube world and integrated similar
> cooling.
It's not convenient. The larger size of heatsink required for a
transistor makes it a larger portion of the total device cost, than in
the case of a tube. If I had to throw away a big, expensive heat sink,
because the tiny chip of silicon attached to it burned out, I wouldn't
like that! I prefer to just replace the tiny little piece of silicon,
with as little packaging around it as possible. And since most people
agree with me in this, the market for power semiconductors with built-in
heatsinks is small.
Paul, to understand the problem of having lots of heat in a small area,
and having to keep this area pretty cool, I suggest that you look up the
thermal resistivity of copper (which is the best material available at
acceptable cost), and that of aluminium, of which most heatsinks are
made, and the thermal capacity of air, and so on, and do some real-world
calculations about how to keep a 1x3cm mounting surface of a transistor
below 90 degrees Celsius, while it is producing 600 watts of heat. Be
sure to include ALL the thermal resitances: The interface between the
transistor and the spreader, that of the spreader, that between the
spreader and the heatsink, the internal one of the heatsink from the
spreader to the fins, and that from the fins to the air, at a certain
airflow. I don't know if you enjoy such number play, but if you do, it
will be enlightening!
I have done it many times, and usually arrive at the conclusion that
using a high power transistor even at half the rated power dissipation
can be quite a challenge! As you relax the power level, or use devices
with lower power density, it gets much easier.
Manfred.
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Visit my hobby homepage!
http://ludens.cl
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