As it has been inferred here, SS Amps can produce a lot of heat. Not that is
more that an Ceramic tube running at 200 deg C
but rather it is concentrated in a very small area. Two MRF154 will create
1800W of heat in a n 1,5 by 3 inches area.
Thus, it is imperative to use some "heat spreader" to allow for a larger
dissipation area BEFORE the heat reaches the Radiator.
I'm usually using a 1/2 by 7 by 11 inch electrolytic cooper plate on top of
the 4 inches thick Aluminum Radiator.
For more demanding applications, as in FM transmitters the cooper surface
under the transistors is Oxide Beryllium coated.
In extreme applications (MIL) the copper Heat Spreader is actually filled
with Liquid N2 holding the die at -100Deg C.
One added benefit of this is that each individual transistor is capable now
of delivering twice the rated useful power.
It is up to your Thermal Management Engineer to provide the most cost
effective/efficient cooling for your AMP.
The Half inch cooper plate and two 145CFM fans will allow you for 45minutes
key down before the SOA is breached.
Alex
PS.
One VERY important factor! The contact surface between the transistors and
the spreader and between the spreader and the
aluminum heat sink must be machined to a precision of better of
100Angstroms.
this is where your money goes when you build one of these amps ... a fact
seldom mentioned :)
If you have a transistor failure and needs to be replaced, the spreader
becomes in-usable and you must replace it :)
Alex
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