[Amps] Heatsinks for SSPAs - Newbie Questions and UK source

[Manfred Mornhinweg]

Conrad,

Just a quick answer, as I'm about to leave for a month-long trip and 
won't read the forum in that time:

> The 
> max dissipation of the device is 500W so I suppose that means that I 
> have to get rid of 225W of heat.

No. If you actually run the amplifier at 250W output and 55% efficiency, 
the dissipated power would be 205W. The maximum power rating of the 
transistor does not define the dissipated power.

But if the efficiency is a bit lower than you expect, then you would get 
close to 225W, and perhaps exceed it.

> The datasheet then goes on to mention 
> thermal resistance from the junction to the mounting base and then the 
> thermal resistance from the mounting base to the heatsink. Is the 
> mounting base the heat-spreader or is that just the mounting flange of 
> the device.

It's the flange. So the junction to base thermal resistance is the 
internal one of the device, and the base to heatsink one is actually the 
one from the device to your heat spreader, assuming you can mount the 
transistor in the same way the manufacturer did to get that rating. This 
rating is often a bit optimistic.

> Anyway armed with that data how on earth do I figure out the heat 
> sinking requirements.

You need to add those thermal resistances to those along your spreader, 
from your spreader into the heatsink, inside the heatsink to the average 
fin surface, and from that fin surface to the air. That sum gives you 
teh total junction-to-air thermal resistance. By taking the highest air 
temperature you will encounter, and the highest allowable junction 
temperature (might be anything from 120 to 200 degrees Celsius, 
depending on the life span and relaiblility you want), you can calculate 
the maximum allowable temperature difference. Since you know the thermal 
power, that allows you to check whether the total thermal resistance you 
got is acceptable.

Typically it's not! ;-)


You can find some more on my web site, here:

http://ludens.cl/Electron/Thermal.html

> Assuming someone enlightens me can anyone tell me a good source of 
> suitable heatsinks in Europe, even better if it is a UK source? 

I import mine from the USA. If you don't find a good source in Europe, 
look here:

http://www.heatsinkusa.com

Manfred, XQ6FOD

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