It was ironic that investigating the use of surface mount transistors for QRP amps (see previous posts using the QRP tag) required a relatively expensive thermal bridge to heatsink the SOT89 package. These thermal bridges are made using aluminium nitrite.
I noticed by accident that some high power surface mount resistors use aluminium nitrite as a substrate. Unfortunately, these are no cheaper than proper thermal bridges.
However, what about regular high power surface mount resistors? Reading various papers I learned they use a ceramic substrate with a thermal conductivity about 1/6th that of Aluminium Nitride. I hastily scratched a break in the resistance film on a few such resistors and constructed a test jig. Sure enough, I had a thermal bridge.
The initial result is very encouraging using two 2512 resistors, open circuit, on each transistor in the parallel 1W amplifier. I cranked up the supply until each transistor was dissipating 2W. The transistor temperature was stable at 125degrees Celsius while delivering 1W continuously. Some more suitable resistors are on the way and a re-design to the pcb means I will be able to properly test this soon.
73's
Richard
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