Numerical evaluation of high power amplifier heat sink design options

Numerical techniques have been used to evaluate various modifications to the design of a cooling system currently used for high power RF signal amplifiers. The system utilises a 300mm long × 220mm wide × 70mm high heat sink in forced turbulent air flows up to 120 litres per second. Using Computational Fluid Dynamics (CFD) software a detailed model of the heat sink, primary transistors and air flow was created. Experimental data obtained from the working system was used to validate the simulation results. The model was then modified to test the effects of various changes to the cooling system including incorporation of heat spreading techniques, elimination of bypass flow and changes to fin geometry. Reductions in transistor junction temperature of between 9°C and 12°C were predicted through the use of a copper heat spreader or full copper heat sink while the elimination of bypass flow reduced junction temperatures by up to 6°C. Strip fin and staggered fin arrangements provided little improvement, however by halving fin pitch and fin thickness (i.e doubling heat transfer area) device temperature reductions of 4°C were possible.