Heat Spreading Enhancement in High Power Amplifier Heat Sinks – Comparison of Measurements with Numerical Predictions

Abstract

A comprehensive study has been undertaken to better understand the thermal conditions within high power, radio frequency (r.f.) signal amplifiers. The majority of the heat generated is dissipated from four high power transistors, mounted directly to a large extruded aluminum heat sink measuring 300mm long × 220mm wide × 72mm high. The transistors each dissipate up to 130W across a footprint of only 1.96cm2 and as such, act as high flux, point heat sources across the heat sink base. At maximum power levels the transistor junction temperatures have been shown to approach the manufacturers rated limit of 200°C [1] when ambient temperatures reach 60°C. The prohibitively high cost of replacing the electronic circuits and cooling air delivery system across the entire transmitter network means that design changes to the heat sink that will reduce device temperatures and can be implemented on a retrofit basis are extremely attractive. Numerical and experimental methods have been used to assess the incorporation of high thermal conductivity materials into the heat sink design to improve cooling performance. Numerical simulation of a copper heat spreader embedded into the base of the existing aluminum heat sink predicted a reduction of transistor junction temperatures of up to 11°C. However, in experiments, a maximum temperature drop of only 5°C was achieved due to the introduction of an additional thermal resistance at the copper/aluminum interface.