Investigation of heat transfer coefficient variation in air cooled hybrid electronic circuits

This paper discusses the problem of heat transfer coefficient variation in air cooled hybrid circuits. The investigations are based on a practical example of a circuit containing a bipolar transistor heat source. Its temperature is measured using the base-emitter junction. Additionally, infrared measurements of circuit surface temperature are taken. The measurements are carried out in a wind tunnel for different values of dissipated power and with variable cooling air speed. The measurement results are analysed allowing the assessment of heat transfer coefficient variation with surface temperature and cooling air velocity. Based on the analyses, compact thermal models are generated for the circuit allowing fast and accurate simulation of circuit temperature in various cooling conditions.