Thermal design of high power semiconductor packages for aircraft electronic systems

The More Electric Aircraft is likely to require more extensive use of power electronics, for which thermal management will be a key issue. This paper presents an approach to designing integrated air cooled heatsinks which is being developed by Loughborough University as part of the CARAD funded Variable Frequency to Constant Frequency (VFCF) Converter project in collaboration with project partners TRW Aeronautical Systems, Mitel Semiconductor, AEA Technology and BAe Airbus. The paper shows how simple models of the heat transfer from heatsink fins, which are based on well established empirical correlations, may be utilised in combination with either simple analytical models or two dimensional finite element models of the heat conduction from the semiconductor die through the multilayer package structure to the base of the fins. These models allow the generation of design curves which may be used to rapidly explore a wide range of design options before selecting potential designs for more detailed evaluation using 3D FE analysis. In systems such as a VFCF convertor the semiconductor devices are switched at high frequency to ensure good input and output current waveforms. The power dissipated in the semiconductors, and therefore the heatsink weight, will however increase with the switching frequency, whereas the associated filtering components will be smaller and lighter at higher frequencies. The optimisation of the overall system weight therefore involves a tradeoff between the heatsinking and filtering requirements rather than just determining the optimum heatsink design for a specific power dissipation.