Enabling Much Higher Power Densities in Aerospace Power Electronics with High Temperature Evaporative Spray Cooling 2008-01-2919

A power electronics module was equipped with an evaporative spray cooling nozzle assembly that served to remove waste heat from the silicon devices. The spray cooling nozzle assembly took the place of the standard heat sink, which uses single phase convection. The purpose of this work was to test the ability of spray cooling to enable higher power density in power electronics with high temperature coolant, and to be an effective and lightweight system level solution to the thermal management needs of aerospace vehicles. The spray cooling work done here was with 95 °C water, and this data is compared to 100 °C water/ propylene glycol spray cooling data from a previous paper so as to compare the spray cooling performance of a single component liquid to that of a binary liquid such as WPG. The module used during this work was a COTS module manufactured by Semikron, Inc., with a maximum DC power input of 180 kW (450 VDC and 400 A). With single phase convective cooling, the coolant must be kept at 25 °C in order to prevent the insulated gate bipolar transistor (IGBT) die temperatures from exceeding acceptable limits at full power. If the coolant temperature is higher (100 °C, for example) the module power rating is reduced by a factor of 4 to 45 kW. Due to the high heat transfer coefficient of the evaporative spray cooling nozzles, the module was run at full load while maintaining satisfactory die temperatures even with the coolant at high temperature. The temperatures of the IGBT dies were measured by electrically insulated type T thermocouples that were placed on the die surfaces by Semikron during the manufacturing process. It was found that water spray cooling yielded IGBT device temperatures about 10 °C lower than WPG did, and both offer a substantial improvement over single phase convective cooling. The ability to cool power electronics with high temperature coolant means that a large ΔT is available for heat rejection to ambient conditions, which translates into a small and lightweight condenser. This higher coolant temperature also means it is possible to reject heat to warm ambient air. Also, the use of lower coolant flow rates enables the use of a smaller and lighter liquid pump. These factors, combined with the higher power density achieved, mean that evaporative spray cooling has significant potential to yield a lightweight thermal management system for aerospace applications.