Evaporative spray cooling of power electronics using high temperature coolant

A pressure atomized evaporative spray cooling nozzle array was used to thermally manage the power electronics of a 3 phase inverter module. The module tested was a COTS module manufactured by Semikron, Inc., and has a maximum DC power input of 180 kW (450 VDC and 400 A) with 25degC coolant. However, the standard heat sink that the module uses is a single phase liquid heat sink and when 100degC coolant is used (as in automotive applications), the maximum module power is de-rated to 45 kW so that the IGBT chips will not overheat. The module tested here incorporated a custom heat sink that allowed for the use of spray cooling nozzles, which were designed and developed by RTI. The spray liquid was a 50/50 mixture of water and propylene glycol (WPG) at a temperature of 100degC. The sprays impinged directly onto the bottom surface of the DBC boards to which the power electronics were mounted. This arrangement, combined with the high heat transfer coefficient of evaporative spray cooling, greatly reduced the thermal resistance of the power electronics material stack up, but did so without directly wetting the electronics. The results of this work were that the unique evaporative spray cooling nozzle design and patented electronics interface design allowed the module to be run to full power while keeping the IGBT junction temperatures acceptable, despite the high coolant temperature. The junction temperatures of the IGBT's were measured by electrically insulated type T thermocouples placed on top of the devices, and the thermocouple readings at the full load were within several degrees of one another. Consistent and uniform junction temperatures are an important factor in long term device reliability. For the standard heat sink, which uses single phase liquid cooling, the pressure drop and flow rate required for maximum heat removal would be 17 psi and 5.3 GPM. For the pressure atomizer spray nozzles, the module would require a pressure drop and flow rate of 40 psi and only 2.7 GPM.