Insulated Metal Substrates (IMS) have become a popular solution for cooling GaN transistors. The small packages of GaN transistors require effective heat spreading which can be achieved through copper spreaders soldered to the IMS. This is however where the limitations of IMS set in. The good thermal performance is achieved by polymer layers with thicknesses in the range of micrometers. In combination with large copper spreaders, this results in an excellent thermal connection. Often overlooked, this also creates an excellent capacitor which experiences the fast voltage slopes of the module output. The result are increased switching losses and excessive common mode currents through the heatsink. A fast and accurate design procedure is presented to select the right insulation material and heat spreader without the need of vast solution-space FEM simulations.
P. Weiler, TU Eindhoven