Design Limitations of Heat Spreaders for Gallium Nitride Power Modules

This paper analyses the heat spreading capability of insulated metal substrates used for cooling of gallium nitride power modules. An analytical design procedure is used to determine the optimal heat spreader area and thickness. The capacitance across the insulation layer connecting the high-side switch and heatsink severly impacts the module’s switching performance and this effect must be considered during the design of the heat spreader. It is found that for any given insulation material, the thermal resistance and electric capacitance are directly linked. A geometry independent figure of merit is introduced, denoting a material’s heat conductivity with respect to its insulation capacitance. For very high frequency operation, results show that insulated metal substrate solutions may not suffice and the step to ceramic insulation is necessary. Several heat spreaders and insulation materials have been characterized experimentally, with a ceramic insulator reaching a power density of 631 W x cm(exp -2) at an insulation capacitance of 2.5 pF.