Insulated metal substrates have become a popular solution for cooling gallium-nitride transistors in half-bridge modules. In order to achieve a good cooling solution some limits need to be kept in mind. The small packages of GaN devices require effective heat spreading to fully utilize both the transistors high current handling capability and high operating frequency. A large spreading area and thin insulation yield a low thermal resistance. This geometry, however, also leads to a large parasitic capacitance which experiences the high slew rates of GaN, resulting in increased switching losses and large current peaks through the heatsink. The link between the cooling capability and parasitic capacitance of any given insulator can be expressed by a constant independent of the geometry, which allows to select the optimal insulator for any application.
Pelle Weiler, TU Eindhoven
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