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A power electronic device's lifetime depends on its maximum operating temperature and the temperature swings it is subjected to. Heat sinks employing phase change materials (PCMs) can be employed to achieve a temperature reduction, but only for a limited duration. This makes such heat sinks appropriate for use in applications with high peak loads but with low duty cycles. The heat sink is modeled using the thermal resistors and capacitors (RCs) network approach, and an optimization procedure for designing a hybrid air-cooled heat sink containing PCM is developed, yielding a maximum possible temperature reduction for a given application. It is shown that air-cooled heat sinks employing pure PCMs are best suited for applications with pulses width lengths of several minutes with a period of several tens of minutes. In order to achieve a faster response of the PCM, the concept of PCM-metal foam is explored and modeled. Experimental data is presented which confirms the validity of the thermal RC network approach.