Energy efficiency is becoming a key issue in the development of thermal management devices. To this end, thermal performance of plate fin and pin fin heat sinks incorporating a solid-liquid phase change material (PCM) has been evaluated, under periodic power inputs simulating actual heating conditions. Experiments were performed using Wood's metal (50Bi/27Pb/13Sn/10Cd, melting point: 70.0°C) under forced convection in a wind tunnel, and also with an integrated fan. A periodic on/off power of 35 W and constant air velocities in the range of 0.5-1.5 m/s were examined. Three different periodic power patterns were utilized. A thermal stabilization period was observed during the phase change from solid to liquid, during which the temperature rise was arrested. The resulting extended operating period, prior to reaching a prescribed maximum temperature, resulted in an improvement in the allowable passive thermal operation time of the heat sinks. The experiments for fan-cooled heat sinks showed that energy savings by using PCM heat sinks were in the range of 5.4-12.4%. Computational predictions were performed using an implicit enthalpy-porosity approach, and were in good agreement with experimental data.