Abstract: In an electric vehicle, the traction motor is driven from a battery by way of a controllable electric power switcher. In operation of the switcher, the power losses of the switches depends upon the power being handled; during acceleration the power is high, and at constant speed on level ground the power is small. Thus, the power “dissipated” by the switches varies with time. A cooler transfers heat from the switcher. During acceleration, the cooler may not be sufficient to limit the instantaneous switch temperature. A phase-change heat “sink” coupled to the switches absorbs heat from the switches during hard acceleration, and returns the heat to the cooler under more constant-speed conditions.