The problem of an influence of cooling conditions of power semiconductor devices on properties of selected DC−DC converters is considered. The new version of electrothermal average model of a diode-transistor switch for SPICE (Simulation Program with Integrated Circuit Emphasis) is used in the investigations. This model makes it possible to take into account thermal inertia of semiconductor devices as well as mutual thermal interactions between these devices. The investigations are performed for boost and buck converters containing the power MOS (Metal-Oxide-Semiconductor) transistor and the diode. Computational results obtained using the proposed model are shown and discussed. Particularly, an influence of thermal phenomena in the diode and the power MOS transistor on the converters output voltage and internal temperature of the semiconductor devices is considered. The correctness of the selected results of computations was verified experimentally.