Thermal load of power device is closely related to the function and durability of the converter system. Activity of the presented model of an Insulated gate Bipolar Transistor (IGBT) is monitored and analyzed on the computer and in the laboratory. Based on the collected information, it is proved that thermal losses in switching converters are consisted of conduction and switching losses. Conduction losses are product of the collector current and on-state voltage of the transistor. Switching losses are turn-on and turn-off losses that exist during the change of transistors condition from the on to off state and from the off to on state of transistor. Current rises slowly in the transistor during the turn-on phase, while the voltage across transistor falls rapidly. This makes the product of current and voltage lower than the same product in turn-off state where the collector current starts to fall after the voltage across the transistor starts to rise and reches its maximum. Because of this, generated peak power while turn-off phase is larger than the one generated while turn-on phase and it is visible on the generated wave forms of switching losses of IGBT. It is also proved by simulation that higher operating temperature and higher switching frequency of IGBT cause raising of the thermal losses significatly.