Reliability consideration is always important among the manufacturers of power modules and converters. Before using of power electronic converters into the related application, it is necessary to predict its reliability over time. In the meanwhile, the power loss and heat generated within the power semiconductors play a key role in the lifespan of the whole system. In this paper, a method for assessing the reliability of a step-down DC-DC converter is employed based on the thermal modeling of power semiconductors. As is evident from the used reliability approach, the junction temperature of power semiconductors – diodes and insulated-gate bipolar transistors (IGBTs) – is the most influential factor on the lifetime of power converters. Therefore, the simultaneous influence of switching frequency and duty cycle is analyzed at the same time as a factor for evaluating reliability. A cut-off of 150°C is considered for the maximum allowable junction temperature for the examined IGBT power module. The results show that a failure can be expected after 46,000 hours of operation of the considered power converter. Additionally, 3D curves are presented to illustrate the influence of duty cycle and switching frequency on the reliability of circuit’s components and the overall system. The obtained results confirmed that an increase in switching frequency from 1 kHz to 10 kHz can decrease the circuit’s lifetime almost 22%.