Power hybrid assemblies including IGBT (insulated gate bipolar transistor) are widely used in the application of motor drivers, switching supplies and other power conversion systems. The estimation of the power loss and junction temperature of semiconductor devices has become the major issue with the increase of the current density and high switching frequency of advanced power devices. The paper presents a partially coupled approach to evaluate the power loss and predict working temperature of switching devices. As an example IGBT PWM (pulse-width modulation) inverter is given to demonstrate the application of this approach. Based on the measurement of IGBTs dynamic characteristics, the estimation of power loss considering the junction temperature is introduced. Then the finite element analysis is used to accurate peak junction temperature prediction needed during dynamic operating conditions. In addition, the effect of switching frequencies during transient thermal response is investigated. The new approach developed can be used for accurate rating semiconductor devices or heat sink systems in power circuit design. Results comparison between proposed approach and commercial simulator shows that this approach is effective as a design step.