Numerical simulation of junction temperature time behavior in a circuit simulation is performed by setting up thermal models of power semiconductors and cooling systems, and connecting these models, typically composed of thermal RC-networks, to the calculated power losses. Calculating or estimating accurately conduction losses and, especially, switching losses has been discussed in the literature ... [Show full abstract] but seems to be not well known among many engineers. Therefore, in this paper we will give an overview of this topic and propose improvements of the procedure of loss-estimation in power electronic circuit simulations. The proposed scheme calculates conduction losses and switching losses with minimum effort, high accuracy and does not slow down the numerical simulation. It can be embedded directly in any circuit simulator employing ideal switches. Loss calculations are based on datasheet values and/or experimental measurements. As an example, a 5kW-inverter connected to a drive is set up with each bridge leg realized by a power module, where the characteristic parameters for the loss calculation scheme are extracted from datasheet diagrams.