The thermal reliability of two different high-power insulated gate bipolar transistor (IGBT) modules was studied under both conduction and switching regimes by experimental and numerical approaches. Indeed, the reliability of semiconductor devices is tightly linked to the junction temperatures reached in the IGBT, which is a function of the diode chips present in such devices and its operating condition. However, measurements of the semiconductor temperatures are often difficult to be done, thus requiring modeling and simulation tools to accurately evaluate the instantaneous temperature under different thermal loads. For this reason, a transient 3D heat transfer numerical model of two different IGBT power devices was proposed using the Finite Element Method, assuming that heat is dissipated on the IGBT by natural convection and radiation. These simulations were validated with experimental measurements, obtained through infrared thermography, performed for the IGBT modules either opened or enclosed, varying the heat source, the module orientation and under conduction or switching regime. For the switching regime, power losses due to the gate-closing and gate-opening transitions between conducting and non-conducting states were taken into consideration. The heat losses were modeled assuming the time-dependent heat dissipation obtained by previous electrical simulations, and it was compared with time-averaged heat source solutions. Results showed that averaging the heat source can significantly reduce the computational time, allowing quick design explorations. Overall, the numerical model led to deviations of less than 16% over the transient heating and at steady-state. Finally, the impact of a heat sink attached to the IGBT was studied, demonstrating that the proposed model can be used for accurate and quick investigations of potential failures and for the development of more efficient IGBT devices.