In the laboratory we have developed quasi-resonant converters using Z.C.S.(Zero Current Switching) mode. In these converters, operating at medium power and frequency range ( a few KW and about 100 KHz), we used the IGBT (Insulated Gate Bipolar Transistor) as a resonant thyristor. The experiment shows a frequency limit: higher frequencies lead to component failure by thermal instability for Punch-Through IGBT. We show that the frequency limitation is due to a phenomenon occurring in the bipolar transistor part of the IGBT: The charges injected into the component during the conduction phase have not all disappeared by recombination when the voltage is applied to the components. We have studies the variation of these charges and of the losses versus the temperature of the component in order to explain the thermal instability. The results allow us to predict the maximum working frequency of the IGBT in ZCS mode, while avoiding failure of the studied component. The results are discussed and an IGBT, included in a forward resonant converter, has been experimentally studied to check the results. First, we show the different behaviors of Punch-Through and Non-Punch-Through components. Thermal instability is impossible with NPT components. Such a study is shown to be the most efficient way of choosing the best adapted component to high frequency applications.