The IGBT's characteristics are examined in a single-pulse series resonant circuit under controlled temperature, The aim being to reach the highest possible switching frequency for IGBT's with ratings suitable for industrial use, it is found that both switching energy and on-state losses must be measured, as on-state losses also vary with frequency. It is also found that IGBTs, though having the same datasheet rating, can have properties depending on the manufacturers philosophy giving them totally different behavior at high frequencies. The time needed for the conductivity modulation mechanism to get into action is crucial, giving advantages to devices from manufacturers using low carrier lifetime and high bipolar emitter efficiency, such as Toshiba. The same datasheet ratings both in current, voltage and switching speed can be achieved with high carrier lifetime and low bipolar emitter efficiency, like Siemens devices, which in the resonant circuit will have their advantages under other modes of operation than Toshiba.