Medium voltage (MV) high power applications are usually realized using high voltage semiconductors (3.3kV and above) operated in hard switching mode with low switching frequencies (several hundreds of Hz). However, for high power DC-DC converters employing a transformer for galvanic isolation, it is attractive to increase the switching frequency so that the transformer size can be reduced. In this paper, switching properties of 6.5kV IGBT within a DC-DC LLC resonant converter are presented, highlighting interactions between semiconductor and circuit properties, which both must be simultaneously considered in order to achieve the best utilization of a high voltage power semiconductor operating at higher switching frequencies. Experimental results are presented to illustrate characteristic operating conditions.