Medium voltage (MV) power electronic transformers (PETs) are nowadays considered, for various industrial applications, as a viable replacement for bulky line-frequency operated transformers. In MV applications modularity in the converter design plays a very important role. In this paper a power electronic building block (PEBB) for PET, based on the LLC resonant DC-DC converter is presented. Soft-switching characteristics of a high voltage IGBT are investigated and experimental results, obtained from a full-power laboratory prototype in back-to-back (B2B) configuration, are presented.