Series connection of IGBTs in hard-switching applications

A new circuit topology is presented which makes possible and safe the series connection of devices in very high voltage switching applications. This circuit behaves like a flyback converter and assures the uniform distribution of the voltage among the devices of the series. Within the proposed topology the active devices are hard switched. By means of this circuit the performances of a single IGBT module rated at 3300 V-1200 A are compared with those ones of the series connection of two IGBT modules rated at 1700 V-1200 A. The comparison is performed on an experimental basis and shows that the on-state voltage drop of the series is about twice that one of the single module, but switching losses are much lower for the series connection. Consequently, the proposed topology makes convenient the use of the series connection of IGBTs modules for frequencies larger than 3 kHz whereas the single high voltage IGBT module remains advantageous for frequency lower than 2 kHz. In other words the paper demonstrates that the series connection of the IGBTs can be used to extend the trade-off between conduction and switching losses also to high voltage modules.