Analysis on voltage unbalance between the inner and outer devices in three level IGBT converters

In this article, the voltage unbalance between the inner and outer IGBT of the NPC (neutral-point-clamping) three level converter is studied in detail. The keys of this task are to use a method-"step-by-step method" and to understand the basic conception that the parasitic inductance is relevant to the dimension of the loop and the value in proportion. It can be seen that the very reasons for the unbalance are the unique structure of NPC three level circuit and the parasitic inductance in the snubber circuit. Finally the designing tips for testing the snubber circuit are given. The simulation and experimental results demonstrate the validity of the analysis.