Analysis and experimental evaluation of two low loss auxiliary circuits for the series connection of IGBT devices

The series connection of IGBTs is necessary when the blocking voltage requirements are higher than the rated voltage of commercially available IGBTs. However, voltage unbalances caused by different switching behaviors during the turn off process of series connected IGBTs could lead to the failure of one IGBT if it exceeds its maximum blocking voltage. This paper demonstrates that the proper operation of series connected IGBTs can be achieved using simple and low loss auxiliary switching circuits. In the first part of the paper an analysis of the problems related to the series connection of IGBT devices is provided. In the second part, two effective and low loss voltage balancing methods are analyzed and experimentally verified. Those methods guarantee the safe operation of the series connected IGBTs during the switching transient and especially during the tail current periods.