The use of capsule IGBTs in the series connection

High voltage and current capsule IGBTs have become available. However, to work at 3.3 kV AC and above, it is necessary to use the devices connected in series. Whilst multiple level inverter topologies are available, the direct connection of IGBTs remains attractive. However, the direct series connection of modules has not been popular for various reasons. Capsule IGBTs, however, promise to change this situation. One obvious advantage of capsule IGBTs is the ease with which they may be mounted in series strings. To use IGBTs in the series connection it is necessary to consider the problem of voltage sharing between devices in a string. The voltage sharing issue can be divided into two by considering the transient sharing separately from the steady state sharing. Perfectly matched devices will share in all cases provided the gate drives and timing signals are identical, assuming negligible capacitances to ground. However, close matching high current devices in every detail is unrealistic. This paper describes an experimental procedure for series connection of capsule IGBTs under hard switched conditions and under active voltage control