Active voltage sharing of series connected strings of IGBT devices in bridge applications

A successful use of series connected strings of MOSFETs or IGBTs requires equalizing the dynamic and static voltage sharing across the devices. The dynamic voltage imbalance is generally managed via the slope control of the output voltages by snubber capacitors, or by active balancing circuits on the gate side. In this paper a novel approach based on the control of the currents supplied to the gates, is proposed. The main issues related to the application in bridge configuration of series connected strings of devices are faced. Firstly, the voltage sensing circuit is analyzed in order to meet the needs of bridge circuits. Experimental tests are performed in order to evaluate the impact of the actual control actions on the devices. Advantages and disadvantages of the proposed active voltage sharing circuit are treated extensively, in order to give also experimental evidence to the switching power losses in comparison with conventional approaches.