Novel voltage balancing technique for series connection of IGBTs

For high power electronics applications, high voltage switches are realised by connecting existing devices in series. The main problem of unequal sharing of voltage across the series connected devices can be minimised by using active gate control techniques, snubber circuits and active clamping circuits. The aim of this paper is to present a novel hybrid voltage balancing technique which can achieve good voltage balancing with minimum number of components and minimum total losses (i.e. IGBT losses and balancing circuit losses). This technique minimises the disadvantages in present voltage balancing techniques and it consists of both an active voltage clamp circuit and an optimised passive snubber. The passive snubber capacitance is optimised to have low power losses. The proposed technique was validated by both simulation and experimental work. In simulation, two lkV/50A IGBTs were connected in series and the results showed good voltage balancing with minimum losses. The experimental work was carried out on three 3.3kV71200A IGBTs connected in series. Experimental results showed good voltage balancing across the devices and the possibility of finding an optimum capacitance. Determining of the optimum snubber capacitance is critical in the hybrid voltage balancing technique and the dependence of the optimum capacitance on load is discussed.