Analysis of IGCT current clamp design for single phase H-bridge converters

IGCTs are especially adapted for high-power applications. In Voltage Source Converter (VSC) IGCT applications a series di/dt limiting inductor and a clamp circuit must be used, in order to protect freewheel diodes from excessive current derivatives. The main parameter to be considered in the clamping circuit design is the characteristics of the DC bus current, which may depend both on the converter topology and the chosen modulation strategy. In this paper a current di/dt limiter circuit for a H-Bridge with Bipolar and Unipolar modulation is designed and experimentally validated. It is shown that if the same clamp parameters are used for both modulations strategies, the semiconductor current di/dt in the Bipolar case is 50% smaller than in the Unipolar one. In addition, the turn-on overcurrent and the turn off overvoltage with the Bipolar modulation are twice with respect to the Unipolar ones. Thus, if the aim is to obtain the same overvoltage in both cases, the maximum output current for the Bipolar case will have to be 50% smaller.