Active Clamping Circuit With Status Feedback for Series-Connected HV-IGBTs

Abstract

Active clamping circuits can effectively suppress the voltage imbalance between series-connected insulated-gate bipolar transistors (IGBTs). However, while the active clamping circuit is conducting, extra current is injected into the gate of the IGBT, the gate voltage increases, and the IGBT operates in the active region. Therefore, long time conduction and frequent actions of the active clamping circuit will lead to extremely high switching loss of the IGBT with high collector-emitter voltage and serious switching loss imbalance between the series-connected IGBTs, which imperils the efficiency and safety of the system, particularly in the applications of high-voltage IGBT (HV-IGBT) series connection. In this paper, a novel active clamping circuit with status feedback is proposed for series-connected HV-IGBTs. A high-speed status feedback subcircuit is designed for transforming the operation status of an active clamping circuit into optical signal and feeding it back to the control system. According to the feedback signals, the microcontroller regulates the switching-on time and switching-off time of the series-connected HV-IGBTs' drive signals timely in order to implement voltage balancing and, thus, to restrain the actions of active clamping circuits. By applying this voltage balancing method to the HV-IGBT series connection circuit, the voltage balancing performance is expected to improve, and high switching loss and serious switching loss imbalance are expected to be suppressed. Experimental results verify the validity and superiorities of the proposed voltage balancing circuit and control strategy.