Analysis of an effective voltage sharing method for IGBTs connected in series

Insulated Gate Bipolar Transistors (IGBTs) connected in series is an essential topology for high voltage applications where fast switching is required. However, the IGBTs are unlikely to share the voltage equally with each other due to many factors, such as various time-delays of control signals, different stray inductances in the circuit, and potential snubber effects [1]. Even if using identical IGBTs with the same design and manufacture process and ideally setting perfectly matched components in the circuit, the voltages that the IGBTs withstand are still different in all probabilities. This is very likely caused by their different intrinsic parameters (e.g. gate-collector capacitances, intrinsic gate resistances, carrier lifetimes). An effective voltage sharing application (Temporary Clamp) [2] performed by intelligent Cascade Active Voltage Control (CAVC) [3] has been successfully implemented to solve this unbalanced voltage-sharing problem in the series circuit. The logic behind this application is worth analysis especially because it is concerning with the changes inside IGBT device, such as excess carrier density distribution, carrier storage region, depletion layer. A complete analysis needs a better understanding of the physical structure of the IGBT and knowledge of device modeling. By using the known IGBT dynamic model with meaningful equations [4], a preliminary analysis at the mathematics level will be introduced for the temporary clamp method.