Voltage Balancing Control For Series Connected High-Power SiC MOSFETs

With the proposal of carbon neutralization and other national energy conservation and emission reduction targets,the research on medium-voltage high-efficiency power electronic equipment is booming.Silicon carbide(SiC)power modules,due to its high frequency,high temperature,high power density and other excellent characteristics,are emerging in the application of high power converter with the trend of light weight and high efficiency.However,limited by the craft and cost,commercial SiC power modules cannot meet the requirements of medium and high voltage power applications.Series connection of low voltage rate commercial SiC MOSFET modules is one of the effective methods to improve the breakdown voltage rate and realize medium voltage power conversion.However,voltage balancing is a key challenge of series connection of SiC MOSFET power modules.Therefore,this paper studies the voltage balancing control technology of series connected SiC MOSFET.Firstly,by studying the mechanism of unbalanced turn-off voltage of series connected SiC MOSFET,the voltage imbalance sensitivity(VIS)analysis is used to quantify the intrinsic relationship between signal delay time and unbalanced voltage.Furthermore,the design method of active delay control is studied deeply,and the closed-loop transfer function model between signal delay and unbalanced voltage is introduced to realize the rapid voltage sharing adjustment of series connected SiC MOSFET.The proposed VIS expression,control loop and voltage balancing control method are verified by simulation and experiment.Secondly,a coupled inductor based active gate control circuit for series connected SiC MOSFETs is proposed.The principle and parameter design of the proposed circuit are expounded in detail.The circuit is built by passive components and transistors,which has fast dynamic response.It is independent of the gate drive circuit due to the high output impedence of the current source.It works only when the voltage is unbalanced,and it has almost no influence on the switching waveforms and extra losses can be ignored.The voltage balancing effects of using passive snubber circuit,using passive coupled inductor circuit and using active coupled inductor circuit are compared under different operation conditions by experiments.The advatanges of the proposed method are verified.At last,a voltage balancing method based on gate drive signal combinatorial regulation is proposed,which makes use of the strong adaptability of active delay control method to different operation conditions,and the advantages of active coupled inductor method that almost no influence on main circuit and almost no extra losses.The proposed method further optimizes the voltage balancing control of series connected SiC MOSFETs.