Series Connection Of High Power SiC MOSFETs

High-power medium-voltage converters are the core apparatus of the power conversion systems in critical industries including high-speed train,all-electrical ship,renewable energy generation and DC grid.As the increasing requirement of energy saving and emission reduction,the application of the medium converter is growing rapidly.The SiC device based converters have improved performance in efficiency,power density and low weight compared with silicon device based converters.Thus,the application of SiC MOSFETs in medium voltage converters is the hot topic in both academia and industry.However,limited by the device structure and the fabrication process,the voltage level and power level of single SiC MOSFETs cannot satisfy the demand of medium voltage high power converters.Compared with the single high voltage device,the series connection of the low voltage devices has significant benefits on the conduction loss and the total cost,which is suitable for the medium-voltage converters.As a result,this paper will focus on the series connection of the high power SiC module to achieve high voltage and high-power handling capability,therefore accelerating the application of SiC MOSFET in high power converters.The prerequisite of the series connection of the SiC MOSFETs is to ensure the reliable driving of the single device.The operation mechniasm of device under the influence of the nonlinear parameters and the high-speed transition needs to be clarified firetly to guide the circuit parameter design.Further,one of the most important challenge of the series connection of the power device is the voltage unbalance among the devices.Traditional voltage balance methods merely rely on experiments and the mathematical boundary of the parameter selection is not clear yet.In this paper,to achieve the voltage balance under the high speed switching conditions,the transient switching model is built and the analytical model of voltage unbalance is proposed to assist the development of the voltage balance methods.Based on the analytical model,the accurate control methods of single device and multiple devices are realized.Herein,the main research topics in this paper are concluded as the following four aspects.Firstly,the analytical model of the high-speed switching transients of SiC MOSFET considering the influence of nonlinear parameters is proposed.The switching transient of the device is divided into four stages,in each stage,the equivalent circuit is given and the time-varying status equation is deduced.The nonlinear state equations are solved by the approximations including small signal assumption,average charge simplification and separation of variables.As a result,the analytical expressions of the electrical parameters are achieved.Moreover,the influences of the nonlinear junction capacitor,nonlinear transfer characteristics and the parasitic parameters in the loop are analyzed based the analytical model.Compare with state-of-the-art analytical model,the proposed model is capable of calculating the duration of each stage accurately,which is the basis of the analysis of the series connection.The error between the analytical model and the experiments is below 10%,which is the basis of the analysis of the series conneciton.Secondly,the crosstalk voltage elimination methods are proposed for SiC MOSFETs in the half bridge circuit.To overcome the challenge of the low noise community of the SiC MOSFET,the mechanism of the influence of the Miller capacitance and common source inductance is given and the analytical model of crosstalk voltage is deduced.Based on the proposed model,the relationship between the crosstalk voltage and the gate loop impedance is analyzed and the crosstalk voltage elimination method is proposed accordingly.For the package with relatively large common source inductance,a high off-state impedance gate driver is proposed to eliminate the crosstalk voltage introduced by Miller capacitor and common source inductor synchronously.With the proposed method,the crosstalk voltage is not dependent on the switching speed.The effectiveness and the robustness of the proposed method is verified by experiments.The proposed method ensures the reliable gate driving of the SiC MOSFETs under series connection with extremely high switching speed.Thirdly,the analytical model of the voltage unbalance is deduced and a series of voltage balance methods are proposed.The relationship between the time delay of drive signal and the unbalance voltage is built based on the analytical model of the device.Based on the analytical model,the voltage balance methods are proposed.As to the dynamic resistor-capactior snubber method,the parameter selection boundary is given based the sensitivity of the voltage unbalance.As to the active gate drive voltage amplitude compensation method,a voltage balance method based on the coupling inductor is proposed with high response speed.As to the active gate signal delay method,the model of the control loop is built and the design method of the control system is introduced to ensure the stable and fast response of the control loop.The proposed voltage balance methods ensure the safe operation of the series connection and improve the efficiency of the device.Last but not least,a 10kV/200 A SiC power electronics building block(PEBB)based on series connection of eight SiC MOSFET is designed under the support of the proposed techniques.The engineering scheme of the modular design is given and a flexible power conversion prototype is built.A comprehensive evaluation on the proposed module is conducted on the switching time,the switching loss,the temperature dependence and the voltage sharing.The measured data demonstrates the low switching loss and stable temperature behavior characteristics of the proposed module.The total switching loss at 5kV/200 A is only 100.42 m J.Then a performance scan tool is developed based on the measured data and adopted to evaluate the semiconductor loss in a medium voltage high power converter in wind applications.At last,the proposed module is verified in the leg test platform,which demonstrates the potential of the wide adoption of the series technique.