Research On SiC MOSFET Zero-Voltage-Switching Three-phase Inverter

Three-phase inverters are widely used in wind power system,PV generation system,uninterrupt power supply,electric vehicles,railway traction,motor drives etc.SiC MOSFET brings the opportunities of increasing the conversion efficiency,switching frequency and power density of three-phase inverter in comparison with its Si IGBT counterpart.However,the switching loss of SiC MOSFET at hard switching rises quickly with the increment of the switching frequency above 50 kHz.To further increase the power density of SiC MOSFET three-phase inverter,soft-switching technique may be used.The dissertation focuses on the impact of applying the Zero-Voltage-Switching(ZVS)Space-Vector-Modulation(SVM)to three-phase two-level SiC MOSFET inverter.The loss distributions,conversion efficiencies and volumes of passive components of both a 20 kW SiC MOSFET hard switching three-phase inverter and a 20 kW SiC MOSFET ZVS-SVM three-phase inverter have been compared under switching frequency range from 50 kHz to 300 kHz.Meanwhile,a new metric called "Efficiency Stiffness" is proposed to compare different inverters with respect to the efficiency performance against switching frequency characteristics.Two 20 kW SiC MOSFET three-phase inverter prototypes are built to verify the proposed design.The SiC MOSFET ZVS-SVM three-phase inverter could operate at higher switching frequency and achive higher power density than the SiC MOSFET hard switching three-phase inverter with the same conversion efficiency.High voltage overshoot of SiC MOSFET and high thermal stress of resonant inductor are the two critical issues in the SiC MOSFET ZVS-SVM inverter with high switching frequency.Sensitive loop of oscillation after the short circuit stage is identified and mathematic model of oscillation is also derived.A power module including seven SiC MOSFET bare dies with low stray inductance is designed and manufactured for ZVS-SVM inverter instead of the existing seven discrete TO-247 package SiC MOSFETs in order to reduce the voltage overshoots on the switches.A 20 kW SiC MOSFET ZVS-SVM three-phase inverter based on the 7-in-1 power module is also built to verify the design.Besides,in order to reduce the power loss of the resonant inductor caused by large amplitude of current at hundreds of kHz excitation frequency,design of the inductor with distributed air-gap and optimal winding thickness are studied and validated by simulations and experiments.A Zero-Voltage-Switching(ZVS)sinusoidal pulse width modulation(SPWM)method for three-phase four-wire inverter is proposed in order to achieve higher efficiency and power density.The ZVS SPWM scheme is introduced by considering the various combinations of the polarities in three-phase filter inductors currents and analysis of operating stages is presented.ZVS condition of the ZVS SPWM scheme is derived and discussions of ZVS condition for typical three-phase loads are also provided.In addition,the resonant parameters design and loss analysis are briefly investigated.Finally the proposed ZVS SPWM scheme is verified on a 10 kW inverter prototype with SiC MOSFET devices.ZVS of all main switches and auxiliary switch can be realized and the conversion efficiency has been raised.A summary of this dissertation is given at last.The main contributions of this dissertation are summarized,and the future works are presented.