Research On Key Issues Of Soft-switching Multilevel DC/AC Converter

DC/AC converters are widely used in industrial applications such as new energy power generation,power conversation systems,data center,and EV charging station,which the conversion efficiency and power density are important indicators.To improve the converter performance,the multilevel technique and soft-switching technique are developed.Among them,the multilevel technique can effectively reduce the filter size,improve power quality,and reduce the voltage stress requirement of power devices.The soft-switching technique can effectively reduce switching loss,improve efficiency,and relieve EMI.This dissertation aims to provide a solution for the high efficiency and high power density DC/AC converter by combining the multilevel technique and soft-switching technique.This dissertation first investigates two-level soft-switching technique,and proposes a ZVS multiphase DC/AC converter and generic modulation scheme.By installing a simple auxiliary circuit on the DC bus,the ZVS operation of all switches on any phase switch legs can be realized.Utilizing EA-PWM,the auxiliary circuit operates at a fixed frequency,which only needs to operate once in every switching period.It not only simplifies the control but also reduces the loss.To study the soft-switching operation principle,the multiphase switch legs are classified and grouped into four classes according to the modulation signals and polarity of the filter currents.The influence of these four classes of switch legs on the soft-switching operation is further analyzed and the ZVS conditions are derived.Meanwhile,a logic diagram of the drive signals generation in EA-PWM is designed.Taking a ZVS two-stage three-phase PV inverter as an example,the proposed theoretical analysis is verified.The ZVS conditions and current stress of power devices under two different modulation signals DPWM and CPWM are compared,and the resonant parameter design is given.Besides,the loss model of the ZVS inverter is established.The experimental results prove the ZVS operation of all switches can be realized,and the efficiency of the inverter is significantly improved.Based on two-level soft-switching technique,the three-level soft-switching technique is further studied.The ZVS three-phase T-type inverter and its ZVS-PWM scheme are proposed.The proposed soft-switching scheme ensures that the voltage stress of horizontal switches and auxiliary switches is still half that of vertical switches.This dissertation focuses on analyzing the soft-switching operation principle and process,deriving ZVS conditions,and obtaining general expressions for the duty cycle of the positive and negative auxiliary switches.According to the voltage and current stress of power devices,the selection criteria of power devices and the design of the resonant parameter are given.Furthermore,the issue of midpoint voltage balance under soft-switching operation is studied.Through the mathematical derivation,it is proved that the midpoint voltage fluctuates at a period of 2π/3 and keeps in balance.To avoid excessive voltage overshoot,the commutation loop of the ZVS inverter is analyzed,and the power circuit is optimized.The parasitic inductance of the commutation loop is extracted by the simulation.To suppress the current surge on the resonant circuit,an improved ZVS-PWM scheme based on the phase-shift carriers is proposed,and the related control diagram is designed.Finally,an experimental prototype of the Si C ZVS three-phase T-type inverter is built.Steady and dynamic experiments under different loads are carried out.The inductors such as middle voltage,THDi,efficiency,etc.are tested.The theoretical and experimental results of the resonant inductor current,voltage stress,and loss breakdown are compared.This dissertation further investigates the five-level soft-switching technique and proposes a ZVS five-level ANPC inverter and its ZVS-PWM scheme.The soft-switching operation process is analyzed and ZVS conditions are derived.The influence of the soft-switching operation on the voltage balance of the flying capacitor is studied,and a control scheme for the dynamic voltage balance of the flying capacitor under soft-switching operation is proposed.Furthermore,the voltage and current stress of power devices are analyzed and the design of the resonant parameter is provided.To verify the proposed theoretical analysis,an experimental prototype of the ZVS five-level ANPC inverter is built.The experiments demonstrate the ZVS operation of the switches can be realized and the dynamic voltage of the flying capacitor voltage can be regulated,which features good dynamic performance.Finally,the work of this dissertation is summarized,and future works are introduced.