Design And Optimization Control Of Dual Active Bridge DC-DC Converter Based On SiC MOSFET

Nowadays,electrical energy conversion systems mainly use the line-frequency transformer to achieve electrical isolation and voltage matching.However,bulky,heavy,low-efficiency and noisy line-frequency transformer hinder the efficiency and power density improvement of power conversion systems(PCSs).With the development of semiconductor technology and magnetic materials,more and more attention has been paid to power electronic transformers(PETs)due to no need of power frequency transformers.Therefore,the PETs are considered to be the developing trend of PCSs.Because of the high power density,high efficiency and small size,PETs are widely used in electric vehicles,distributed generation and train electrical traction system.At the same time,wide bandgap semiconductor devices,such as silicon carbide(SiC)and gallium nitride(GaN),have attracted much attention due to their excellent characteristics such as high interruption voltage,high switching frequency,high temperature resistance,fast switching speed and low switching loss.The core parts of PETs are cascaded pulse rectifier(CPR)and dual active bridge(DAB)DC-DC converter.In the single-phase PETs system,the DC input voltage of the DAB DC-DC converter contains a ripple component whose frequency is twice the line voltage frequency(named as the second harmonic),due to the use of a single-phase converter.An external LC resonant circuit can suppress the voltage ripple,which increases the volume and cost of the converter.Therefore,a DAB DC-DC converter based on SiC MOSFET was built with improved power density and efficiency.A control method,the minimal current stress control based on input voltage feedforward,is proposed to improve the efficiency and dynamic characteristics of the converter,and to solve the impact of second harmonic on the DAB converter.Firstly,the working principle of DAB converter and the phase-shift control method,as well as their advantages and disadvantages are analyzed.The state space model of DAB converter under triple phase shift(TPS)control,and the small signal model of DAB converter are solved.Secondly,according to the input-output model of DAB converter,the expression of phase shift D is deduced.By adding PI controller,virtual direct power(VDP)control and load current feedforward methods for improving the dynamic performance of DAB converter in existing literature are obtained.The similarities and differences of these control methods are compared.Furthermore,the peak inductance current value is defined as the current stress.The expression of minimum current stress is solved by Lagrange multiplier,thus the control model of the minimum current stress(MCS)control is obtained.Thirdly,the relationship between input and output of DAB converter under input voltage fluctuation is analyzed,and it is proved that PI closed-loop has nearly no effect on output voltage fluctuation.The VDP and MCS control method cannot completely suppress the output fluctuation voltage.The influence of the PI control based on input voltage feedforward(PI-IVF)method on the output of DAB converter is analyzed.It is theoretically proved that the PI-IVF method can suppress the output fluctuation voltage.Combining the input voltage feedforward control with the MCS control,a minimum current stress control based on the input voltage feedforward(MCS-IVF)is proposed,which can improve the dynamic performance and efficiency of the converter and suppress the output fluctuation voltage.Finally,in order to further improve the efficiency of the converter in hardware,SiC MOSFET is applied to DAB converter.The principle of switching spike in DAB converter is analyzed.It is proved that the diode on-voltage drop plays a decisive role in reducing the switching spike of the converter.A DAB converter experimental platform based on SiC MOSFET is designed.On the experimental platform,the dynamic performance,output fluctuation voltage suppression and efficiency of each method of DAB converter are compared.The experimental results show that PI-IVF control and MCS-IVF control can effectively suppress the output fluctuation voltage,while MCS control fail to completely suppress the output fluctuation voltage.In efficiency experiment,MCS-IVF control and MCS control are obviously better than other methods.In general,the proposed MCS-IVF control can optimize the DAB converter in three aspects: dynamic performance,output fluctuation voltage suppression and efficiency.