Research On High-frequency DC-DC Converter In Electric Vehicle Based On GaN Transistors

With the great degree of globe industrialization and numerous number of globe cars,tranditional automobile industy has been greatly restricted by the depletion of oil resources and atmospheric pollution problem caused by vehicle emissions.Electric Vehicle(EV)becomes the world focused emerging industries because of its feature of zero emission.The vehicle-mounted converter,including vehicle-mounted bidirectional DC-DC converter and vehicle-mounted DC-DC converter,is an important part of electric vehicle's power system.Gallium nitride(GaN)power transistor have the advantages of faster switching capability,smaller parasitic parameters and better electric parameters,which make them more suitable to achieve higher power density in DC-DC converters in EV.Based on the incomplete ZVS of high-voltage GaN transistor while working in ZVS region of LLC resonant converter and the phenomenon that it will get into ZCS region under overloaded or shorted condition,this paper discusses the uniqueness of working status of high-voltage GaN transistor in ZCS region compared to the traditional Si MOSFET,and also gives the comparison experiments between ZVS region and ZCS region of LLC resonant converter which proved high-voltage GaN transistor can work in ZCS region without any protections,but it is not suitable to work in that region for a long time.Considerring the narrow space of EV,LLC bidirectional converter has been selected as the research object of bidirectional DC-DC converter whose working frequency is high in order to reduce the volume.Based on the forward and backward working characteristics of LLC bidirectional resonant converter,the superiority of high-voltage GaN transistor in LLC resonant converter has been analyzed compared to Si MOSFET and resonant tank parameters also have been optimized.In order to improve the ability charge of power battery placed on EV,control strategies of improved burst mode under ligh load condition and smooth mode transition when electromagnetic braking have been presented.A 1MHz prototype has been fabricated,which has the peak efficiency of 97%,based on high-voltage GaN power transistors.Consiferring high ouput-current of vehicle-mounted DC-DC converter,achieved the optimized design of transformer and inductor and the reduction of their height.Based on the wide range of input voltage and the dramatic change of load,peak current mode has been selected through which dynamic properties and rejection of input disturbance have been significantly improved.Compared to traditional Si MOSFET,the superiority of GaN transistor under hard-switching condition has been analyzed in this paper.A 1.2kW prototype has been fabricated which is suitable for the input-voltage ranging from 90 V to 120 V.Moreover,the comparison of power loss analysis between GaN-based converter and Si-based converter has been done,which proves GaN transistors can greatly improve the efficiency of vehicle-mounted DC-DC converter.