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

Facing with the ever severe energy crisis and environmental pollution problem, energy-saving and enviroment-friendly electric vehicle(EV) has become more attractive to customers. As the bridge connecting high-voltage power battery and low-voltage storage battery, the DC-DC converter in EV has higher demand for high efficiency and power density. Gallium nitride(GaN) power transistors 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.In this thesis, several commonly used topologies are analyzed and compared based on the features of wide input voltage and output load range in the DC-DC converter. The two-staged converter consisting of Buck and half-bridge LLC resonant converter is chosen as the main topology. Because of the large output current, multi-phase interleaved LLC resonant converter is used in the post-stage and the advantage of decreasing output current ripple is discussed. To analyze the current sharing problem, the output gain characteristic of LLC resonant converter is derived based on fundamental harmonic analysis method. The current sharing problem caused by the mismatch between resonant components is calculated according to gain characteristic. The method to optimize the parameter of the resonant converter is given to improve the output current unbalance.Low voltage enhancement GaN power transistor has the disadvantages of narrow drive voltage range and low turn on threshold voltage. In this thesis, the influence of parasitic parameters on the driving circuit in low voltage enhancement GaN power transistor is analyzed and some suggestions are given to improve the driving waveform. About the high voltage GaN power transistor, the working principle of cascade structure is analyzed and the influence of parasitic parameters on the driving circuit caused by the packages is discussed. Further this paper indicates the small junction capacitor feature is beneficial to the decreasing of the dead time and improving the conversion efficiency.Based on the theoretical analysis, a 330 V input, 12V/150 A output, 1MHz prototype is fabricated and the efficiency comparison experiment with Si MOSFET is finished in the lab. The experimental results show that GaN power transistor has superior high frequency switching capability and efficiency advantage compared to that of Si MOSFET. Finally, a detailed power loss analysis is conducted, based on which, the further optimization objective is given.