Wide-Range Multi-Level LLC Resonant Converter For On-Board Charger

While the extensive use of fuel vehicles brings convenience to people’s lives,it also exacerbates problems such as the energy crisis and environmental pollution.In order to solve these problems,countries all over the world are actively promoting the research and use of electric vehicles.As the core component of electric vehicles,energy storage batteries need to have a wide output voltage range and high efficiency in the charging process.These performances rely on the DC/DC converter with a wide range and high-efficiency.However,traditional DC/DC converters often fail to meet the above requirements.Based on this problem,this paper studies a multi-level LLC resonant DC/DC converter for electric vehicle on-board chargers.The converter outputs two levels of V_in and 2V_in in half of the inverter cycle.With phase shift control,the voltage gain can be extended to twice,and the converter can realize the ZVS of the primary side power tube and the ZCS of the secondary side rectifier tube to meet the gain range and efficiency requirements of the on-board charger at the same time.The multi-level LLC resonant converter adopts fixed frequency modulation and always works at the resonant frequency point,avoiding the large-scale frequency modulation of the traditional PFM,which is conducive to the design of the magnetic element of the converter and the improvement of power density.Different from the traditional frequency conversion LLC resonant converter,the double gain range of the multi-level LLC resonant converter is not affected by the quality factor and inductance ratio.This feature can greatly simplify the selection and design process of the converter resonant parameters and facilitate the adoption of increasing the magnetizing inductance to limit the amplitude of the excitation current;in addition,compared with the traditional full-bridge or half-bridge LLC resonant converter,the application of multi-level helps to reduce the amplitude of the resonant current;the above factors will effectively reduce the switching loss and conduction loss of the converter,thereby improving the conversion efficiency.This article first introduces the circuit topology,modulation strategy and working principle of the multilevel LLC resonant converter,and gets the voltage gain curve of converter using the fundamental harmonic approximation.Then by comparing with the gain and resonant tank parameter characteristics of the full-bridge LLC resonant converter,the advantages of the converter are explained.Subsequently,the selection principle of resonance parameters is introduced from the aspects of ZVS realization conditions,device stress and converter efficiency.Finally,based on the application background of the on board charger,an experimental prototype with an input voltage of 380V,an output voltage of 250V～420V,and a power of 3.3k W was built,and the working principle and characteristics of the converter were experimentally verified.The experimental results show that the converter can reach the gain range demand of the car charger,and the ZVS of the primary side switch tube and the ZCS of the secondary side diode can be realized in the full load range,and its peak efficiency reaches 97.3%,and it maintains a high level effectiveness during the entire working process.