| With the rapid development of electric vehicles in our country,the DC charging technology for electric vehicles has been widely concerned and studied in recent years.And the power module of DC charging pile as one of its core parts has become the focus of relevant research.The power module of DC charging pile is generally composed of the front AC/DC converter and the rear DC/DC converter.In this paper,the rear DC/DC converter with wide output voltage range is studied.The research and development of the DC/DC converter for DC charging pile is faced with many problems such as wide output voltage range,high power density and high efficiency.As one of the common schemes,LLC resonant converter has good performance in soft switching,which is a unique advantage in reducing switching loss,reducing electromagnetic noise and improving power density.However,the output voltage range of LLC resonant converter in the soft switching operation is still limited under the traditional control mode.Aiming at the problem of the wide output voltage range demand for the DC/DC converter in the charging process of electric vehicles and the soft switching challenge faced by the converter,the following work is carried out in this paper:First of all,the basic properties of LLC resonant converter under the frequency conversion control and phase shift control are discussed in detail by means of Fundamental Harmonic Analysis(FHA)method respectively,and the voltage gain expressions of the converters under different control modes are obtained.Furthermore,the influence of parasitic capacitance in the circuit on voltage gain is analyzed,and a hybrid control method is proposed to solve the difficulty of voltage regulation caused by parasitic capacitance,so as to broaden the output voltage range of the converter.Secondly,the main switching modes of LLC resonant converter are analyzed,the equivalent circuits are established,and the time domain expressions of related physical quantities in each switching mode and the constraints of mode transition are obtained by the time domain analysis method,so that the accurate time domain model of the converter is established.Besides,the possible operating modes of the converter and the boundary conditions between them are summarized,so as to determine the distribution of each operating mode of the converter.Then,the soft switching conditions of LLC resonant converter are analyzed and summarized,and the working states of the converter at the ZVS operating boundary are analyzed in detail.Furthermore,the mathematical ZVS boundary conditions are proposed,and the soft switching workspace of the converter under different control modes is calculated by combining the ZVS boundary conditions and the time domain model.Finally,an LLC resonant converter prototype with maximum output power of1.2k W was built according to the design requirements.Meanwhile,the process of parameter design is summed up,and the key components are selected according to the parameter design results.The output voltage range of the prototype is expanded by the hybrid control method combining full-bridge frequency conversion control,full-bridge phase shift control and half-bridge frequency conversion control.And the soft switching operation of the prototype in the full load range is realized by controlling the output current of the converter always less than the output current at ZVS boundary and the switching frequency always less than the resonant frequency.And the maximum efficiency of the prototype is 96.3%. |
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