Design And Research On High Efficiency CLLC Resonant Converter

As an important component of new energy vehicles(NEVs),the development of electrical vehicles(EVs)has been strongly supported by many countries with effective measures.In order to achieve the goal of low-carbon,intelligence and informatization in 13 th five-year plan,it is vital to replace oil with electricity for the innovation and adjustment of vehicles industry,which is the important way for China to become a powerful country in automobile industry.The onboard power converter manages the energy flowing,controls the charge and discharge of battery,which influence the life of EVs.Isolated DC / DC converter is an important unit of vehicle power converter,which increasingly requires high power density,high efficiency and wide output range.In this paper,the working principle of CLLC resonant circuit is firstly analyzed.The power transmission characteristics,soft switching realization and working states under different conditions are analyzed in time and frequency domain.Based on the time domain analysis,working modes are listed according to different working conditions,and the conditions that circuit need satisfy under different modes and its criterion are given.On the basis of time domain,the influences of excitation inductance ratio on gain and mode switching,soft switching and converter loss are analyzed and optimized.Then,a hybrid control strategy of multiple phase shifts and frequency conversion for CLLC resonant converter is proposed.In order to solve the problem that single control parameter is unable to meet the requirement of wide output and load range,some control parameters are added to adjust the output gain,including the internal phase shift angle among different bridge arms of input-side inverter,the internal phase shift angle among different bridge arms of output-side rectifier and the external phase shift angle of neural point voltage in the full bridge between input and output sides.The circuit can achieve wide output voltage through a narrow switching frequency range under the condition of satisfying the converter efficiency.At the same time,it can be used at low gain point under light load to avoid high switching frequency.In this paper,the RMS current,switching frequency and efficiency of the resonator with different phase shifts under light load are analyzed.When the internal phase shift angle of input side and the external phase shift angle between input and output sides meet a certain linear relationship,the switching frequency can be reduced and the efficiency can be improved.Next,a synchronous rectification strategy(SRs)based on the time domain analysis is proposed to reduce the conduction loss of converter.Through the time-domain analysis,the fitted linear equation can be given to present the phase-shift angle relationship of input-side and outputside switches with the change of switching frequency.The switches on the output-side rectifier can achieve the synchronous rectification under different working points,which significantly improves the global efficiency.Finally,a 2k W CLLC resonant converter prototype is designed and tested in this paper.Experimental results show that the proposed control strategy can achieve high working efficiency in a wide output voltage range,and all the MOSFETs can realize soft switching and synchronous rectification.In addition,the proposed control strategy is able to reduce system complexity in the meantime of fulfilling the high efficiency of CLLC resonant converter.