Research On Vehicle-Mounted Bidirectional DC-DC Converter Based On Symmetric CLLC Topology

With the growing concern about global warming,electric vehicles(EVs)are gaining more and more attention as an alternative to internal combustion engine driven vehicles,and the EV market is booming and its acceptance is increasing.For EVs,an on-board charger(OBC)is a power electronic device fixed to the vehicle to provide safe charging and discharging.Bidirectional DC/DC converter is the core part of OBC,and its core design goal is to achieve wide range output voltage regulation,increase power density,realization of soft switching characteristics,and loss reduction,so it is relevact to study DC/DC converter with good bidirectional capability.The conventional bidirectional symmetrical CLLC resonant converter resonant cavity parameters are designed symmetrically and the bidirectional operation process is the same.However,it is difficult to output a stable input/output voltage when facing the situation of asymmetric charge/discharge gain caused by the large range of battery voltage changes during charging,and the CLLC converter has the problems of bi-directional switching band asymmetry and the operating frequency is far from the resonant frequency.At the same time,the traditional symmetrical design for the analysis of the operating frequency prediction based on the fundamental analysis method,in the face of the switching frequency away from the resonant frequency will have a large error.To address the above situation,this paper analyzes the bidirectional voltage gain characteristics based on the symmetric CLLC resonant converter separately,so that the parameters of the resonant circuit are designed asymmetrically and the parameter design freedom is introduced,so that the converter can optimize the operating band during the charging and discharging process and realize a wide range of output voltage and soft switching characteristics.This paper firstly elaborates the circuit principle and operation mode of bidirectional CLLC resonant converter in charging process and discharging process,and derives the voltage gain equation and soft switching characteristics of the converter working in bidirectional process.According to the theoretical analysis,the traditional symmetric design using fundamental wave analysis method leads to the problem of low accuracy of the switching frequency model,the design of experiment method is used to optimize the switching frequency model and design the main parameters of the converter,and establish the simulation model to verify that the CLLC resonant converter studied in this paper can achieve a wide range of voltage output,compared with the traditional symmetric design to improve the minimum switching frequency,reduce the maximum switching compared with the conventional symmetric design.Finally,a low-power experimental platform of the converter is built,and the results of the simulation and experiment are compared and analyzed to show that the converter achieves bidirectional power transfer and soft switching characteristics with narrower switching band range and more band overlap in the face of wide range voltage output and asymmetric voltage gain.