Research On LLC Resonant Converter For Vehicle Charger

With the development of power semiconductor devices,the power electronic converters are more and more widely used in electric vehicles,new energy power generation,motor speed regulation and other fields.In recent years,due to the vigorous development of new energy vehicles,as an indispensable infrastructure,on-board charging equipment has attracted more and more attention.In order to enable the grid power to flow bidirectionally with the vehicle power to reduce the user’s electricity cost,more and more researchers have begun to study vehicle chargers.As the core part of vehicle charger system,DC/DC converter must have high performance and efficiency.LLC resonant converter is widely used in vehicle charger system because of its high working frequency and easy to realize soft switching,which can achieve high efficiency while greatly reducing the volume of the converter.This paper has started research on study the LLC resonant converter used in the vehicle charger system.The problem of LLC resonant converter can not be controlled by pulse frequency modulation(PFM)under light load,this paper has used mixed modulation to solve it.It means uses pulse width modulation(PWM)to control the converter under light load and uses PFM to control the converter under heavy load.At the same time,based on the relationship between the input and output impedance of the converter,this paper has used the fundamental analysis method derived the DC gain formula of the converter in forward and reverse operation,analyzed the realization conditions of soft switching.Based on the extended description function method,the small signal models of the forward and reverse operation of the converter has been established respectively,the corresponding transfer function has been derived,and the corresponding controller has been designed according to the transfer function.Aiming at the shortcomings of the traditional control mode,the adaptive control strategy has been introduced to further optimize the system.Aiming at the problem that it is difficult to determine the switching frequency when the converter adopts PWM control,a switching frequency optimization algorithm has been designed,which can calculate the optimal switching frequency of the converter according to the change of input voltage.The simulation model has been built by using the simulation software to verify the correctness of the designed controller and switching frequency optimization algorithm.Finally,this paper has summarized the calculation methods of converter parameters,built the experimental platform with GaN devices,measured and analyzed the key waveforms of the converter.This paper has tested the transmission efficiency of the experimental platform,and verified the hybrid modulation mode and adaptive control strategy studied.