Research On Multi-Mode Control Strategy For Bidirectional CLLC Resonant Converter With Wide Voltage Gain Range

With the continuous development of electric vehicles,new energy generation,and energy storage,the demand for the efficiency and wide voltage gain range of bi-directional DC-DC converters has been increasing.The resonant converter has certain advantages in improving converter efficiency compared to other types of isolated DC-DC converters due to its unique soft switching characteristics.In this thesis,the CLLC resonant converter with primary and secondary symmetry is selected as the research object,its wide voltage gain range,high efficiency bi-directional energy flow and soft-startup problem are studied.The main research contents are as follows:Firstly,the working principle and working state of the CLLC resonant converter in frequency control mode and phase-shifted control mode are analyzed,then the equivalent circuit model of the converter is established by fundamental analysis,and the influence of the converter resonance parameters on output voltage gain under different control modes is analyzed,and the theoretical basis for expanding the voltage gain range is provided.Secondly,aiming at the problem of limited voltage gain range and inability to achieve wide range voltage output when retaining soft switching characteristics in the primary side switch of the converter,a multi-mode control method based on variable circuit topology is studied,which enables the converter to switch between full-bridge circuit topology and half-bridge circuit topology by controlling the switching drive signal,so that the converter can operate in two modes of high voltage gain and low voltage gain respectively,thus widening the output voltage gain range.Furthermore,in order to improve the efficiency of CLLC resonant converter in full-bridge topology mode,a composite control strategy based on efficiency optimization is proposed.The control strategy is based on the traditional frequency control and phase shift control,establishes the power loss model of the converter.With the minimum loss of the converter as the objective function,the efficiency seeking algorithm is used to analyze the converter loss under different gain and load conditions,and determine the switching frequency and phase shifted angle to achieve the optimal converter efficiency.In addition,in order to suppress the inrush current of CLLC resonant converter during the start-up process,an improved soft-startup method based on phase-shifted control is investigated.Based on the monotonic characteristic between the output voltage gain and the phase-shifted angle of the converter,the output voltage is nearly linearly changed by controlling the change of the phase shift angle of the converter,so as to suppress the inrush current during the startup process.Finally,the simulation and experimental platform is designed and built to simulate and verify the studied multimode control strategy.The simulation and experimental results show that the improved soft-startup scheme with phase-shifted control can achieve smooth start-up of the CLLC resonant converter and suppress the inrush current during the start-up;the multimode-control can achieve wide voltage gain range of the converter under different input voltage levels;the composite control based on efficiency optimization reduces the converter loss in the buck range,thus improving the system efficiency in the whole gain range.