Research On Topology And Control Strategy Of Symmetrical Half-Bridge Three-Level LLC Resonant Converter

The bidirectional DC-DC converter based on LLC resonant circuit has less switching loss,higher switching frequency,lower EMI and smaller volume compared with conventional PWM converter.So this paper puts forward a symmetric half-bridge three-level LLC resonant converter,which can be applied to the isolated DC-DC module of the power electronic traction transformer.It can realize voltage transformation,isolation and energy bidirectional transmission.Meanwhile,it possess all the advantages of three-level structure and LLC resonant converter.Therefore,it will become the future research trend.In this paper,the working principle of symmetrical half-bridge three-level LLC resonant converter is studied.At first,its equivalent model had been gotten based on the First-order Harmonic Approximation method,with its gain characteristics,impedance characteristics and the multiple conditions of realizing the soft switching being analyzed as well.Then the working process of three level LLC resonant converter has also been analyzed under three different working states.In order to improve the efficiency of the system,the converter is expected to work in the region of resonant point,which can realize zero voltage turn-on of the primary switches and zero current turn-off of the secondary switches.Moreover,according to the gain characteristics,the process of resonant network parameter selection has also been analyzed in detail in the paper.In this paper,the working principle of Pulse Frequency Modulation(PFM)and Burst PFM is analyzed in detail.And it uses Burst PFM as it works in no-load or light load,and and it adopts PFM mode to realize the stable control of the output voltage under working other modes,which can realize soft-switching under all over the load range and improve the system efficiency.The symmetrical half-bridge three-level LLC resonant converter simulation model based on PFM Control is set up with the Matlab/Simulink software.The working process of the experimental system model and the engineering prototype model is simulated,as they are respectively working in the stage of switching frequency less than?equal to and greater than resonant frequency.The correctness of the theoretical analysis is verifiedThe selection process of the switches,driving resistance and resonant network parameter was analyzed in detail.A 450W three level resonant converter experimental prototype based on SiC MOSFET is built.The validity of the software design is verified With FPGA being the main controller,the validity of the software design is verified.And the correctness and feasibility of the system are verified through the experimental results.