Optimal Design Of Full-bridge LLC Converter

With the development of power electronics technology,there are higher requirements for the efficiency and stability of power supplies.It is found that the LLC resonant converter has gained wide attention because it can realize the ZVS of the primary switching transistor and the ZCS of the secondary diode in the full load range.However,when the input voltage fluctuates,the pulse of the resonant current and the resonant capacitor voltage in the LLC converter is large,and the output voltage has a slow response speed and a large fluction range,which limits the use in the voltage fluctuation environments such as photovoltaic and wind power.Based on the full-bridge LLC converter,this paper proposes a hybrid control strategy.The specific control strategy is as follows: 1)When the input voltage is stable,the PID controller is used.The frequence modulation control and phase shift control will be combined;2)When the input voltage fluctuates,a digital direct controller is used.Quick adjustment of the output voltage is achieved by direct frequency modulation control or direct phase shift control.Firstly,the working mode of the full-bridge LLC converter is analyzed.Based on the phase plane trajectory analysis,the frequency modulation control and phase-shift modulation control of the full-bridge LLC resonant converter are analyzed.When the input voltage fluctuates,the analysis of the time domain waveform and phase plane trajectory changes provides a theoretical basis for the design of the hybrid controller.Secondly,the gain of the full-bridge LLC converter is analyzed by FHA.Based on phase plane analysis and time domain model,the concrete implementation scheme of hybrid control strategy is given.The structure of hybrid controller is designed and the function of each module in the structure is introduced in detail.The specific working mode of the digital direct control module,and the calculation method of the change of the frequency and phase shift angle in the module are introduced.The voltage gain curve of the converter under different control modes is plotted with MATLAB.At the same time,the resonant network of the converter is optimized,and the parameters of the resonant inductor,resonant capacitor and transformer are designed and selected.Then,the design of the control circuit for full-bridge LLC converter is based on TMS320F28335.The hardware circuit design part includes a sampling circuit and a driving circuit.The software design section includes the overall solution,interrupt service subroutine,PID controller design,and soft start.At the same time,based on the phase plane trajectory,combing frequency modulation control with phase-shift control,a soft start process is proposed.Finally,a prototype of 20V/100 W full-bridge LLC converter was built,which can meet the input 50V~100V.The simulation waveform and experimental waveform of the converter under the input voltage fluctuation environment are given to verify the feasibility of the proposed control strategy.Moreover,some suggestions for the improvement are given.