Analysis Of LLC Resonant Converter With PWM Control Strategy And Synchronous Rectification

With the development of communication and aerospace technology, switching power supply hasbeen widely applied. The trend of miniaturization of power electronic devices makes power density ofswitching power supply increasing. High-frequency switching is the basic means to improve thepower density. However, the increasing of switching losses limits the efficiency of converter. In orderto reduce the high switching losses, resonant soft-switching technology has been proposed.Compared with other resonant converters, LLC resonant converter under wide input and wide loadconditions can achieve the primary switch ZVS turning on and secondary rectifier diode ZCS turningoff. Advantages of high efficiency make it be taken into consideration widely. LLC resonant convertercontrol methods commonly use pulse frequency modulation (PFM) and pulse width modulation(PWM). PFM controlling has higher efficiency, while the variable frequency makes it difficult tooptimizing converter design. Meanwhile, the circuit structure of PWM control is simple, and givesattention to both converter voltage regulator and current limitation. Based on stable frequencycharacteristics, it is favorable to design of magnetic components, which can be applied in limitedinput voltage range and simple circuit structure required situation. This thesis discusses the workingprinciple of full-bridge LLC resonant converter with PWM control, and analyzes the convertercharacteristics of output with time-domain method. The output voltage gain curve is calculated bymaple, and prototype with390V input,60V/60A output is designed to verify the accuracy of analysis.Moreover, to improve the efficiency of the converter, loss analysis is investigated in detail.According to loss analysis, this thesis proposes a compound control strategy combiningvariable-frequency and fixed-frequency mode, which reduce conduction loss of switch. This thesisalso proposes synchronous rectifier method, which reduce conduction loss of rectifier.The thesis analyzes the working principle of full-bridge LLC resonant converter with compoundcontrol, expound the reform in start-up, voltage regulator, and current limitation, and complete thedesign of prototype and experiment. In the end, this thesis introduces the synchronous rectifiermethod fitting for LLC resonant converter, and analyzes the advantages and disadvantages of methods.Based on analysis above, synchronous rectifier method which fits for low voltage and large currentsituation is pointed out. Besides, experimental results are represented to verify analysis.