Research On LLC Resonant Converter For Wide Voltage Range Applications

LLC resonant converter has some advantages such as simple structure,soft-switching,high efficiency,high power density and low EMI.Therefore,it is widely used in data centers,electric vehicle on-board chargers,photovoltaic power generation,and wind power.However,in the above applications,the resonant converter always operates in a wide input/output voltage range.The conventional frequency-controlled LLC resonant converter is not suitable for wide voltage applications because of limited gain adjustment,large circulating current,wide frequency variation range,complex resonance parameter design,soft switching limited by frequency range,which significantly reduces the high efficiency and high power density of the resonant converter.Moreover,the application of the resonant converter is also limited.Therefore,this paper takes LLC resonant converter as the research object,focusing on the goals of high voltage gain adjustment,low circulating current,and high conversion efficiency in wide input and wide output voltage applications,and obtains corresponding solutions.The main works of this thesis are as follows:(1)Taking the resonant converter for wide voltage applications as the focus point,the research ideas and solutions at home and abroad are summarized.In addition,its research ideas and solutions are classified.Then,the operation principle of frequency-modulated LLC resonant converter is analyzed in detail,and the voltage gain and efficiency characteristics of the resonant converter are explored.As a result,the defects and problems of conventional frequency-controlled resonant converter are essentially analyzed.The rationality of the analysis is verified by simulation and experimental results.(2)In order to meet the high efficiency and wide input voltage regulation requirements for data center and server power supply applications,a topology morphing control modulation strategy and a hybrid control strategy of topology morphing and frequency control are proposed.In the normal mode,the resonant converter operates at the resonant frequency point,which is the optimal efficiency point.In the hold-up mode with wide input voltage situation,the researched modulation strategy and hybrid control method are adopted to adjust the output voltage of the resonant converter.The operation characteristics of the resonant converter for server power applications are analyzed in detail.The effectiveness and superiority of the modulation strategy and the hybrid control strategy are verified by simulation results and comparative research.(3)In order to meet the wide output voltage charging requirements for on-board charger applications,a fixed-frequency PWM control strategy is researched.The resonant converter always operates at the resonant frequency point.In the thesis,the fixed-frequency PWM control technology and the operation principle of the resonant converter are analyzed in detail.The design of resonance parameters can be simplified with this method.Because a larger magnetizing inductance can be designed to reduce conduction and turn-off losses,the conversion efficiency of system can be improved.The feasibility is verified by simulation and experimental results.(4)In order to achieve the wide output voltage regulation for on-board charger applications,based on a resonant converter with hybrid rectification structure,a switching control strategy and a common pulse PWM control are given,and its control principle and operation principle are analyzed in detail.The high gain regulation is realized with the proposed method.Moreover,the frequency range is effectively narrowed and the circulating current is reduced.For above given methods,the feasibility is verified by simulation and experimental results.Moreover,the superiority of the improved solutions is highlighted through comparative research.