Control Improvement and Efficiency Optimization of Four Switch Three-Level Half-Bridge LLC Converter
Chen Jie, Liu Changyong, Deng Wenming, Gao Yu, Ruan Xinbo
Ministry of Industry And Information Key Laboratory of More-Electric Power System Nanjing University of Aeronautics & Astronautics Nanjing 211106 China
Abstract：The ram air turbine (RAT) is an emergency power supply for aircraft when the engine and the auxiliary power system fail. It is the final guarantee to ensure the safe landing of the aircraft. The output voltage amplitude and frequency of permanent magnet synchronous generators vary widely due to the wide speed range of the RAT. Since most onboard emergency electronic devices or loads are powered by DC voltage, it is necessary to rectify the AC voltage to an uncontrolled DC voltage and then convert it to a desired DC voltage by a cascaded DC-DC converter. Therefore, the input voltage range becomes very wide for the DC-DC converter. Similarly, such a wide input voltage range also exists in spacecraft solar arrays and photovoltaic power generation. Thus, converters operating with an extensive wide range of input voltage are necessary. Currently, various topologies and control strategies are suitable for wide input voltage ranges. Among them, the three-level half-bridge (TLHB) LLC resonant converter has a relatively simple topology and low voltage stress. Consequently, this paper analyzes the operational principle, control strategy, and parameter optimization of the TLHB LLC resonant converter. Firstly, the paper compares different structures of LLC resonant converters suitable for wide input voltage ranges and the commonly used control strategies, analyzes the merits and drawbacks of various topologies and control strategies, and gives the selection consideration of the topology. Considering the broad input voltage range, the two-stage topology is adopted. The four-switch TLHB resonant LLC topology is employed as the front-end converter. It operates in an open-loop state with a fixed resonant cavity input voltage frequency, with the advantage of easy parameter optimization and loss reduction. The second stage uses the synchronous rectification Buck circuit. Secondly, the operating mode of the LLC converter is analyzed using the constant switching frequency dual mode control strategy. The mathematical model of the TLHB LLC resonant converter is obtained based on the first harmonic approximation (FHA) method, and the relationship between the main parameters and output gain of the resonant converter is analyzed. The impact of switching frequency on the over current phenomenon is revealed, and a soft switching control strategy is proposed for a smooth transition between the two control modes. Additionally, the voltage imbalance mechanism of input bus capacitors is analyzed, the influence of the duty cycle on the capacitor voltage balancing is investigated, and an input bus capacitor voltage balancing control strategy is proposed. The loss distribution of the TLHB LLC resonant converter is analyzed, and the expressions of power losses and several key currents are quantitatively derived. The optimized parameters of the converter are solved using the cuckoo search algorithm regarding the relative loss rate of the converter as its objective function. The optimized parameters significantly improve the converter’s efficiency. Finally, a 500 W four-switch TLHB LLC resonant converter is developed. Experimental results validate the correctness and effectiveness of the proposed control strategies and the parameter optimization design.
陈杰, 刘昌咏, 邓文明, 高宇, 阮新波. 四开关三电平半桥LLC变换器的控制策略与效率优化[J]. 电工技术学报, 2024, 39(24): 7833-7845.
Chen Jie, Liu Changyong, Deng Wenming, Gao Yu, Ruan Xinbo. Control Improvement and Efficiency Optimization of Four Switch Three-Level Half-Bridge LLC Converter. Transactions of China Electrotechnical Society, 2024, 39(24): 7833-7845.
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