Topology, Control, and Design of LLC Resonant Converters

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Hu, Zhiyuan
load sharing , switched-mode power supplies , current sensing , charge control , Resonant power conversion , transient response , interleaving
Data centers and supercomputers have become the backbone to support today’s scientific researches, economic developments, and individual lives. The power consumption of data centers and supercomputers are enormously high, bringing the urgency of improving energy efficiency of the power conversion systems. The LLC resonant converter emerged in recent years. As an element of the front-end AC-DC power conversion systems, it brought significant efficiency improvement and has been popularly deployed. However, surrounding the LLC topology there are still several problems unsolved, including: (a) interleaving problem, (b) current sensing problem, (c) poor dynamic performance problem, and (d) peak gain design problem. The works in this thesis include several original ideas to solve above problems: Firstly, an SCC-LLC topology is proposed, featuring constant switching frequency operation to solve the interleaving problem. Secondly, theoretical analysis reveals that the constant frequency operation compromises the converter’s operation range to some degree. A new control strategy featuring variable switching frequency is proposed to achieve lower cost and better performance than its constant frequency counterpart. Thirdly, upon solving the interleaving problem, it is recognized that existing current sensing methods bring inaccuracy to the load sharing performance, as well as low bandwidth to the current-mode control. A cycle-by-cycle average input current sensing method is proposed obtain per-cycle average input current based on sampling the resonant capacitor voltage, which is simple, accurate, with no delay, and virtually has no cost. Fourthly, inspired by the cycle-by-cycle average input current sensing method, a Bang-Bang Charge Control (BBCC) method is proposed to achieve very fast dynamic performance. The feedback loop bandwidth can achieve 1/6 of switching frequency at all operation conditions. Lastly, it is recognized that the design method of the LLC converter lacks an accurate and comprehensive mathematical solution. An accurate design algorithm is derived based on time-domain analysis to identify all the possible designs that provide the exact peak gain. The results of this algorithm will help identify the optimal design. Simulation models are developed to prove the accuracy of the proposed theories and algorithms. Prototype circuits are built to demonstrate the advantages of the proposed circuits and control methods.
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