Parameter Optimal Design of Full-Bridge CLL Resonant Converter Considering Backflow Power Factor
Huang Hewei1, Cao Taiqiang1, Pan Guangxu2, Cao Weizhong3, Zheng Min1, Yang Xiaoming1
1. School of Electrical and Electronic Information Xihua University Chengdu 610039 China; 2. Civil Aviation Chengdu Electronic Technology Co. Ltd Chengdu 611430 China; 3. Chengdu SIWI High-Tech Industry Company Limited Chengdu 610097 China
Abstract：Resonant converters are widely used in distributed power systems, notebook computers, communication equipment, and new energy electric vehicles due to their excellent performance. However, the parameters of resonant elements are the main factors affecting the gain, loss, volume of the converter, and overall performance. Therefore, optimizing these parameters can lead to improvements in converter performance. Among many resonant topologies, the LLC resonant converter has received significant attention and is widely used in practical applications. This paper studies a new resonant topology converter with a T-type CLL structure resonant tank. Its operating characteristics are very similar to the traditional LLC, which can be realized in the ZVS of the primary-side MOSFETs and the ZCS of the secondary-side diodes. The advantage of the resonant converter is its high conversion efficiency. Nonetheless, the converter cannot avoid generating backflow power during the operation, and the increase in backflow power can lead to a sharp increase in the conduction loss and current stress of the converter switch, ultimately reducing conversion efficiency. Therefore, this paper analyzes and optimizes the backflow power generated by the CLL resonant converter. Firstly, the operating modes of the full-bridge CLL converter are analyzed based on the backflow power definition. It is pointed out that backflow power is generated in both positive and negative operating modes. In addition, a time-domain expression for backflow power is derived, though its complexity makes direct calculation and characterization challenging. Secondly, two insights emerge from the modal analysis of backflow power: (1) Backflow power is essentially reactive power circulating in the circuit and is not directly equivalent to power loss. (2) Power loss due to the substantial backflow power in the circuit is dissipated mainly within the equivalent resistance of the active and passive components in the reactive path. As a result, direct calculation is unnecessary, and it suffices to characterize backflow power using a relevant variable. Thirdly, by analyzing the AC equivalent model of the CLL converter and combining it with the modes that generate backflow power, it is derived that backflow power can be indirectly characterized by the phase shift angle of the resonant tank’s input voltage and current. Accordingly, a simple characterization method for the backflow power of CLL resonant converters is proposed. Backflow power is reduced by optimizing resonance parameters within the constraints of voltage gain and ZVS, thereby enhancing conversion efficiency. Finally, the design method is verified by simulation and experiment using two sets of resonant parameters. The results show that conversion efficiency is improved by about 1.8% at the resonant frequency point under full-load conditions.
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2023, Vol. 38 
