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
Resonant converters are widely used in distributed power systems, notebook computers, communication equipment, and new energy electric vehicles with their excellent performance. However,the parameters of resonant elements in resonant converter are the main factors affecting the gain,loss,volume of the converter,etc. Therefore, the performance of the converter can be improved to some extent by optimizing the resonance parameters.
Among many resonant topologies, experts and scholars at home and abroad have studied the LLC resonant converter the most, and in the application of LLC is also the most widely used, while in this paper, we will study another new resonant topology converter, whose resonant tank is a T-type CLL structure, and whose operating characteristics are very similar to those of traditional LLC, and the same can be realized in the ZVS of the primary-side MOSFETs as well as the ZCS of the secondary-side diodes.The biggest advantage of the resonant converter is its high conversion efficiency, while the converter can not avoid the generation of backflow power during the operation, andthe increase of backflow power will lead to a sharp increase in the conduction loss and current stress of the converter switch, thus reducing the conversion efficiency.To address this problem, this paper will analyze and optimize the backflow power generated by the CLL resonant converter.
Firstly, a detailed analysis of the operating modes of the full-bridge CLL converter is conducted based on the definition of backflow power, which pointed out that backflow power is generated in both positive and negative operating modes, in addition,a time-domain expression for the backflow power is derived, but the high complexity of the defining equation itself makes it difficult to calculate and characterize the backflow power. Secondly, two things can be seen from the modal analysis of the backflow power: (1) The backflow power is essentially reactive power circulating in the circuit and is not directly equivalent to power loss. (2) The power loss due to the large amount of backflow power in the circuit is consumed only in the equivalent resistance of the active and passive components in the reactive path. Therefore, there is no need for a direct calculation, and it is sufficient to characterize it with a variable that is indicative of the backflow power.Thirdly,by analyzing the AC equivalent model of the CLL converter and combining it with the modes that generate the backflow power, it is derived that the backflow power can be indirectly characterized by the phase shift angle of the resonant tank input voltage and current.On this basis, a simple characterization method of the backflow power of CLL resonant converter is proposed, and the backflow power is reduced by optimizing the resonance parameters under the constraints of voltage gain and ZVS to improve the conversion efficiency.Finally, the design method is verified by simulation and experiment using two sets of resonant parameters, and the results show that the conversion efficiency is improved by about 1.8% at the resonant frequency point and under full-load conditions, which verifies the correctness and feasibility of the theoretical analysis.
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