Abstract：The unidirectional magnetization of the basic forward conversion topology will lead to the transformer’s saturation, and various magnetic reset measures have been proposed. Among them, the primary side magnetic reset only consumes the excitation energy on the resistance or feedback to the input side, so the excitation energy can not be fully utilized. Therefore, it is not beneficial to improve energy transmission efficiency of the converter. The secondary-side reset forward-flyback conversion topology composed of four diodes on the secondary side can forward energy and excitation energy to the load, thus improving the energy efficiency. However, the excitation inductor and the forward inductor affect the power transmission and distribution characteristics and the switch current stress of the conversion topology. Therefore, this paper studies the relationship between power transmission characteristics, switch current stress and the magnetic components parameters. The design method of magnetic components parameters is proposed by comprehensively considering the influence of forward and flyback transmission power ratio on the electrical performance of the conversion topology. For the forward-flyback converter, the forward inductor can only work in DCM. Compared with the minimum excitation inductor current in one cycle with zero, its working mode can be divided into Magnetizing current continuous conduction mode (MCCM) and Magnetizing current Discontinuous Conduction Mode (MDCM). Its power transmission characteristics and its relationship with a load resistance RL under different modes show that both the forward power PFW and flyback power PFB increase with the decrease of the RL when the conversion topology works in the MDCM. When the RL is reduced to make the conversion topology enter the MCCM, the PFW increases to the maximum value and no longer increases with the decrease of the RL, while the PFB still increases with the decrease of the RL. The critical excitation inductance Lmc corresponding to PFW=PFB is defined by analyzing the influence of excitation inductance on the power transmission and distribution characteristics of the conversion topology. For the given output power, the PFW increases, and the PFB decreases with the increase of excitation inductance. When the excitation inductor equals Lmc, and the conversion topology works in MDCM, the PFW equals PFB. The PFB increases, and the PFW decreases with the increase of the transformer turn ratio. Regarding the influence of power transmission distribution on the switch current stress, for the given output power, the switch current stress decreases with the decrease of PFW. Therefore, reducing the PFW can reduce the switch current stress. However, reducing the PFW will increase the PFB, while increasing the PFB requires increasing the transformer air gap to reduce the magnetizing inductance, which will increase the transformer loss. Therefore, reducing the PFW is not beneficial to improving the efficiency of the conversion topology. Regarding the influence of the distribution of PFW and PFB on the electrical performance of the conversion topology, reducing the PFW is beneficial to reducing the current stress of the switch, and reducing the PFB can improve the efficiency of the conversion topology. Therefore, to improve the efficiency of the conversion topology and not cause the current stress of the switch to be too high, a component parameter design method is proposed to guarantee that PFW=PFB when the output power reaches the maximum within the given load range. Experimental results verify the correctness of the theoretical analysis and the feasibility of the proposed design method. The proposed design method for component parameters can develop a high-performance forward-flyback conversion topology and promote its popularization and application.
刘树林, 沈一君, 刘旭, 庹汉宇, 王成, 王文强, 吴星. 正-反激变换拓扑的功率传输分配特性及设计考虑[J]. 电工技术学报, 2023, 38(18): 5006-5016.
Liu Shulin, Shen Yijun, Liu Xu, Tuo Hanyu, Wang Cheng, Wang Wenqiang, Wu Xing. Power Transmission Distribution Characteristics and Design Considerations of Forward-Flyback Conversion Topology. Transactions of China Electrotechnical Society, 2023, 38(18): 5006-5016.
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