Abstract：Featuring low isolation capacitance and ease of integration into PCBs or ICs, toroidal transformers have increasing applications in modern low-power isolated DC/DC converter designs. common-mode (CM) characteristics of toroidal transformers, the main propagation path of CM noises, have yet to beinvestigated. Also, corresponding suppression methods for toroidal transformers are required. A CM equivalent lumped capacitance model of a toroidal transformer is crucial for CM noise modeling and prediction in the early design stage of a power converter. This paper addresses the CM model for toroidal transformers in low-power isolated power converters and proposes several methods to mitigate CM noise. Four commonly used winding structures (uniformly distributed single/double layer structure, split winding structure, and superimposed double-layer structure) are illustrated as a uniform CM equivalent lumped capacitance model. This model, characterized by its symmetry, relies solely on one parameter—the structure capacitance, which can be easily derived through simple measurement. Considering leakage inductance, a novel two-step validation approach is introduced to verify the transformer modelutilized in traditional PWM and resonant converters. Moreover, the proposed transformer model can assess CM noise directly in the early stages of transformer design. The concept of equivalent noise sources outlines acomprehensive noise prediction procedure. As a result, an accurate prediction of CM noise for any isolated power converter using toroidal transformers is facilitated. Additionally, suppression methods for toroidal transformer implementation are discussed. The conditions of converters with toroidal transformers naturally achieving zero CM noise areanalyzed, highlighting several topologies. CM suppression methods are proposed to reduce the CM noise current, including optimal turns ratio or improved terminal configuration, compensation capacitor, balancing winding, and shielding winding (using shielding copper or shielding-cancellation wire). A winding structure suitable for split winding structure application is proposed to eliminate CM noise cancellation and reduce isolation capacitance. The proposed transformer model is verified on different toroidal transformers. Two prototypes are constructed: a conventional PWM converter (flyback with high-side rectification) and a resonant converter (series-resonant converter). The noise prediction results are consistent with the measurement. All discussed noise mitigation methods are experimentally verified on the prototypes, achieving significant CM noise suppression effects of over 8 dB mitigation in the low-to-mid frequency range, suggesting a possible CM filter size reduction.
[1] Sun Bingyao, Burgos R, Boroyevich D.Ultralow input-output capacitance PCB-embedded dual-output gate-drive power supply for 650 V GaN-based half- bridges[J]. IEEE Transactions on Power Electronics, 2019, 34(2): 1382-1393. [2] Baktash A, Vahedi A.Calculation of parasitic elements in toroidal core transformers[J]. IEEE Transa- ctions on Plasma Science, 2014, 42(6): 1690-1696. [3] Li Haiwang, Zhu Kaiyun, Lei Kaibo, et al.Integrated MEMS toroidal transformer with Ni-Zn ferrite core for power supply on chip[J]. IEEE Transactions on Power Electronics, 2022, 37(9): 10075-10080. [4] 谢立宏. 隔离型变换器共模传导干扰的建模与抑制方法研究[D]. 南京: 南京航空航天大学, 2018. Xie Lihong.Research on modeling and suppression of common mode conducted interference in isolated converter[D]. Nanjing: Nanjing University of Aero- nautics and Astronautics, 2018. [5] 孔鹏举. 开关电源共模EMI噪声抑制技术的研究[D]. 北京: 清华大学, 2009. Kong Pengju.Common mode EMI noise reduction techniques in switch mode power supplies[D]. Beijing: Tsinghua University, 2009. [6] 王议锋, 陈晨, 陈博, 等. 一种基于串联输入并联输出型LLC的噪声抑制磁集成方法[J]. 电工技术学报, 2022, 37(9): 2319-2328. Wang Yifeng, Chen Chen, Chen Bo, et al.A magnetic integrated method for noise suppression based on input series output parallel LLC[J]. Transactions of China Electrotechnical Society, 2022, 37(9): 2319-2328. [7] 林苏斌, 陈为, 董纪清, 等. 开关电源变压器传导共模EMI磁电综合模型[J]. 中国电机工程学报, 2017, 37(8): 2436-2445. Lin Subin, Chen Wei, Dong Jiqing, et al.Magneto- electric composite model of transformer for conducted common-mode EMI in switching-mode power supply[J]. Proceedings of the CSEE, 2017, 37(8): 2436-2445. [8] Zhang Huan, Wang Shuo, Li Yiming, et al.Two- capacitor transformer winding capacitance models for common-mode EMI noise analysis in isolated DC-DC converters[J]. IEEE Transactions on Power Elec- tronics, 2017, 32(11): 8458-8469. [9] 徐策, 张方华, 董光冬. 考虑磁场频率特性的变压器共模EMI宽频建模方法[J]. 中国电机工程学报, 2022, 42(13): 4675-4686. Xu Ce, Zhang Fanghua, Dong Guangdong.Wideband modeling method of transformer common-mode EMI considering frequency characteristics of magnetic field[J]. Proceedings of the CSEE, 2022, 42(13): 4675-4686. [10] Salomez F, Videt A, Idir N.Modeling and mini- mization of the parasitic capacitances of single-layer toroidal inductors[J]. IEEE Transactions on Power Electronics, 2022, 37(10): 12426-12436. [11] Himanshu, Singh H, Kumar P S, et al. High frequency transformer's parasitic capacitance minimization for photovoltaic (PV) high-frequency link-based medium voltage (MV) inverter[J]. Electronics, 2018, 7(8): 142. [12] 王一帆, 刘雪山, 奉榆杰. 浮动式Boost DC-DC变换器共模噪声分析[J/OL]. 电工技术学报, http://doi.org/10.19595/j.cnki.1000-6753.tces.232092. Wang Yifan, Liu Xueshan, Feng Yujie. Analysis of common mode noise in floating Boost DC-DC converters[J/OL]. Transactions of China Electro- technical Society, http://doi.org/10.19595/j.cnki.1000-6753.tces.232092. [13] 彭金融, 毛行奎, 崔文玲, 等. 基于插入无源二端口网络的噪声源阻抗提取方法研究[J]. 电工技术学报, 2023, 38(2): 523-532. Peng Jinrong, Mao Xingkui, Cui Wenling, et al.Research on noise soure impedance extraction method based on inserting passive two-port network[J]. Transactions of China Electrotechnical Society, 2023, 38(2): 523-532. [14] Yao Juntao, Wang Shuo, Zhao Hui.Measurement techniques of common mode currents, voltages, and impedances in a flyback converter for radiated EMI diagnosis[J]. IEEE Transactions on Electromagnetic Compatibility, 2019, 61(6): 1997-2005. [15] 吴岳哲, 郑成君, 杜子凯, 等. 推挽变换器共模EMI噪声的预测与抑制[J/OL]. 电源学报, 1-12. 网络首发http://kns.cnki.net/kcms/detail/12.1420.tm.20230619.1715.012.html. Wu Yuezhe, Zheng Chengjun, Du Zikai, et al. Common-mode EMI noise prediction and suppression for push-pull converter[J/OL]. Journal of Power Supply, 2024, Early Access Online. [16] 何杰, 刘钰山, 毕大强, 等. 开关变换器传导干扰抑制策略综述[J]. 电工技术学报, 2022, 37(6): 1455-1472. He Jie, Liu Yushan, Bi Daqiang, et al.Review of conducted electromagnetic interference suppression strategies for switching converters[J]. Transactions of China Electrotechnical Society, 2022, 37(6): 1455-1472. [17] 李虹, 张冲默, 丁宇行, 等. 基于混沌脉宽调制的有源EMI滤波器高频抑制效果优化设计方法研究[J]. 中国电机工程学报, 2022, 42(13): 4642-4651. Li Hong, Zhang Chongmo, Ding Yuhang, et al.Optimization of high frequency noise suppression effect of active EMI filter based on chaotic spread spectrum PWM method[J]. Proceedings of the CSEE, 2022, 42(13): 4642-4651. [18] Li Yiming, Zhang Huan, Wang Shuo, et al.Investigating switching transformers for common mode EMI reduction to remove common mode EMI filters and Y-capacitors in flyback converters[J]. IEEE Journal of Emerging and Selected Topics in Power Electronics, 2018, 6(4): 2287-2301. [19] Fei Chao, Yang Yuchen, Li Qiang, et al.Shielding technique for planar matrix transformers to suppress common-mode EMI noise and improve efficiency[J]. IEEE Transactions on Industrial Electronics, 2018, 65(2): 1263-1272.
2024, Vol. 39 
