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.
Hui Qi,Ren Xiaoyong,Chen Qianhong. Common-Mode Electromagnetic Interference Noise Modeling and Suppression for Low-Power Isolated Power Converter Using Toroidal Transformer[J]. Transactions of China Electrotechnical Society, 2024, 39(22): 7126-7138.
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