采用环形变压器的小功率隔离型DC-DC变换器共模电磁干扰噪声建模与抑制

Common-Mode Electromagnetic Interference Noise Modeling and Suppression for Low-Power Isolated Power Converter Using Toroidal Transformer

环形变压器具有隔离电容小、易于实现印制电路板(PCB)和集成电路(IC)等优点,在小功率隔离DC-DC变换器设计中的应用日益广泛。实际产品设计中,针对共模EMI噪声的准确预测和有效抑制是至关重要的,而变压器是隔离变换器共模EMI噪声的主要传播通路。然而,目前鲜有针对环形变压器共模电磁干扰(EMI)特性的相关研究,也缺乏针对性的噪声抑制方法。该文针对多种常见于小功率隔离型DC-DC变换器应用的环形变压器对称绕组结构,推导证明其具有统一的对称共模等效集总电容模型。该模型的参数易于获取,且可直接用于预测共模EMI噪声。分析了常用隔离变换器拓扑采用环形变压器时的共模噪声特性,同时给出噪声预测方法,并经实验验证了模型的正确性和有效性。此外,还提出多种环形变压器的共模EMI噪声抑制方法并加以分析,所提方法已在两台样机上得到验证,共模噪声的抑制效果显著。

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.