Common-mode (CM) conduction electromagnetic interference (EMI) is a challenge for switched-mode power supply (SMPS) designers. A flyback converter is a traditional topology used in low-power isolated applications. The size and cost of EMI filters are important considerations for higher power density and lower cost of SMPS design. In most cases, the CM current is generated by voltage pulsation (dv/dt) assigned on the primary winding and the secondary winding. The CM current conduction paths can be mainly categorized into two categories. One is the interwinding parasitic capacitance of the transformer, and the other is the parasitic capacitance between the semiconductor switches and the protective ground. This paper proposed an evaluation method to measure the equivalent CM noise capacitance of the two main CM current conduction paths, respectively. Then, this paper proposed a CM noise cancellation scheme based on the transformer winding design combining the measurement technique and the FEM simulation tool. Planar transformers with PCB windings also have advantages in low profile, good heat dissipation, and good stray parameter consistency. Based on these considerations, an 18-W adapter is designed and tested to verify the effectiveness of the proposed evaluation method and the design scheme. INDEX TERMS Common-mode (CM), flyback, EMI, planar transformer, equivalent capacitance.

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In isolated power converters, the transformer is a key part of voltage transformation and isolation. Since common-mode (CM) noise is rather difficult to suppress compared with different-mode (DM) noise, more and more scholars are paying attention to the characteristics of CM noise, especially in high-frequency CM noise behaviors. CM noise can be further divided into conducted CM noise and radiated CM noise, and the main focus of this paper is on conducted CM noise. The CM coupling capacitance of the transformer is one of the main contributors of CM noise, which has been verified in many previous studies. Hence, eliminating the CM noise in a transformer coupling path can significantly lower the whole CM noise level of the converter. Professional conducted electromagnetic interference (EMI) testing instruments are quite expensive. In this paper, a general transformer evaluation technique for CM noise behavior is proposed. Only a signal generator and oscilloscope can achieve transformer CM noise behavior evaluation. PCB planar flyback transformers are designed, and a series of noise spectrums and voltage waveforms can verify the effectiveness of the proposed transformer evaluation method. The flyback adapter porotype can pass the EMI standard limited line EN55022 class B by the proposed evaluation method.