Determination of Transformer Shielding Foil Structure for Suppressing Common-Mode Noise in Flyback Converters

Flyback converters are widely used for implementation of low-power and multiple-output isolated power supplies. In flyback converter, the power semiconductor devices in the primary and secondary circuits are electromagnetic interference sources, which will induce common-mode noise (CMN). Transformer shielding foil is usually employed for suppressing the CMN in flyback converters. However, transformer shielding foil with inappropriate structure helps little for CMN reduction. In fact, the shielding effectiveness of flyback transformer is affected by the noise sources and the shielding foil structure parameters (length, width, and position). In order to improve the shielding effectiveness, this paper proposes a method to determine the shielding foil structure of the flyback transformer. This is realized through analysis and modeling of the CMN cancellation behavior of the flyback transformer. The method is validated by a comparison of CMN measurement results.