Common-Mode Noise Modeling and Reduction for Multi-Output Flyback Converters

Flyback converters are widely used in low power applications with wide range input and multiple outputs. Compared with the conventional spiral wire winding transformer, the planar transformer has smaller volume and is easier to implement flat structures for high power densities. However, due to the larger relative area and the smaller distance between two layers, planar transformers have larger interwinding capacitance which makes the CM noise increased significantly. In this paper, an actual multi-output flyback converter is taken as an example. Firstly, the CM equivalent lumped capacitance model of multi-winding planar transformer is established. The CM noise propagation paths and the spectrum characteristics of the dual-switch and single-switch flyback converter are analyzed and compared. An expression for the value of the cancellation capacitance is derived. Finally, the effectiveness of the proposed method is verified by simulation and experiment.