Shielding-cancellation technique for suppressing common mode EMI in isolated power converters

In an isolated power converter, the inter-winding capacitances of the power transformer are critical parasitic capacitances for the generating of common mode (CM) noise. To suppress the CM noise in isolated power converters, shielding-cancellation (SC) technique is proposed in this paper. The SC technique is realized by introducing noise cancellation on the basis of shielding, and it is easy to be implemented, suitable for various isolated power converters and independent of the transformer inter-winding capacitance. To verify the effectiveness of the SC technique on suppressing the CM noise, a flyback converter prototype is built, and the CM noise of the converter is measured and compared using three different transformers. It is revealed that the CM noise of the converter, which utilizes the transformer with SC technique, is lower than that using the transformer without shielding or with traditional shielding layers.