Common-Mode Noise Cancellation by an Antiphase Winding in Multilayer Isolated Planar Transformer

Common-mode (CM) noise is always present in switch-mode power supplies. All International Electrical Commission class I appliances must have their chassis directly connected to the earth ground, which provides a low impedance path for CM noise to transfer from primary to secondary via the isolated transformer. A proposed planar transformer construction is analyzed and a comparison of CM noise cancellation performance with wire-wound transformers is presented. Planar transformers are found to have better noise cancellation throughout the conducted electromagnetic interference range, thanks to a better winding plane matching to the primary CM noise source winding plane. A planar transformer with two primary winding planes is investigated and a construction with antiphase winding is proposed for best CM noise attenuation.