An evaluation method of transformer behaviors on common-mode conduction noise in SMPS

The electric coupling between primary and secondary windings of the transformer is usually evaluated by the capacitance between the primary and secondary windings which is tested by one-port test devices, such as LCR meter. This method can't completely characterizes the EMI behaviors of the transformer on common-mode noise, because this method does not account electrical potential distribution along the winding turns and the shielding copper foil effect in the transformer. A new evaluation method is proposed in this paper which can overcome the traditional shortages on CM noise by spectrum analyzer, EMI receiver or network analyzer. The method provides an effective test tool to optimize the structures of the windings and shielding of a transformer in design and to control the quality of the transformer in mass production with its CM EMI behaviors. Finally, the experimental results verify the proposed method to be correct and feasible.