Identification and improvement of stray coupling effect in an L-C-L common mode EMI filter

The high frequency attenuation performance of an EMI filter is often deteriorated greatly by the stray electromagnetic field existing in interior or exterior of a filter. In this paper, the stray electromagnetic coupling factors in an EMI common mode filter are investigated by different experiments after theory analysis. A novel metal insulated grids method is proposed to separate electric and magnetic field effect between two CM chokes. The parasitic parameter influence of passive components on insertion loss is also analyzed and compared with stray coupling effect. In the end, a novel equivalent paralleled capacitance decrease design is introduced and validated by experiment. Some coupling restrain methods are introduced in this paper too