Research On The Design Of Planar EMI Filter With Passive Components

EMI filter plays an important role in suppressing conduct EMI. However, the large volume, lowpower density, large parasitic parameters of the choke and the filter capacitor influence the highfrequency performance of the conventional EMI filter. In this paper, the integrated planar EMI filter isstudied; its electromagnetic characteristic and the design procedure are also proposed.The theory of electromagnetic integration and the extraction of the electromagnetic parametersare discussed and the numerical models of each part are established. Furthermore, the electromagneticcoupling among different parts of EMI filter is studied. The validity and accuracy of numbericalmodel are verified by emulation and experiments.Based on the equivalent circuit, the relationship between the transformation of the CM noise andthe DM noise and the asymmetry of the impedance is studied. Conventional EMI filter design is basedon the decoupling of CM and DM. In order to improve this design, a new integrated “LC” unit withbetter symmetry impedance is proposed.In which, the dissymmetry of resistance as a result of staggerwinding is reduced obviously; what’s more, the grounding resistance is minished. The S parameter testmethod of two-port network provides a higher accuracy and more convenience way to achieveinsertion gain of EMI filter.The generating mechanism and the suppression of the mix-model noise are investigated based onthe PFC. Its effect on the design of DM capacitor will be illustrated in this paper. The measuring ofthe DM noise based on the balance capacitor is proposed and viability is verified by experiments.Furthermore, key factors of designing differential capacitance are concluded.The implementation of the planar EMI filter based on an89W PFC+Flyback is proposed and themain structure of the filter is designed. Based on the insertion loss experiments, the superiority of theplanar EMI filter is verified compared with the conventional EMI filter. In a word, the design methodof planar EMI filter in this paper is meaningful in futher work.