The Suppression Of Electromagnetic Noise Based On Multi-stage Planar EMI Filter

EMI filter plays an important role in restrain EMI. Compared with single-stage EMI filter,multi-stage EMI filter has the advantages of small component parameters, well high frequency performance, high performance to price ratio and so on. Therefore, based on the multi-stage planar EMI filter, the design method of multi-stage EMI filter, and three phase EMI filter are studied in order to improve the performance of EMI filter. In addition, the design method of impedance mismatching network under the condition that impedance of noise source and load were mismatched is analyzed and applied to the filter.In this paper, the serious electromagnetic interference(EMI) of switching power supply is analyzed in detail to get the relationship between EMI noise and power electronic device. The CM and DM equivalent circuit of switching power supply were proposed after investigation into the EMI noise generation. And the EMI noise can be precisely predicted in the whole conduction range, which was validated by the experimental results.For some special occasion like the EMI noise is serious, the single-stage EMI filter can not meet the demand of noise suppression, and the multi-stage EMI filter is an important choice to solve this trouble at this time. The two-stage EMI filter topology is taken as an example to introduce the method of using the voltage attenuation(G) instead of insertion loss(IL) to evaluate the effect of EMI filter.Affected by the impedance of noise source and load, the performance of EMI filter is often poor.In order to eliminate the adverse effect, the design method of an EMI filter network with impedance mismatching network is proposed in this paper. According to the value of the impedance of noise source and load, the impedance mismatching network will be defined into two kinds: L-type network and C-type network. Using the characteristics that different impedance proportion has different value of reflection loss, the optimized running state of EMI filter will be achieved. In addition, In order to reduce the volume of the EMI filter, the design method of the EMI filter network is applied to the planar EMI filter. Finally, an experiment is taken to verify the effectiveness and feasibility of the EMI filter network.According to the characteristics of load impedance in the power grid side and source impedance in the switching power supply side, the ideal equivalent circuits of common-mode and differential-mode filters are built. And the relationship between the insertion loss of three phase EMI filter and frequency is analyzed under the condition that impedance of noise source and load were different. Finally, the installation experience of three phase EMI filter is summarized.