Abstract: A two-stage LC filter for switching-mode converters was designed using the "three-point frequency" design method. Theoretical analysis and experimental measurements of current harmonics were performed, and the results are in good agreement. The filter effectively suppresses electromagnetic interference (EMI). Finally, the theoretical calculations and experimental data are presented. Keywords : Electromagnetic interference, Filters, Harmonics analysis, Harmonics suppression. 0 Introduction Switching power supplies are the most widely used, but EMI is the most serious. The main sources of EMI in switching power supplies are: Firstly, in the rectification stage, due to the large capacitance of the filter capacitor, the rectifier tube only conducts near the peak value of the AC voltage. At this time, the capacitor flows through a large charging peak current, generating rich harmonic components [1]; Secondly, since the switching frequency of the DC/DC converter is between tens of kHz and hundreds of kHz, the switching tube current contains rich harmonic components, and it is the main source of electromagnetic interference in the switching power supply [2]. Since the EMI of the switching power supply is mainly conducted interference, the main means to suppress it is to use a filter [3]. The design of EMI filters is completely different from that of general signal filters, and special design methods must be adopted. This paper adopts a design method that is completely different from that of signal filters. A two-stage LC filter is designed using the "three-point frequency method", and the filter effect is satisfactory. For details, please click: Electromagnetic Interference Filter Design of Switching Power Supply