Study On Conducted Emi Prediction And Suppression Of A DC/DC Semi-isolated Power Supply

A switched mode power supply(SMPS)has the advantage of high efficiency,high power density and low volume,so it is commonly used in many kinds of fields.However,SMPS is easier to produce electromagnetic interference(EMI)with a very high magnitude for its high switching frequency of power semiconductor components.EMI may exert huge influences on the equipment's communication and contaminate environment.Electronic equipment also should have the capacity of resisting EMI,so it is necessary to ensure that SMPS is tested to identify if it matches the EMI standards.In this study,a new semi-isolated DC/DC SMPS is designed firstly.Based on the principle of its work state and technical requirements,parameters of main circuit components are designed.After calculating circuit's transfer function,PID compensator in controlling circuit is proposed and designed to satisfy the margin request of system stability.Then a prototype with 110 kHz frequency is made.The measured waveforms verify function realization of this SMPS.Since components in high frequency perform unique characteristics,it is necessary to analyze their impedance in the range of conducted frequency.Models of passive components in high frequency are built up with impedance analyzer.Models of active components are established by software tool.The transformer's modeling methods combine calculation and measurement.A LCR meter helps extract parasite parameters of PCB.Total circuit simulation model is established with sub-models all above.According to conducted EMI three factors,noise sources are identified.Then common mode(CM)and differential mode(DM)pathways are analyzed respectively using the superposition principle.Measured CM and DM noise frequency spectrum are compared with simulation results.Both of them match with each other very well.This proves the accuracy of the EMI pathway model.A traditional EMI filter is designed to suppress EMI.Furthermore a new type of filter is proposed based on special structure of the circuit.Two kinds of EMI filters are tested and achieve good suppression effect.