Prediction And Suppression Technology Of Conducted Electromagnetic Noise In Isolated DC/DC Converter

With the development of high-frequency,high-power density,and high-precision trends,the importance of electromagnetic interference(EMI)in power converters has become increasingly prominent.Compared with the non-isolated type,isolated DC / DC converters need to establish impedance high-frequency modeling and differential mode(DM)noise models and common mode(CM)noise models for high-frequency transformers during EMI noise analysis,which increases the complexity of the problem.There are research difficulties in the EMI problem of isolated DC / DC converters.This article takes the Flyback converter as the research object,takes the multi-port output Flyback converter CM conduction noise mechanism and optimizes the high-frequency transformer CM noise suppression method as the research center,launched a series of research,mainly including the following parts:1.Proposing a high-frequency modeling method for discrete components based on the principle of nonlinear fitting.Perform impedance analysis of the resistors and capacitors in the Flyback converter system by measurement,use nonlinear fitting tools to model high-frequency discrete components,accurately extract their parasitic parameters;The high frequency model was analyzed.2.Making the model of high-frequency transformers and optimizing their high-frequency models.Analyze and compare various equivalent circuits based on "concentrated parameters" for high-frequency transformers.Based on the traditional equivalent circuit of the transformer,the high-frequency modeling of the transformer is carried out through the finite element simulation method,and the simulation method is optimized to raise up the simulation speed without accuracy loss.Through the impedance measurement method,the impedance characteristics of each port of the high-frequency transformer are analyzed,and a high-frequency model of the transformer based on physical characteristics is proposed,and the high-frequency transformer is modeled by the model.Compared with the traditional model,the optimized transformer high-frequency model achieves high accuracy in a wider frequency band.3.Analyzing the CM equivalent circuit of the Flyback converter and predicting the CM noise.This paper analyzes the generation and transmission mechanism of CM conducted noise of the Flyback converter,and simplifies the CM noise source and noise model based on the substitution theorem and superposition theorem.Using Saber simulation software,the CM conduction noise simulation circuit of the Flyback converter is established and the CM noise spectrum characteristics are obtained by simulation.The correctness is verified through experiments,and the "consistency" relationship between the time domain noise source signal and the CM spectrum characteristics is obtained.Using simulation circuit,the simulation determines the effectiveness and applicable range of changing the main capacitive parasitic parameters of the CM equivalent loop to suppress the CM noise of the system.4.Proposing a method of suppressing CM noise by adding balance capacitors,shielding and grounding.The capacitive effect of the prototype high-frequency transformer is analyzed.Based on the assumption of the linear distribution of parasitic parameters and winding voltage,the winding turn-layer voltage distribution and the transformer CM noise current formula are derived.Put forward three kinds of CM noise suppression measures including adding balanced capacitor,copper foil shielding,and magnetic core grounding.Based on the equivalent capacitance elimination theory,the balanced capacitance method is analyzed.Based on the CM noise loop analysis theory,the three suppression methods are analyzed,and the three suppressions are verified through experiments.Comparing the experimental results of the three suppression measures,the suppression effects and limitations in different frequency bands are obtained.