Passive Device Modeling And Spectrum Calculation For Conducted Emi Model Of Switching Power Supply

Switching power supply is a kind of power supply which uses modern power electronic technology to control the time ratio of switching on and off to maintain stable output voltage.While switching power supply realizes high-frequency and miniaturization,it also brings many problems.One of the most important problems is that there is high-frequency switching action when the power supply works,which leads to the fast change of voltage and current,which will produce strong harmonic and peak interference in the switching power supply module.These interference signals will be coupled to other devices or systems through parasitic inductance and capacitance in the circuit,which will seriously affect their normal operation.Therefore,it is significant to establish a prediction model of the conducted EMI(electromagnetic interference)for analysis and suppression of CE(conducted emission)from switching power supply.In view of the current situation that the conducted EMI simulation model of the switching power supply is not perfect,this thesis focus on the model of transformer and EMI filter in switching power supply,and the FFT(fast Fourier transformation)considering the resolution bandwidth effect of the spectrum analyzer.This thesis studies the transformer model in the conducted EMI model of switching power supply.The transmission paths of DM(differential mode)interference and CM(common mode)interference on transformer are analyzed theoretically,and it is pointed out that the port impedance of transformer model represents the path impedance of DM interference,and the effective capacitance of CM between windings reflects the transmission path of CM interference.This thesis introduces and refines a model which can accurately describe the impedance of the transformer port in the conducted emission frequency band,analyses and calculates the effective capacitance of CM considering the actual potential distribution of the transformer winding,puts forward a CM/DM universal model for transformer in the conducted emission frequency range,gives the calculation method of all parameters in the model,and verifies the accuracy of the model by the impedance analyzer test.The study of equivalent circuit model of high-power three-phase common-mode EMI filter is carried out.Based on the traditional EMI filter modeling method,the corresponding CM model is established by considering the near-field coupling inside the filter.The key near-field coupling parameters affecting the filter performance are theoretically analyzed and simulated.The parasitic parameters of PCB are extracted by electromagnetic analysis software.Based on the measurement of impedance analyzer,the high-frequency models of three-phase CM choke,DM choke and capacitance are analyzed and established.The effectiveness of the model is verified by the common-mode insertion loss of EMI filter measured by vector network analyzer,and the effect of the electromagnetic coupling parameters on the common-mode insertion loss of the filter is discussed.The fast Fourier transform considering the resolution bandwidth effect of the spectrum analyzer is studied.The working principle of the spectrum analyzer is analyzed,and it is pointed out that in order to obtain the same results as the spectrum analyzer test when calculating conducted EMI spectrum based on FFT,the necessity of the resolution bandwidth should be considered.This thesis introduces a FFT algorithm considering the resolution bandwidth of the spectrum analyzer.Hanning window function is used to simulate the characteristics of IF(intermediate frequency)bandpass filter in the spectrum analyzer.Based on matlab,a periodic pulse signal is calculated considering IF bandwidth.The spectrum of the signal is measured by a spectrum analyzer and the calculation result is verified.