Common-mode Conducted Electromagnetic Interference Modeling And Prediction For Isolated Power Converters

With the international and national electromagnetic compatibility (EMC) regulations have become more strictly, modeling and prediction of electromagnetic interference (EMI) issues has been an important research topic. This dissertation concentrates on the development of modeling methodology to predict common-mode (CM) conducted EMI in isolated power converters.This dissertation presents a general modeling approach, called multi-channel "noise source coupling path" model, to investigate the conducted EMI couplings in power converters. This model is based on frequency domain modeling technique, and is independent of converter structure. In this model, the noise source is characterized as a one-port network, and the coupling path is characterized as a two-ports network. With the extracted spectrum of noise sources and transfer function (equivalent impedance) of coupling paths, the interference spectrum of the converter can be evaluated directly in frequency domain.Lack of high frequency model of transformer CM noise coupling path is the main reason for the inaccuracy of CM noise predicting at higher frequencies in isolated power converters. To solve this problem, this dissertation proposes the distribution model of transformer CM noise coupling path. In this model, only the parasitic parameters of the transformer related to CM noise coupling paths need to be extracted. So, this modeling approach is simple and the parameters can be easily extracted. The high frequency characteristics of the transformer CM noise coupling path can be well described by this model, because both of the distributed parameters and the high frequency losses in the core and windings are included. Because the transfer function (equivalent impedance) of complex noise coupling path is usually hard to be extracted directly, this work presents the transfer impedance simulation method. This simulation method can describe the noise coupling path and the operation of the converter completely, it is general and can be applied to all the noise coupling paths. With the combination of the transformer CM noise coupling path distribution model and the transfer impedance simulation method, the equivalent impedance of the CM noise coupling path can be extracted, and the CM noise caused by isolated power converter can be well predicted, which is validated by measured results.In order to fully describe the high frequency characteristics of the transformer and establish general transformer high frequency model, this dissertation works on traditional transformer with spiral windings, and proposes the transformer multiconductor transmission line (MTL) model. This transformer MTL model can completely describe the electromagnetic fields in the transformer, and can be used in different applications for all transformer structures with spiral windings. This work also applies the transformer MTL model to CM noise coupling path modeling. And the parasitic parameters matrices of the transformer are extracted by finite element method (FEM) based tool-Ansoft Maxwell2D. This modeling approach not only can predict the CM noise caused by the converter, but also can relate the geometry of transformer to both of the transformer CM noise coupling path impedance and the CM noise generated by the converter. Experimental results indicate that, with the transformer MTL model, the CM noise coupling path impedance of the transformer can be well extracted, and the CM noise caused by the converter can be well predicted in the whole conducted EMI frequency range (150kHz-30MHz).The EMC design and efficiency optimation of power converters usually can’t be satisfied altogether. This dissertation works on the RCD snubber in Flyback converter, the transient operation procedures of RCD snubber are analyzed, and the transient operation models are established. The influences of RCD snubber designing on the spectrum of noise source, EMI performance of the converter, voltage spikes of the transistor, and efficiency of the converter are all revealed. And a novel design procedure for RCD snubber is derived to make an optimal tradeoff between EMC performance and efficiency. Experimental results verified the theoretical analysis and optimal design process.