Analysis And Suppression Of Conducted Electromagnetic Interference In DC Power Convertor Systems Of Hybrid Electric Vehicles

Compared with traditional vehicles, hybrid electric vehicles have obvious EMC(Electromagnetic Compatibility) problems. The high-power electronic devices onvehicle will bring about strong EMI (Electromagnetic Interference), which willnegatively interference these devices and other systems on the same vehicle. In order tosolve the EMC problems of high-power electronics devices of hybrid electric vehicles,this dissertation mainly focuses on the EMC problem of high-power DC/DC converter,studying the EMI measurement methods of this system, mechanism of EMI production,EMI transmission routes, EMI predictive model and optimal schemes. The research ideaand approach of this dissertation can be applied to other on-vehicle systems with seriousEMC problems, thus offering theoretical and practical guide to EMC problems ofhybrid electric vehicles. The main research contents and results of this dissertation arelisted as follows.1) Consumable ferrite is employed to investigate EMI of DC/DC converter lines,so that EMI signals of multiple lines can be independently measured and the separateEMI levels of DC/DC converter lines can be obtained. Also, wavelet analysis is appliedto decompose EMI signals and to obtain corresponding wavelet coefficients. Based onwavelet coefficients on each decomposition scale, three characteristic parameters ofEMI signal in frequency-domain can be calculated, including cumulative energyparameter, mutation parameter and unbalance parameter.2) Based on the EMI production mechanism of DC/DC converter system, anequivalent circuit model producing EMI vibration wave is constructed. This model canrepresent main properties of EMI wave, and the mathematic equation of vibration wavecan be deduced. In the study of EMI transmission routes, a model that describes theEMI coupling route through the transformer is established on the basis of transformer’smain structure and the distribution of parasitic capacitance between the primary andsecondary coils. From this model, the mechanism of EMI coupling from primarywindings to secondary windings has been studied, and three approaches to reduce EMIcoupling effect have been qualitatively analyzed.3) A conducted EMI predictive model of DC/DC converter has been constructed.This model actually is consist of many sub-models, including cable model based ontheory of transmission lines,“object-field-circuit” model of transformer and iron-core inductor based on multiple-software simulation, and parasitic capacitance calculationmodel based on the finite element method of electrostatic field. All these sub-models areintegrated to form a system model, and conducted EMI level has been calculated by thisintegrated system model. The accuracy of this system EMI predictive model has beenverified via practical test comparison.4) According to the different EMI transmission paths of DC/DC converter system,the CM equivalent circuit and the DM equivalent circuit are separately constructed. It isconcluded that parasitic capacitance between power components and chassis is animportant factor to affect the CM EMI level of this system. Therefore, a new approachchanging parasitic capacitance has been proposed to reduce EMI. In simulation, thereare3optimal methods to reduce system CM EMI, namely increasing thickness ofthermal conductive adhesive between heat sink and heat-conducting plate of powercomponents, decreasing the permittivity of thermal conductive adhesive and decreasingthe geometry shapes of heat sink on power electronics modules. According the labcondition, the validity and effectiveness of the first two optimal methods have beenverified.5) The design process of EMI filter of DC/DC converter has been studied. Aftermeasuring interference source impedance of DC/DC converter system and EMI levelsof power lines, a high-frequency equivalent circuit of EMI filter has been constructed,and necessary parameters have been extracted. The effectiveness of filters is verified viasimulation of insertion loss. Meanwhile, the designed EMI filters are applied to DC/DCconverter system to verify their filtering performance, and the result shows thesedesigned filters can effectively suppress the conducted EMI level on power lines ofDC/DC converter system.