Analysis And Suppression Of Conducted EMI In Power Electronic Equipment

Fast switching actions makes power electronic equipment a significant source of EMI to nearby devices. This thesis focuses on EMC design of power electronic equipment, analysis of conducted EMI generation and propagation mechanisms, suppression of common-mode (CM) EMI by using balanced circuits and optimized driving signals. In order to ensure good EMC performance of power electronic equipment, EMC design should be carried out in the early stage of development, since this brings lowest cost. Three factors of generating EMI in power electronic equipment are analyzed, then the techniques which suppress EMI are given, such as grounding,isolation, optimal physical layout and wiring, shielding and filtering. The origin of conducted EMI in switching power supplies is discussed. The generating mechanisms of differential-mode (DM) EMI, CM EMI, non-intrinsic DM EMI in flyback converters are analyzed, then the equivalent DM and CM EMI models are built according to the basic coupling paths. Because conventional switching converters have been usually using unbalanced circuit topologies, the CM current flows through the parasitic capacitance of the switch to ground in the power circuit and generates the CM EMI. A balanced switching converter circuit, which is an effective way to reduce the CM EMI from its source, is discussed. The conducted EMI generation and propagation mechanisms of full-bridge converters that are widely used in the industry are analyzed. The DM EMI is related to the switching current and the CM EMI is caused by the CM current flowing through the parasitic capacitance of the device arms to the ground. When the bipolar PWM modulation strategy is adopted, the CM currents generated by the two arms should cancel each other. Due to different transmission delays of the drive pulses for the diagonal switches and high frequency oscillation of the CM current, the actual CM currents cannot cancel each other. The propagation path of CM EMI is analyzed, then it is concluded that timing is critical for the cancellation of the CM current. In order to suppress CM EMI from its source, this paper presents a method of optimizing drive pulses through pre-shifting the pulse edges. Experimental results show that this method can effective suppress CM EMI without any additional hardware. The effects of parasitic parameters on the CM current spectrum are also analyzed. To lessen high frequency CM EMI, suppression of surge voltage across the switches is studied.