EMI Suppression with Switching Frequency Modulated dc-dc Converter

Usually a large amount of conducted electromagnetic interference (EMI) of a switched-mode power supply is generated by fast acting switches with sharp switching rise-and fall-times. Certain dc-dc converter topologies have peculiar input current and switching voltage wave shapes containing an infinite number of switching frequency harmonic components. However only the dc component of the input current contributes towards the average real power transfer. The remaining power contained in the harmonics is considered as conducted EMI interference and must be prevented from emanating into the supply. Furthermore, high frequency components present in the switching voltage can generate leakage current flow through converter parasitic elements thus aggravating the EMI interference. A great deal of research has been done to mitigate such conducted interference generated in de-de converters. Certain approaches in mitigating conducted EMI interference have been reported recently where the interference is suppressed at its origin. These have been carried our by selectively eliminating certain harmonics in accordance with existing EMI standards by programmed pulse width modulation or by suppressing the peak energy of a given switching frequency while redistributing the energy to additional harmonics by spread-spectrum methods. However co obtain a perfect suppression of conducted EMI, it is desirable chat these novel methods must be applied along with traditional techniques such as EMI filtering.