Conductive EMI reduction in switched-mode power converters

Switched-mode power converters (SMPC) used in specific applications (i.e. commercial, automotive or military) have to be designed not only to meet electrical and safety requirements, but also electromagnetic compatibility (EMC) regulations (CISPR, VDE, EN, etc.). In this paper, problems of conductive electromagnetic interferences (EMI) are highlighted. Two different DC/DC converter topologies were compared based on EMI emission level: high side switch and half-bridge synchronous rectifier. Some parasitics and topology based EMI sources in device are presented. Frequency analyze with discrete Fourier transform (DFT) is also shown and compared with actual EMC measurement. Simulations are used to find out the sources of EMI and to test different precautions for EMC improvement. Practical solutions to EMC problems are also proposed. Levels of conductive EMI are estimated with DFT and verified with measurements based on EN55025 or CISPR25 standard.