RESEARCH ON CONDUCTED EMI OF AN OFF-LINE PWM SWITCHED MODE POWER SUPPLY

In this paper, the conducted EMI characteristic of an off-line PWM power supply is analyzed in detail. Considering the high frequency switches as main EMI sources, a group of simple equations can be deduced, which describe the typical relationship among EMI emissions, the noise sources and the coupling paths. Based on the equations, a novel measurement is proposed to characterize the conducted EMI coupling paths. According to the equations and the characteristics of the coupling paths, the different roles of dv/dt and di/dt in high frequency switches on conducted EMI emissions can be analyzed concretely. An equivalent common mode conducted EMI model is built according to the basic coupling paths and the main stray parameters of the converter. This model avoids complicated stray parameters extraction process and is useful for the design of EMI filter.