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Investigation and reduction of line frequency common mode voltages at the outputs of AC/DC power converters

Although existing standards such as IEC62684 and EN 301 489-34 limit the line frequency common mode (CM) voltages at the output of AC/DC converters, the mechanism and characteristics of line frequency CM voltages at the outputs of AC/DC converters are unclear and the technique to reduce it is not well investigated and developed. In this paper, the line frequency CM voltages at the outputs of AC/DC converters are analyzed and a modeling technique is developed. Simulations and experiments were conducted to validate the developed technique. An AC/DC Flyback converter is taken as a study example. The paper also analyzed the effect of the start-up circuit at the primary side on the CM voltages. The relationship between leakage current and CM voltage is also discussed. The techniques to reduce the output line frequency CM voltages is also investigated and developed.