The effect of duty cycle on SMPS common mode emissions: theory and experiment

The theoretical time- and frequency-domain waveforms of common-mode conducted emissions are derived for a switched mode power supply (SMPS) of arbitrary duty cycle. The dependence of common-mode emissions on duty cycle and switching frequency is derived, modeled, and verified through experimental data. A special fly-back converter was built to isolate the switching transistor heat-sink parasitic capacitance from other potential sources. Data on the common-mode emissions from this source were taken utilizing the line impedance stabilization network measurement method. Time-domain and frequency-domain data were taken for different supply operation parameters (states). The varying common-mode emissions of the supply were compared to that predicted by an earlier model. The magnitudes of the common-mode emissions are shown to be independent of switching transistor risetime. Using Fourier analysis of the common-mode waveform, the model predicts the variation of emissions with duty cycle value.<>