Common mode RFI of a HF power converter: phenomenon, its modelling and its measurement

The search for higher switching frequencies in power electronics allows magnetic components to be minimized but also enhances the spectrum of electromagnetic interference engendered by interaction of the power structure and its active components; hence the need for investigation in the interference phenomena. The authors deal with the modelling and the measurement of one of the phenomena occuring, the common mode current. Its origin and its propagation in a chopper structure are highlighted, and a simple equivalent circuit is derived enabling noise level prediction. The exciting signal is the voltage over the switch. Its Fourier transform applied to the transfer function yields the noise signal time domain waveform and the corresponding frequency spectrum level. These theoretical results are compared to power circuit simulation and to time and frequency domain waveforms monitored on an experimental set-up. Further investigations could include more accurate models of the components and be extended to other structures