Abstract: The average current controlled boost PFC converter may operate in full-continuous, part-discontinuous and full-discontinuous current modes in a half line cycle, and the conducted EMI spectra of the converter appear differences in these operation modes. Analyzing the worst conducted EMI spectra of the converter, the input voltage and load conditions for the different operation modes were discussed, and the waveform of the voltage across the drain and source of the power switch was analyzed. By analyzing the characteristic of the harmonic amplitudes of the voltage across the drain and source, it is revealed that, the worst case of the fundamental switching frequency harmonic of the conducted EMI appears in the full-continuous current mode, and the worst cases for the other harmonics appear under the light load conditions.