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This paper investigates the use of switching frequency modulation in the reduction of the differential-mode conducted electromagnetic interference (EMI) of the offline boost power-factor correction converter. In particular, the EMI benefits obtained with modulation frequencies at and in excess of twice the line frequency are considered, incorporating the influence of average and quasi-peak (QP) detectors into the analysis and measurements. It is concluded that there is no significant change in the QP measurement unless the modulation frequency exceeds half the resolution bandwidth; however, up to 8-dB reduction in the average measurement, with no change in the QP measurement, can be obtained with modulation at twice the line frequency, for a switching frequency deviation of 20%. A digital modulation algorithm was devised and implemented using a digital-signal-processor-controlled converter. The reduction in the average detector measurement obtained for the digital modulation was comparable to that predicted for the linear modulation, but it delivers a more well-defined and predictable conducted EMI spectrum.