Abstract: The switching frequency of the Boost PFC converter in critical conduction mode varies in a line cycle.As a result,the conducted EMI spectra is difficult to be diagnosed,which makes the design on EMI filter complicated.By analyzing the harmonic spectrum of the voltage across the drain and source of the power switch,effect of the varied switching frequency on the peak,quasi-peak and average conducted EMI spectra of the CRM boost PFC converter was revealed.It is indicated that the fundamental harmonic dominates the low frequency noise of the conducted EMI spectrum.Based on the relationship between the duty cycle and the fundamental harmonic amplitude of the voltage across the power switch,the input voltage and load condition for the worst case of the conducted EMI spectrum of the CRM boost PFC converter was derived.It is demonstrated that the worst spectrum in the condition is identical.The theoretical analysis was verified by the experiment results.