Control conditions to improve conducted EMI by switching frequency modulation of basic discontinuous PWM preregulators

In this paper the authors analyze the operation conditions to control basic discontinuous AC-DC preregulators with variable switching frequency. The correct use of variable switching frequency control can lead to a reduction of conducted EMI generated. The basic PWM converters (buck, boost, buck-boost, sepic, Cuk and zeta) are studied. The authors achieve results which allow them to operate the converters under the condition proposed maintaining a very small low-order harmonic-current content. Some simulation examples using a buck-boost as DCM-PFP show the amplitude reduction of the high order harmonic content. Experimental results using a sepic converter show the feasibility of the approach