Power factor correction and harmonic filtering for AC/DC converters

The mains current in an AC/DC converter contains periodic current pulses, due to the action of the rectifier and the output buffer capacitor. The high current peaks cause harmonic distortion of the supply current and low power factor. The harmonic currents can almost be eliminated completely by introducing active power factor correction. The design parameters and the output voltage characteristics of such a converter are investigated in general and verified by experimental results. This principle achieves a power factor near to unity, but is costly and voluminous. Investigations of the harmonic current limits defined in the standard IEC/EN61000-3-2 lead to a harmonic filtering approach, rather than power factor correction, as harmonics are not completely avoided, but reduced to the permitted level. This paper investigates the required filter inductance for compliance with the standard for class D equipment. A novel inductor, with a self-adapting filter characteristic, is introduced by the authors as the most cost effective and least voluminous solution. The new sloped air-gap (SAG) inductor is investigated and its performance in a power supply is verified by experimental results.