Quasi-active power factor correction with a variable inductive filter:: Theory, design and practice

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. Introducing active power factor correction in the form of a boost pre-regulator can reduce the level of harmonics. Passive power factor correction in the form of an input filter inductor is less expensive but the component is bulky because it must be sized to handle the full power range of the supply. This paper investigates the use of variable inductance (the inductance varies with current), which provides adequate harmonic reduction. Three types of inductors are investigated: an inductor with a fixed air-gap operating with a saturated core, a swinging inductor which has a stepped gap and a novel inductor construction with a sloped air-gap (SAG). Results are presented for a 200W power supply and it is shown that the SAG inductor has the best performance in terms of harmonic response and size.