Theoretical study of the limitation of frequency excursion inhysteresis-mode boost-converter input stages in high power-factor AC-fedpower supplies

摘要

Switched-mode power supplies employ either variable switch dutyncycle or variable switching frequency (sometimes both) associated withnsome feedback circuitry to deliver the required output power. Intrinsicnto both techniques is the generation of electromagnetic interferencen(EMI) and the spectrum of this unwanted interference, conducted as wellnas radiated, is wide. The authors present a theoretical basis for thendesign of a variable-output, variable-power switched-mode converternoperating in the hysteresis mode at a fixed switch frequency and withnlimited current ripple; this makes the EMC filter circuits drasticallynsimpler and cheaper. Instead of adjusting the high-frequency switchingnparameters, the technique involves the real time, responsive modulationnof the hysteresis envelope. Suggestions for modulating the boostninductance are made. The former technique requires real time computerncontrol of the feedback system (not a significant problem at 50 Hznsupply frequency and today's digital processing speeds); the latterninvolves biasing the `operating point' of a nonlinear inductance with anpermanent magnet. The latter technique is not theoretically capable ofndelivering the required behaviour alone; this is demonstrated by a briefnqualitative experiment where the technique alone has enabled reductionnof switching frequency variation by a factor of two