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

Abstract

Switched-mode power supplies employ either variable switch duty cycle or variable switching frequency (sometimes both) associated with some feedback circuitry to deliver the required output power. Intrinsic to both techniques is the generation of electromagnetic interference (EMI) and the spectrum of this unwanted interference, conducted as well as radiated, is wide. The authors present a theoretical basis for the design of a variable-output, variable-power switched-mode converter operating in the hysteresis mode at a fixed switch frequency and with limited current ripple; this makes the EMC filter circuits drastically simpler and cheaper. Instead of adjusting the high-frequency switching parameters, the technique involves the real time, responsive modulation of the hysteresis envelope. Suggestions for modulating the boost inductance are made. The former technique requires real time computer control of the feedback system (not a significant problem at 50 Hz supply frequency and today's digital processing speeds); the latter involves biasing the ‘operating point’ of a nonlinear inductance with a permanent magnet. The latter technique is not theoretically capable of delivering the required behaviour alone; this is demonstrated by a brief qualitative experiment where the technique alone has enabled reduction of switching frequency variation by a factor of two.