Synchronized zero-crossing-based self-tuning capacitor time-constant estimator for low-power digitally controlled DC-DC converters

Abstract

A hardware efficient method and system for estimating the time constant and measuring the current of the output capacitors in digitally controlled switch-mode power supplies is introduced. The estimator emulates the equivalent RC circuit of the output capacitor with a much smaller version, placed in parallel, and adjusts its own resistance until the two circuits have the same time constant. The adjustment is based on a novel zero voltage crossing detection method and on synchronization with the digital pulse-width modulator operation. The effectiveness of the new estimator is verified with a 5 V to 1 V/5 A, 500-kHz buck converter prototype utilizing an optimal response controller. The experimental results show accuracy within a few tens of nanoseconds in the detection of capacitor zero-current crossing points, corresponding to a smaller than a 1.5% error in the time constant estimation, and, compared to an imperfectly tuned system, about 40% smaller voltage deviation during transients. © 1986-2012 IEEE.