Modulation Improvements for High-Phase-Count Series-Capacitor Buck Converters

This article presents three modulation improvements for the series-capacitor buck (SCB) converter and its topological derivatives. The first consists of various phase activation sequences (PHACTSs) which raise the maximum input-to-output voltage conversion ratio of an
N
-inductor,
N
-phase SCB converter beyond the traditional limit of 1/
N
²
, without incurring any additional voltage stress to the switches. Phase counts up to 16 are analyzed with conversion ratios increasing by a factor of up to 7. Due to the inherent link between the converter's maximum attainable output voltage and maximum output current slew rate, these PHACTSs offer a significant improvement to the load-voltage transient response. Utilizing the flying capacitors that link adjacent inductors, a second modulation technique is introduced that effectively increases the digital pulse-width-modulator's (DPWM) output-voltage resolution, by a factor of
N
, by employing a novel method of minimum duty increments (MDIs). Despite the commonly-held assumption of automatic steady-state inductor-current-balancing present in an
N
-inductor SCB, large-signal modelling reveals that slight current imbalances inevitably arise, even in lossless configurations, with three or more output inductors. After elucidating its origin, this article introduces a third modulation technique that can reduce these inductor current imbalances through a particular implementation of MDI. A discrete prototype of an 11-inductor, 48 V-to-1.0 V, 275 A-load, SCB converter was fabricated to experimentally demonstrate and validate the simulated results of the increase in both the output voltage ceiling and DPWM resolution, as well as to evaluate the MDI-DPWM output-voltage linearity. Finally, the maintenance of both inductor current balancing and low switch-voltage-stress is experimentally substantiated when using MDI.