Modulation Scheme for an Effective Increase in the Number of Levels of DC-DC Multi-Level Flying Capacitor Converters

Abstract

This paper introduces a novel modulation scheme for dc-dc multi-level flying capacitor buck converters that effectively increases the number of converter levels. It is shown that, in general, an N-level flying capacitor converter can be split simultaneously into an $\frac{{N + 2}}{2}$ and an $\frac{{N + 3}}{2}$ level flying capacitor converter. This modulation scheme creates unequal differences between the flying capacitor voltages which can be used to reduce the voltage swing at the switching node and thus the size of the inductor. The voltage swing becomes equivalent to an $\frac{{{N^2} + 3}}{4}$ level converter for some conversion ratios. Additionally, this modulation scheme has passive flying capacitor voltage balancing, simplifying controller design. A 5-level flying capacitor converter was modulated to create a 3,4-level converter with the effective voltage swing of a 7-level converter. The functionality of this modulation scheme was confirmed with a 24V-to-9V, 45W 5-level buck converter confirming a 35% reduction in the inductor ripple and, thus, allowing for the equivalent inductor reduction.