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This paper introduces a constant frequency mixed-signal single mode controller for multi-level dc-dc flying capacitor (ML-FC) converters that practically achieves minimum output voltage deviation transient response and, at the same time, maintains the stability of flying capacitor (FC) voltage. The output voltage loop of the controller is a simple modification of a conventional voltage-mode pulse-width modulated controller, where the polarity change of the output capacitor current is used to synchronize operation of two modified pulse width modulators. During load transients the controller achieves one cycle recovery of the inductor current, with near minimum output voltage deviation and without mode toggling occurring in many other fast recovery methods. To avoid runaway effect and stabilize the FC voltage, two methods, based on modification of the duty ratio and the capacitor current zero-crossing points, are introduced. Controllers effectiveness is verified with a 15 W, 12 V to 3.3 V, three-level FC buck prototype operating at 400 kHz switching frequency (800 kHz ripple frequency).

Recently, it has been shown that geometric programming, a class of convex optimization problems, can be used to optimize converters quickly and accurately. Some however, such at the seven-switch flying capacitor step-down converter (7SFC), designed to operate in three modes as three distinct topologies in order to maximize efficiency for the given operating point, cannot be modeled as geometric programs (GPs) on the converter level. This paper presents a hybrid optimization procedure, generalizable to other topologies, that uses a GP coupled to a fast circuit simulation to quickly optimize and design a 7SFC power stage IC for multiple operating points.

This paper introduces two methods for limiting the voltage stress during start-up across switches of multi-level flying capacitor (ML-FC) step-down dc-dc converters. For a general N-level converter, the presented methods reduce the voltage stress to (N-1) times lower value than that of a conventional buck, allowing lower voltage rating transistors with smaller specific on-resistances to be used. These methods require no active control of switches on initial start-up and rely on segmentation of the input filter capacitor or the utilization of flying capacitors as part of voltage dividers. The speed of these schemes is limited only by the size of the flying capacitor, parasitics in the conduction pathway, and the quality of the start-up diode and the low side switch body diodes. The methods have been verified on a 3-level buck 24V-to-5V, 20W prototype, with input voltage rise times of less than 1µs showing the effectiveness of the start-up circuits.

This paper introduces a practical single-mode quasi constant-frequency controller that provides practically minimum output voltage deviation during load transients. This control scheme eliminates the need for two separate controllers for steady-state and transient operation and the associated potential mode transition problems which are characteristic of numerous fast response solutions. The controller only relies on the detection of changes in the output capacitor current polarity and a simple PI compensator to provide fast transient response and tight voltage regulation. The effectiveness of this mixed-signal controller is demonstrated with a 600 kHz, 15 W single-phase buck converter based experimental prototype. During load transients, the inductor current is shown to be corrected within one switching cycle with near-minimum output voltage deviation.