Stability Properties of the 3-Level Flying Capacitor Buck Converter Under Peak or Valley Current Programmed Control

Abstract

This paper investigates basic stability properties of the three-level flying-capacitor Buck converter when operated under current-mode control (CMC). The proposed analysis is developed for both peak CMC (P-CMC) and valley CMC (V-CMC), and addresses both the static instability of the inductor current and the flying capacitor (FC) voltage runaway phenomenon. Conditions for avoiding current subharmonic oscillations are derived first, along with the expressions of the minimal compensation ramp, which guarantee current stability throughout the entire operating range. As for the stability of the FC voltage, P-CMC results to be inherently unstable unless the converter operates with a relatively large peak-to-peak inductor-current ripple, a feature not yet recognized in the literature. However, V-CMC results to be inherently stable once the static instability of the current is eliminated with an external compensating ramp—a property that has been highlighted in the literature but never formally proven. The proposed analysis is verified both using a simulation model and experimentally using a 3.3-V, 500-mA, 500-kHz prototype.