Closed-Loop Design and Transient-Mode Control for a Series-Capacitor Buck Converter

This paper explores the large-signal and small-signal dynamics of a series-capacitor (SC) buck-type converter and introduces an optimal closed-loop control scheme to accommodate both the steady-state and transient modes. As opposed to a conventional buck converter, where time-optimal control is realized by a single on-off cycle, in the SC-buck topology, there is a need to distribute the switching phases to satisfy the charge balance of the SC. The new control method merges a voltage-mode controller for the steady-state operation and a nonlinear, state-plane based transient-mode controller for load transients. A detailed principle of the operation of the SC-buck converter is provided and explained through an average-behavioral model and state-plane analysis. The operation of the controller is experimentally verified on a 20 W 12 V-to-1 V converter, demonstrating voltage-mode control operation as well as minimum-deviation and time-optimal responses for load transients.