A Multivariable Approach of Small-Signal and Large-Signal Design Techniques in a Current Mode Series Capacitor Buck Converter for Fast Recovery

Current-mode control (CMC) can achieve fast transient performance in a series capacitor buck (SCB) converter; however, a frequency-domain design approach is insufficient to fully explore its performance potential. Using a multivariable design framework, this paper presents small-signal and large-signal design techniques for an SCB converter under one-degree-of-freedom (1DOF) and two-degree-of-freedom (2DOF) CMC implementations. In the 1DOF case, only the primary phase inductor current is sensed, whereas both inductor currents are sensed in the 2DOF case. The 2DOF design technique achieves robust compensation with fast recovery and a well-damped response. Furthermore, it is demonstrated, for the first time in an SCB converter, that employing a single voltage controller with optimal gains allows the large-signal design technique to achieve near-time-optimal recovery within the series capacitor voltage constraint while maintaining stable periodic behavior. A comparative case study highlights the effectiveness and superiority of the proposed design techniques over existing linear and nonlinear control methods. A hardware prototype of an SCB converter is implemented with a nominal 12V to 1V input/output, 30A load current, and 1MHz switching frequency. Simulated and experimental results for load and reference step transients are provided for both the 1DOF and 2DOF CMC implementations.