Fast Transient State Feedback Digital Current Mode Control Design in Series Capacitor Buck Converters

Series capacitor buck (SCB) converters offer high efficiency and high power density even at a higher step-down ratio by combining a switch capacitor network in a conventional buck converter. However, the challenges remain in identifying suitable modeling, control and design methods in achieving fast transient performance while maintaining the desired voltage levels of the flying capacitors. In particular, structural changes in the system dimension due to the dynamic reconfiguration of the flying capacitors, make it difficult for modeling and analysis of digitally controlled SCB converters. By considering the full order state space model of the SCB, this digest proposes a unified discrete-time (DT) framework to analytically derive DT large-signal and small-signal models, which takes into account structural changes. The accuracy of the model is verified using SIMPLIS simulation. A digital current mode control (DCMC) technique is considered for a 12 V to 1 V operation, a novel state feedback based design approach is developed. For a 30 A load step, the proposed method results in
$25\ \mu\mathrm{s}$
settling time and nearly 75 mV under/ overshoot with a 25% improvement in the transient performance compared to that using a conventional output feedback design approach. The proposed method is scalable with the increasing phase count, which is further verified considering two modules of the SCB converter. Improved performance using the proposed design method is demonstrated using SIMPLIS simulation.