Dual-Loop Control for Synchronous-Conduction-Mode Tapped-Inductor Buck Converter

Tapped-inductor (TI) buck converter is capable of high step-down conversion with extended duty cycle over traditional buck converter. Additionally, its synchronous conduction mode (SCM) can completely eliminate turn-on losses, allowing high switching frequency operation and subsequent high power density. Nevertheless, the converter efficiency under SCM is refrained by circulating current and therefore a dual-loop (DL) control is proposed in this paper. Based on a time-domain model for zero-voltage-switching (ZVS) process, the DL control has a variable-frequency (VF) outer loop to minimize both ripple-current conduction loss and dead-time loss. While the VF loop moderately adjusts switching frequency, an inner voltage-mode-control (VMC) loop regulates output voltage. To verify the idea, a MHz-level, 15-W, DL-controlled TI buck prototype is built. Experimental results demonstrate the efficiency-boosting effect of the VF loop especially under light load. Finally, the derived plant model of TI buck converter under VMC is introduced in the paper along with the feedback design for output regulation.