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Coupled-inductor buck converters are more effective in high step-down applications compared to conventional buck converters given the same circuit complexity. A coupled-inductor buck converter can adopt synchronous conduction mode to fully mitigate turn-on loss and operate at high frequency. However, this typically induces large circulating currents and harms the converter's light-load efficiency. To alleviate these issues, this paper analyzes the mechanisms behind synchronous conduction mode and then proposes a dual-loop control method solution. The variable-frequency outer loop of the proposed control minimizes conduction and deadtime loss, while the voltage-mode-controlled inner loop provides the necessary output voltage regulation. Experiments are conducted with a compact prototype converter using switching frequencies higher than 2 MHz, and the prototype is verified under load transients to validate both control loops. The results show that the light-load efficiency can be boosted by more than 8%.