A Soft-Switched High-Conversion-Ratio Quasi-Resonant Flying Capacitor DC-DC Converter

This article presents a new flying capacitor-based quasi-resonant DC-DC converter topology for high step-down, low-power applications, ranging from fractions of a watt to tens of watts. The converter has a structure similar to that of a conventional three-level flying capacitor buck, but it operates in a fundamentally different manner, offering a favorable trade-off for many targeted applications. By allowing a small flying capacitor to fully charge and discharge between 0 and V-in, using a unique switching scheme, switching losses are drastically reduced at the expense of requiring transistors rated for the full input voltage. The converter can operate in both discontinuous conduction mode (DCM) and continuous conduction mode (CCM). In DCM, soft switching is achieved on all edges, independent of operating conditions, while in CCM, soft switching occurs on most edges. The capacitor charging process causes the converter to draw a fixed amount of energy per switching cycle, resulting in a unified small-signal model and a transfer function with a Q(o) factor limited to 1/root 2, simplifying voltage-mode compensator design compared to standard buck solutions. The effectiveness of the introduced solution is verified through simulations and experimental prototypes, processing up to 50 W of power with varying inductor values. These prototypes were designed to explore trade-offs for different applications while maintaining a low converter volume and high power processing efficiency. Experimental results for a 48 V-to-1 V/4 A converter demonstrate peak efficiencies of 81.74% at 3 W, 87.36% for 48 V-to-2 V/10 A at 6.4 W, 89.87% for 48 V-to-3.3 V/10 A at 10.6 W, and 91.33% for 48 V-to-5 V/10 A at 16 W.