Isolated AC–DC Converters for High-Density USB-PD Adapters: A Comparative Review of Topologies and Future Trends

The transition of the USB Power Delivery (USB-PD) standard from fixed 5–20 V profiles to the dynamic extended power range (EPR) of up to 48 V and 240 W has redefined the constraints of isolated AC–DC conversion for high-density adapters and chargers. Delivering high efficiency simultaneously at low (5 V) and high (28–48 V) outputs, while maximising volumetric power density and meeting stringent no-load and average-efficiency regulations, has become the central design challenge. This review provides a structured, comparative analysis of the isolated topologies competing in the sub-240 W adapter space: the quasi-resonant and active-clamp flyback converters, the two-transistor flyback, the resonant hybrid (asymmetrical half-bridge) flyback, and conventional and reconfigurable LLC resonant converters. Each is assessed against a common set of metrics (soft-switching capability, output-voltage-range flexibility, control complexity, component count, and achievable power density), supported by a quantitative meta-analysis of representative published prototypes. The enabling roles of wide-bandgap gallium nitride (GaN) devices, planar magnetics, and advanced digital control are examined. The analysis delineates the region of the power and output-voltage space in which each topology is optimal and proposes a common figure of merit for benchmarking future high-density adapters.