Magnetic devices, and associated methods of manufacture, using flex circuits. Conductive flex circuit traces, or combinations of such traces with conductive printed circuit board or other substrate traces, form windings around toroidal ferromagnetic cores. Bending the flex circuit into a partial loop or a full loop forms partial or full windings respectively. Bonding or flow soldering electrically connects the windings together and to a printed circuit board or other substrate. The methods yield transformers with high conversion efficiency, are compatible with conventional printed circuit boards and readily available high-volume assembly equipment, and avoid the higher cost of manually made windings.