A transformer can include a flexible substrate having at least a first conductive layer and a dielectric layer. The transformer can further include an unbroken toroidal core of a magnetic material. The magnetic material can include material with a relative magnetic permeability greater than unity. The substrate can include a plurality of planar extensions arranged to provide respective windings encircling the core when the planar extensions are folded and attached back to another region of the substrate. Adjacent windings can be conductively isolated from each other. The flexible substrate can further include a second conductive layer separated from the first conductive layer by the dielectric layer. The first conductive layer and the second conductive layer can be coupled via a plurality of interconnects so that the respective windings are formed when the planar extensions are folded and attached back to the another region of the substrate.