Planar transformer advantages
The turns in a planar transformer are flat foil patterns on a printed circuit board, which limits the number of turns possible. At the same time, the greater magnetic cross-sectional area allows for fewer turns. And, the flat form of the magnetic core materials provides a larger surface area for power dissipation.
The printed circuit nature of the windings results in a high degree of consistency in spacing between turns and layers. As a result, interwinding capacitance is consistent and winding interleaving allows for reduced ac conduction losses. And, as with any other printed-circuit layout, creepage and clearance spacing is used to meet dielectric breakdown requirements. Taking all of this into account, planar transformers offer excellent efficiency and a high degree of reproducibility.
Traditional transformer advantages
Were it not for the demand for higher frequencies, it’s somewhat doubtful that planar transformers would be considered as an alternative to traditional wound-wire magnetics. For all of their apparent benefits, and even in those high-frequency applications, traditional wound-wire transformers still offer a number of important benefits. Planar magnetics take up a great deal more circuit-board footprint than traditional transformers. So, unless power dissipation and/or headroom are major design considerations, then designers will typically go with a standard transformer.