Planar cores are low profile cores. The core material is almost exclusively ferrite material. The core design is intended for use with windings etched on a printed circuit board, thereby eliminating the winding of a separate coil. Etching of the windings puts a limit on the number of available turns hence the operating frequency must be high to avoid core saturation. If the turns requirement is sufficiently high, some designers might cement a thin coil to the board under the core. Since the typical planar core user does not need a coil, Butler Winding has little experience with planar cores.