Planar Ferrite Cores for Switching Power Supplies TDK Planar Ferrite Cores for Switching Power Supplies are available in a wide range of sizes from 10mm to 25mm and have a -30°C to +105°C temperature range. The Planar series features a low-profile shape suited to PCB mounting and comes in a range of various shapes. TDK Planar Ferrite Cores are ideal for low-profile DC/DC converters, transformers, and coils for use in planers.