Editing Transformer (section)

====Solid cores====
Powdered iron cores are used in circuits such as switch-mode power supplies that operate above mains frequencies and up to a few tens of kilohertz. These materials combine high magnetic permeability with high bulk electrical [[resistivity]]. For frequencies extending beyond the [[Very high frequency|VHF band]], cores made from non-conductive magnetic [[ceramic]] materials called [[ferrite (magnet)|ferrites]] are common.<ref name="McLyman2004-3-9"/> Some radio-frequency transformers also have movable cores (sometimes called 'slugs') which allow adjustment of the [[coupling coefficient (inductors)|coupling coefficient]] (and [[bandwidth (signal processing)|bandwidth]]) of tuned radio-frequency circuits.