Toroids are the core type geometry of choice for optimizing performance. A toroid of round cross-section offers better performance than one of rectangular cross-section, but for practical and economic reasons toroids of rectangular cross-section are much more prevalent. The symmetry of their circular geometry minimizes the amount of external magnetic flux produced. Consequently they produce much lower amounts of unwanted electromagnetic interference. Unlike other core types, turns can be wound along the entire length of the core thereby allowing more turns per layer. The mean turn length will be shorter than that of other core types of equal power capability hence lower winding resistance and lower winding losses. Compared against other core types, a toroidal coil has a lot of surface area from which it can dissipate heat hence it cools much better than other core types. Cooler windings result in higher efficiency and may allow more utilization of the cores capability.
Because of its circular nature, the magnetic path of a toroid is an unbroken continuous path unless intentionally broken. There is no air gap in the magnetic path (unless intentionally added) hence optimal use can be made of high permeability materials. Ferrite toroids and stacks of stamped lamination rings are examples of this. A tape wound core is the next closest example. The flux in each layer wound on the core can make a full revolution and then continues onto the next layer, but the magnetic flux must eventually pass from layer to layer encountering an air gap between layers in the process. The gap occurs because the tape strip is not perfectly flat. The layer to layer passage is distributed the surface area of an entire revolution, hence the magnetic reluctance of the gap becomes very small and usually can be ignored. A tape wound core can utilize the advantage of grain oriented materials (such as grain oriented silicon steel) while stamped rings cannot.
In some applications it is desirable to have an air gap in the core path. For mechanical reasons, it is cumbersome to add air gap to a toroid. Large air gaps produce undesirable flux fringing. Powdered cores combined the magnetic material with a non-magnetic binder material. Magnetically, the binding material acts like an air gap, but this gap is distributed throughout the entire core. Because of this distribution there are no flux fringing effects. The binder(s) also reduce eddy currents.
Toroids are manufactured in practically all soft magnetic materials. Toroid Cores can be coated with insulation to provide electrical isolation between the core and the winding(s). Some toroid cores are boxed to provide isolation. Some toroid cores are boxed because the core material is sensitive to stresses produced by the winding processes.