Toroidal core:

In the recent years, the toroidal core has become norm for winding radio frequency transformers. The toroid is donut shaped ring of the powdered iron or ferrite. The coils are wound around donut. The primary and secondary can be wound one over the other, or they can be wound over different parts of core. As with the other transformers, when windings are atop the other, there is much more inter winding capacitance when they are separate.

 
Figure-- Toroidal-core transformer.

The toroids confine practically all magnetic flux within core material. This permits toroidal coils and transformers to be placed close to other components without inductive interaction. Also, a toroidal coil or transformer is mounted directly on the metal chassis, and the operation will not be affected (assuming that the wire is insulated).

A toroidal core gives considerably more inductance in each turn, for same type of ferromagnetic material, than the solenoidal core. It is common to see toroidal coils or transformers which have inductances of 10 mH or even 100 mH.