Toroidal transformers are passive electronic components, consisting of a circular ring magnetic core of high magnetic permeability material such as iron laminations, iron powder, or ferrite, around which wire is wound.
Although there is a fundamental difference between the function of an inductor and transformer, they both rely on electromagnetic induction for their operation and they are both constructed from similar materials.
Because of their superior electrical performance, the use of toroidal shape cores has increased greatly. The advantage of the toroidal shape is that due to its symmetry the amount of magnetic flux that escapes outside the core (leakage flux) is low, therefore it is more efficient and thus radiates less electromagnetic interference (EMI) which must be reduced to the levels defined by international standards to ensure that one equipment is able to work with other equipments.
Toroidal inductors and transformers are used in a wide range of electronic circuits: suppressors, power supplies, and amplifiers, which in turn are used in the vast majority of electrical equipments: TVs, radios, computers, mobile phones, HiFi.