Toroids have the best possible shape from the magnetic point of view. The flux path is completely closed so the capabilities of the ferrite are fully exploited. Especially for high permeability ferrites the effect of even a minor airgap in the magnetic circuit can spoil up to 50% of the effective permeability. A further advantage is the very low leakage field which makes it a suitable shape for power and pulse transformers. Ring cores are mainly used for pulse- and wide band transformers and interference suppression coils but also in special power supplies.