Toroidal cores: Widely used in potential transformers, current transformers, voltage transformers, and various audio equipments; Due to its particular structure, it has better performance than EI cores;
The graphic on right for your reference.
Our actual manufacturing scope as below:
ID (Inner diameter) 5mm - 1200mm
OD(out diameter) 6mm - 1800mm
We could also customize cores including the air gap according to customers requirements. 
