Transformers are to transfer energy or signals in an electrically isolated way. While power transformers ought to have a low loss core material, signal transformation may work good enough with cheaper material. Mostly ferrite is being used and there are at least two windings. More secondary windings enable to trigger more switching transistors like in a bridge circuit. 

In order to meet electrical strength insulation layers or multiple isolated wires is being used.

A special variation is flyback chokes, a combination of transformer and storage choke. In this case metal or iron powder cores may be adequate, as well. However, ferrite is mostly used. 

Advantages of toriods vs. E-core types and its relatives are

  • small dimensions
  • very good heat distribution and dissipation
  • good coupling

Technical specifications, materials and areas of application:

Ferrite (X)

  • lowest cost material
  • application with little demand

ferrite (V)

  • low cost
  • applications with high demand
  • low loss
  • higher temperature possible

tape wound core (BB)

  • highest saturation flux density
  • limited area of frequency (50Hz, audio)

amorphous / nanocristalline tape wound cores

  • high saturation flux density
  • high linearity
  • limited area of frequency (50Hz, audio)

metal powder (HF)

  • flyback

Available versions:

  • Standard versions for toroids
  • E-core types and their relatives (ETD, RM, etc.)