Transformers In SMPS
Torroidal Transformer
Toroidal transformers are built around a ring-shaped core, which, depending on operating frequency, is made from a long strip of silicon steel or permalloy wound into a coil, powdered iron, or ferrite core. A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer's efficiency by reducing the core's reluctance. The closed ring shape eliminates air gaps inherent in the construction of an E-I core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimizes the length of wire needed, and also provides screening to minimize the core's magnetic field from generating electromagnetic interference.
Line Filters / Common Mode Chokes
Switch mode power supplies result in conducted interference because they generate radio interference voltage on the mains side. This can interfere with other equipment supplied with mains power. Line filters help to suppress the generated radio interference voltage. These can easily be designed from passive components such as current-compensated line chokes and X / Y capacitors.