Magnetic Core - Inductors and Transformers

Motivation

For realizing any practical embedded system use of Magnetics (Inductors and Transformers) are unavoidable as it is a integral part of SMPS required for control supply. In other words I can also say selecting or making Magnetics is more critical and time consuming part of complete SMPS realization. To design transformers and Inductor, we will have a short overview of magnetic cores. 

What is a Magnetic Cores?

Magnetic cores are basically a piece of magnetic material made in a particular shape. This magnetic material provide low reluctance path to magnetic flux. (Here is word reluctance for magnetic flux is similar to resistance for electric current. That is lesser the reluctance more is the flux. Air itself have very high reluctance.) 

These low reluctance magnetic cores provide low reluctance path hence guide the flux in particular path. Confining magnetic flux in particular path solve one two or both purposes. 

1. Coupling electrical coils wounded around the path i.e. on the core. By using this magnetically coupled link, the coils can exchange the electrical energy. This is a typical application of Transformers. Please refer the image blow: 

2. Storing controlled amount of energy in the magnetic flux. This is a typical application of Inductors. Inductor cores generally have gaps, either a intentionally given small air gap e.g. gaped ferrite cores (please see image below), or distributed gap like in iron dust powder cores. 

TYPES

Laminated Core

These are made up by staking thin lamination of Iron material. These cores can carry high magnetic flux density without getting saturated, but operating frequencies are quite low (as high core losses occurs at high frequency). Generally used on power frequencies (50 Hz in India)  Almost all power transformers are makeup of Laminated cores. 

Ferrite Cores 

These are made of Ferrite material in many difference shapes (EE, EI, RM, Toroids, etc). Below image shows ferrite cores of EE shape. 

These are having low saturation flux density typically 0.2 web/m^2. Because of having very low core losses, it can be operated at very high switching speeds i.e. hundreds of kHz. These are used as Inductor with a small gap in the magnetic path, or as a part of high frequency transformer with no gap. Some application like Flyback converter needs a transformer with primary winding working as a Inductor also. In such case also a gaped ferrite core is used. 

Powder Cores

Power cores are made of Iron powder dust and are mostly available in Toroidal shape. It have mixture of magnetic material and non magnetic materials. Therefore, there would be evenly distributed gap between magnetic grain. These core are used in the Inductor application generally required in DC to DC converters and SMPS application.