The Basics of Magnetic Materials
This article highlights the basics of magnetic materials — the control, transfer, and conditioning of electric power.
Magnetic components form a key aspect in several power electronic devices. They are used for the control, transfer, and conditioning of electric power. Designers are always on the lookout for newer materials, topologies, and processes in order to improve performance.
In order to be able to implement the designs for inductors and transformers, it is extremely important to understand the nuances of the magnetic materials used and the technology required to accomplish the same. Read on to learn more about the basics of magnetic materials and their classification, core materials, and shapes.
Magnetic Materials and Classifications
The elements found in nature that are attracted by a magnet are referred to as magnetic in nature. Typically, they can be magnetized so that they behave as magnets themselves. Most permanent magnets that currently exist are known to be made of five types of material: ferrite, alnico, flexible rubber, and rare earth magnets like cobalt and neodymium . Surprisingly, each of these exhibits extremely different characteristics.
The design of magnetic components is proven to be a crucial part of the design of power electronic systems. In order to realize the designs of the magnetic components like inductors and transformers, it is essential to know the details of the magnetic materials and relevant technology to facilitate implementation. This is crucial in order to accomplish efficient and functional components.
In order to better understand the magnetic materials, let us look into the basic classification of the same. Magnetic materials are classified as magnetically hard or magnetically soft materials . Magnetically hard materials can be magnetized by a strong magnetic field and will remain magnetized indefinitely. In contrast, magnetically soft materials can be easily magnetized but the induced magnetism is temporary.
When placed in a magnetic field, the response of the material differs significantly based on the differences in the atomic structure. In simple terms, the magnetic behavior is determined by the number of unpaired electrons in each atom . Based on the unpaired electrons in a material that helps generate a net magnetic field, most of the materials can be classified as ferromagnetic, diamagnetic, or paramagnetic. A simple representation of the same is shown in Figure 1 .
Figure 1: Classification of magnetic materials
Ferromagnetic materials have a few unpaired electrons in their atoms and thus generate a weak net magnetic field as evidenced in iron, cobalt, and nickel. Diamagnetic materials are known to repel any externally applied magnetic field and do not generate their own magnetic field, like most of the elements in the periodic table. Paramagnetic materials have a very small susceptibility to the magnetic field. Most of the elements fall under this category as they are slightly attracted to a magnetic field but are generally termed to be non-magnetic in nature.
Classification of magnetic materials that are typically used for the fabrication of magnetic components can be made as ferromagnetic materials that have iron as the base, ferrimagnetic materials, or ferrites that have iron oxide as the base, and superparamagnetic materials made from powdered iron like permalloy. Both ferromagnetic and ferrimagnetic materials can be further categorized as hard or soft materials.
The choice of magnetic materials for a specific component depends on the intended functionality. The classification studied previously gives an insight into the variety of magnetic materials available for use.
Typically, magnetic materials used as cores should have high relative permeability . As a rule of thumb, hard materials are used for realizing permanent magnets whereas soft materials are employed for implementing inductors and transformers.
Based on the design of the power electronics component and the choice of core material, different core shapes can be employed . There are a multitude of core shapes readily available off the shelf and the choice of the same based on the application in hand is a tough design decision to make.
Figure 2: Sample magnetic cores of different core shapes
Table I discusses several core shape options while contrasting their advantages and shortcomings .
Table I: Comparison of different core shapes and their characteristics
|Winding Cost||Bobbin Cost||Assembly||Heat Dissipation||
|Pot Core||High||Low||Low||Simple||Poor||Excellent||Low leakage flux|
|E Core||Low||Low||Low||Simple||Excellent||Poor||High transmission power/unit volume|
Round central leg, facilitates easier winding
|High||Medium||Good||Poor||Preferred for high voltage applications|
|Toroids (Ring cores)||
|High||None||None||Good||Good||Preferred in current transformers|
L. Umanand, S.R. Bhat, “Design of Magnetic Components for Switched Mode Power Converters”, Wiley Eastern Limited.
Marian K. Kazimierczuk, “High-Frequency Magnetic Components”, John Wiley and Sons, Ltd.