Overview

Standard off-the-shelf transformers and inductors were described in Chapter 11. This chapter will describe magnetic materials and techniques for constructing custom transformers and inductors. The primary design requirement is to minimise losses, but to do this we have to consider copper losses, core losses, magnetic saturation, size and construction. Since this book is about designing LED drivers, only the basics of magnetic materials will be given here. For more detail, the reader should consult specialist books on the subject.

An inductor can be made from a coil of wire, wound on a bobbin, and surrounded by a soft magnetic core material. By soft, the meaning is that magnetisation is easy and demagnetisation occurs when the magnetising force is removed. A hard magnetic core is like a permanent magnet; it has high remnance (magnetic field remaining once the magnetising force has been removed). Most magnetic materials have some remnance and the field strength required to return the magnetic flux to zero, to overcome this remnance, is called the coercivity . On a graph showing magnetic flux versus field strength, the curve follows an italic S shape. But when the magnetic field is reversed, the flux does not follow the same curve; it needs more field strength (more energy) to return to the same point and thus forms a fat S shape. The fatter the S, the higher the magnetising losses.

The core can be rectangular or cylindrical in cross-section with two halves that separate to allow the bobbin to be inserted.