Modern electronic equipment has changed a lot since the widespread use of semi-conductor devices. One obvious result has been the reduction in size of the equipment. Because of the nature of solid-state circuitry, which tends to use lower impedances and lower voltages, different methods of coupling and matching between stages have come into widespread use in rf circuits. Airwound coils have largely been replaced with small ferromagnetic devices which seem to be stacked into odd spaces throughout the rf sections. Other cores are used in many power supplies as switching transformers and chokes. The following sections will cover the general aspects of ferromagnetic cores and provide you with sufficient information to allow you to select a suitable core for your application.
Most of the data on this site is related to cores and material made in USA, but by using sensible comparisons with material from other manufacturers, you can get useful information.
The main advantages from using these cores are as follows: higher Q, self-shielding and compactness. Let us consider each of these benefits.
Iron Powder materials are normally used in narrowband (tuned) rf circuits. A core is chosen which is appropriate for the operating frequency and power (this will become clear later). For a given inductance the ferro-magnetic inductor has fewer turns than an equivalent air-cored inductor, because of the permeability of the material used in the core. This also means that the winding resistance is lower, and since both inductors have the same inductance and therefore the same value of inductive reactance, the value of XL/R will be greater for the toroidal inductor. This expression is the same as that for the Q of a coil and so the Q will also be greater. An additional benefit is gained with low voltage equipment which passes dc current through the inductor, by reducing the voltage drop and hence heating in the component. Unlike normal (solenoid wound) coils which have a large external magnetic field, the toroid winding has a field which is almost completely contained within itself. This means that there is almost no magnetic coupling between toroidal coils and other circuit elements. We are therefore free to place toroid coils fairly close to other circuit components without complicated shielding. It should be noted that capacitive coupling can occur, and normal precautions must be taken because of this effect.
Ferrite materials are suited to wideband rf applications where the response of the element must be constant over a wide frequency range. A wideband transformer with a bandwidth of one decade, say 3 - 30MHz, is easily achieved when the correct ferrite material and winding technique are chosen. A balun transformer as used in most TV antenna installations is a good example of a wideband transformer using a ferrite core.
The use of these materials in switched mode power supplies (smps) allow the use of switching frequencies of 20 kHz or more. This results in the use of much smaller transformer and choke components than is possible with the use of direct mains frequency operation.
The benefits mentioned above allow us to create a much more compact circuit layout when these cores are employed.
The following magnetic formulas may be useful when doing core calculations.