Tag Archives: Ferrites

The Function of Ferrites in Electronics

Engineers often use ferrite components in electronic circuits. These ferrite components are nonconductive, ceramic compound materials made with numerous combinations of iron oxides. Electronic components typically use them because of their high electrical resistivity and low eddy current losses. Ferrites can have various properties depending on their condition of synthesis, sintering temperature, composition, and grain size.

Manufacturers classify ferrites based on their crystal structure and magnetic properties. In general, they are of two types—soft and hard. Soft ferrites, made from magnesium, manganese, nickel, cobalt, and zinc, have low coercivity, such that their magnetism changes easily, and they act as conductors of magnetic fields. On the other hand, hard ferrites make very good permanent magnets, owing to their high coercivity.

It is also possible to classify ferrites based on their crystal structure. Typically, there are four groups— spinel, garnet, ortho, and hexagonal. Manufacturers distinguish them based on the molar ratio of ferric oxide to other oxide compounds present in the ferrite ceramic.

Crystallizing spinel ferrite results in a cubic structure with oxygen anions in a closely packed arrangement. Here, a unit cell comprises 32 oxygen ions. The anions form an FCC or face-centered cubic array.

Ferrites typically exhibit a permanent type of magnetism that physicists refer to as ferrimagnetism. This is a phenomenon that aligns the magnetic moments of atoms in both antiparallel and parallel directions. This alignment partially cancels the magnetic field, making the overall magnetic field of a ferrite material weaker than that of ferromagnetic materials.

Various types of ferrites are available. In electronic circuits, engineers typically use them as beads. For a ferrite bead, the resistivity is the strongest in a thin frequency band. This feature makes ferrite beads very useful as frequency-dependant resistors. Above the frequency band, the impedance of the bead begins to appear capacitative.

Other types of ferrites structures are also available for use in electronics. For instance, there are flat ferrites, typically rectangular or disc-shaped. Engineers use them in applications where they need a flat shape, such as power inductors, planar transformers, filters, and power inductors. Flat ferrites are very useful for suppressing radio frequency interference and electromagnetic emissions.

Ferrite rings and sleeves are also available. These are cylindrical-shaped components, suitable for placing around a wire or cable. It acts like a filter that can block high-frequency noise, allowing only low-frequency signals to pass through the wire or cable. Manufacturers choose the inner diameter of the ferrite to closely match the outer diameter of the cable, as this maximizes the benefits of interference suppression. Ferrite rings and sleeves are very useful in applications like data communications, consumer electronics, and power supplies to improve signal integrity and reduce interference effects on circuit performance.

Multi-hole ferrite beads are cylindrical cores with typically 6 through-holes running along the axis of the cylinder. When a trace or wire in a circuit is wound through its holes, the multi-hole ferrite bead behaves as a low-pass filter. It blocks unwanted high-frequency interference signals and allows only low-frequency signals to pass through the wire.

Using Ferrites in Wire Assemblies

The phenomenon of magnetism is prevalent all over the world, along with related concepts like the magnetic field, electromagnetism, and electromotive force. Although these are complex subjects at a higher level, they are easy to understand. However, these are principles on which electric motors operate, the earth’s magnetosphere shields life, and refrigerator doors remain closed.

The wonderful properties of magnetism also help products and applications like cable assemblies. There are well-known magnets like those made of neodymium, and these are permanent magnets with inherent magnetic properties. They comprise elements of Neodymium, Boron, and Iron. Neodymium magnets are among the most powerful permanent magnet types available. In comparison, there are non-permanent magnets also. Typically known as electromagnets, they derive their properties from the passage of an electrical current.

Other types of permanent magnets are also available. The most popular of these is the ferrite magnets, and industries use them for a lesser-known reason. Used in various forms like chokes, cores, and beads, these inexpensive devices greatly help filter electrical noise and get products to comply with EMI/EMC regulations. Countless design applications use them in different form factors and are available from numerous manufacturers. Ferrite magnets comprise a mixture of iron oxide and ceramic magnets. In doughnut-like shapes, they keep control over signal integrity within bundles of wire. For instance, a data cable carrying high-frequency data transmission,  when routed through the magnetic field of a ferrite, can eliminate unwanted electrical noise, as the ferrite acts as a passive EMI filter.

For a ferrite to be effective, the cable must pass through the center of the ferrite and its magnetic field. Looping and routing the wire multiple times through the ferrite helps incrementally improve the signal integrity. While a majority of cables have their wires passing through the ferrites only once, some designs require them to make as many as three loops to meet design objectives. Typically, there are two types of ferrites available that are suitable for cable assemblies—snap-on ferrites and doughnut ferrites.

Snap-on ferrites are the easiest to assemble. These are passive suppression devices with two halves. A plastic clamshell case holds the two halves as it snaps close around the wire. Available in a wide variety of sizes for different cable diameters and performance types, these are excellent devices that can mix and match various types of ferrite to help pass an aggressive test requirement. However, snap-on ferrites can be expensive and require accurate sizing to match the wire’s outer diameter to create an interference fit. As their design is like a clamshell, it is easy to remove snap-on ferrites.

Doughnut ferrites are simpler, being in the shape of a ring or a doughnut. The cable must pass through the center of the continuous circle of the ferrite before the wires terminate into a connector. The doughnut ferrite is therefore a permanent fixture, unlike the snap-on ferrite that the user can remove at any time. Overmolding the ferrite helps to fix its position on the cable while protecting the brittle ferrite magnet from damage.

Are Ferrites Good for Interference Suppression?

Although ferrite beads and sleeves are a common sight on cables, the technique for reducing both outgoing and incoming RF interference is the least understood. To study ferrites, and to do some comparative frequency domain measurements, one needs actual ferrite samples, a specially designed test jig, a spectrum analyzer, and a tracking generator.

Any current flowing through a metal conductor will create a magnetic field around it. The inductance of the conductor transfers the energy between the current and the magnetic field. A straight wire has a self-inductance of about 20 nH per inch. Any magnetically permeable material placed around the conductor helps to increase the flux density for a given field strength, thereby increasing the inductance.

Ferrite is a magnetically permeable material, and the composition of the different oxides making it up control its permeability, which is frequency dependent. The composition is mainly ferric oxide, along with nickel and zinc oxides. Furthermore, the permeability is complex with both real and imaginary parts. Therefore, the line passing through the ferrite has both inductive and resistive components added to the impedance.

The ratio of these components varies with frequency. The resistive part dominates at higher frequencies, and the ferrite behaves as a frequency dependent resistor. Therefore, the assembly shows loss at high frequencies, with the RF energy dissipating in the bulk of the material. At the same time, there are few or no resonances with stray capacitances.

Cables are usually in the form of a conductor pair, carrying signal and return, or power and return. Multi-way cables may carry several such pairs. The equal and opposite return current in each circuit pair usually cancels the magnetic field from the current in the forward line. Therefore, any ferrite sleeve place around a whole cable will have zero effect on the differential mode currents in the cable. This is true as long as the sum of differential-mode currents in the cable is zero.

However, for currents in the cable in common mode, with conductors carrying current in the same direction, the picture is different. Usually, such cables produce ground-referred noise at the point of connection or have an imbalance of the impedance to ground, causing a part of the signal current returning to ground through paths other than through the cable.

For instance, a screened cable, improperly terminated, may carry common-mode currents. As their return paths are essentially uncontrolled, these currents have a great potential for interference, despite being of low levels. Sometimes, the incoming RF currents, although generated in common mode, convert to differential mode and so affect circuit operation. This happens due to differing impedances at the cable interface.

As common mode currents in a cable generate a magnetic field around it, placing a ferrite sleeve around the cable increases the local impedance of the cable and operates between the source and load impedances.

When interfacing cables, low source impedance implies the ferrite sleeve is most effective when adjacent to a capacitive filter to ground. Since the length and layout of a cable will usually vary, engineers take the average value of the cable impedance as 150 ohms.