Ferrite bead inductors are used in many areas of electronic equipment as a simple yet effective form of filter.
Using ferrite as their basis, these inductors are simple to make using a bead, or low cost if they are bought.
Ferrite bead inductors form very effective filters for applications such as reducing electromagnetic interference because of the properties of the ferrite using within the inductor.
Ferrite bead inductor basics
Ferrites provide an excellent core material for a bead inductor. At low frequencies ferrite bead inductors act as a normal inductor. The ferrites have a high permeability and as a result they make excellent high density inductors.
Additionally the ferrites are highly resistive like cores including iron where eddy currents flow causing losses to be incurred. Ferrites have a high resistivity and therefore the inductor is able to pass signals up to relatively high frequencies (dependent upon the ferrite in use) with only the inductive reactance affecting the circuit and not the resistive eddy current induced losses.
However as the frequencies increase the eddy current losses rise. In turn this means that the resistive losses rise above frequencies of 10 to 100 MHz dependent upon the ferrite in question.
The fact that resistive losses rise above a certain frequency makes these ferrite bead inductors ideal for applications such as EMI filtering. The fact that the high frequency signals can be dissipated as resistive losses means that they are not reflected back into the system where they may find other routes to radiated or conduct. They are dissipated as heat rather than circulating elsewhere in the system.
Still higher in frequency the self-capacitance of the inductor takes over and the capacitive reactance dominates. Therefore at frequencies above 500 MHz or so care must be taken to ensure that the inductor is not capacitive in nature as this will enable signals to bypass the lossy nature of the ferrite bead inductor.
For even the highest frequency ferrites, typical insertion loss figures of no more than 10dB can be expected at frequencies of 2 GHz and more, although this is very much a rule-of-thumb.
Using ferrite bead inductors
Ferrite bead inductors are particularly useful in many RF applications. However to gain the most from them it is necessary to understand some of their limitations as well as how to ensure the benefit from their advantages is maximised.A few pointers are included below:
- Correct frequencies: As a general rule, ferrite beads are generally only resistive over one decade of frequency. Accordingly it is necessary to choose the ferrite for the frequency over which resistive absorption is required.
- Beware resonances: Like any other inductor, a ferrite can form part of a resonant circuit. Care must be taken combining ferrite beads with other components that are also reactive in either the inductive or capacitive regions. At low frequencies where X >> R, a ferrite bead inductor has a high Q.
One of the advantages of ferrite beads is that they are particularly easy to install. A single component can be inserted into the equipment, or a ferrite bead can be placed over a wire to provide filtering. This makes they very attractive for using when problems are encountered with equipment that is in service or having been designed, as well as for incorporating into designs at the earliest stages.
Ferrite bead inductors components
Ferrite beads can be obtained or made in a variety of forms.
- Ferrite beads: It is possible to obtain small and large ferrite beads. The wire requiring filtering can simple be wound around the bead, passing the wire through the core a number of times, often only two or three turns are required, although on some occasions the wire is simply passed straight through. Small ferrite beads may even be slipped over component leads to suppress parasitic oscillation - for example they may be slipped over the base of a transistor used as an emitter follower to suppress possible instabilities there.
When current flows in the lead for the ferrite bead inductor, magnetic flux is generated inside the bead. As a result, the ferrite bead functions as an inductor.
- SMD ferrite bead inductors: There is a vast variety of surface mount ferrite bead inductors available as ready usable components with specified performance. These can be chosen according to the performance required and easily added to a new design.
The SMD or chip ferrite bead inductors are made by incorporating the ferrite bead inductors into and SMD format. The coil is built up between layers ferrite and the windings incorporated between the layers. The overall three dimensional SMD chip ferrite bead is then made by a process of integration and firing. The structure used for these ferrite bead SMD inductors is essentially the same as that used for the more widely used monolithic chip inductors. The difference lies in the fact that the ferrite material used is chosen to suppress noise
By fabricating coil structure within the overall SMD ferrite bead inductor assembly, it is possible to achieve a much higher impedance than that of the more traditional leaded ferrite bead inductors. Often these simply have a single wire passed through them and therefore the inductance and effect of the ferrite is much less.
- Clamp ferrite cores: When it is not possible to wrap a wire around the ferrite bead, it is possible to obtain clamp on cores that can be easily clamped to the cable that needs filtering. The clamp approach is used where the connectors on the cable prevent a complete core or bead from being used. Although the wire is not coiled around the core, it is able to increase the self-inductance of the wire or cable sufficiently to have the effect of absorbing the energy of the noise traveling along the wire or cable Often computer monitor and other cables use this approach.
When used to their best, ferrite beads form a particularly effective form of inductor and filter for unwanted signals. They can be particularly effective in their resistive region, where they absorb signals rather than reflecting them back into the circuit where they may cause problems elsewhere. In addition to this they are a low cost component that can be easily added to a circuit.
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