What is a Ferrite Bead and How Do Ferrite Beads Work?

Ferrite beads are passive electronic components that can suppress high frequency signals on a Power Supplies line. They are normally placed around a power/ground line pair that is incoming to a particular device, such as the power cord for your laptop. These beads work according to Faraday’s Law: the magnetic core around a conductor induces a back EMF in the presence of a high frequency signal, essentially attenuating the ferrite frequency response. Standard ferrite beads can be acquired from specialized manufacturers such as Coilcraft, though certain projects may require customized beads.

Ferrites are magnetic materials, and placing this material in a ferrite clamp around the Power Supply/ground line allows provides a source of inductive impedance for signals passing through the line. That might tempt you to think of them as a standard inductor, but they’re more complex than that. In reality, a ferrite bead is a nonlinear component; the impedance it provides changes was the load current and voltage drop across the ferrite change. The simplified circuit model of a ferrite bead will help you understand its frequency characteristics. However, keep in mind that these attributes can change as a function of current and temperature. 

Because ferrite bead impedance is inductive, ferrite bead inductors are used to attenuate high-frequency signals in electronic components. When a ferrite bead choke is placed on the power line connecting to an electronic device, it removes any spurious high frequency noise present on a power connection or that is output from a dc power supply. This ferrite clamp use is one of many approach to noise suppression, such as that from a switched-mode power supply. This application of ferrite beads as a ferrite filter provides suppression and elimination of conducted EMI.

Among the various uses of ferrite beads as filters, an EMI filter bead/power supply filter bead is usually rated for a certain DC current threshold. Currents greater than the specified value can damage the component. The troublesome thing is that this limit is drastically affected by heat. As temperature increases, the rated current quickly decreases. Rated current also affects the ferrite's impedance. As DC current increases, a ferrite bead will "saturate" and lose inductance. At relatively high currents, saturation can reduce the ferrite bead impedance by up to 90%.