Ferrite Filters: Types, Characteristics, and Applications

May 16, 2022 Cadence PCB Solutions

Key Takeaways

  • Ferrite filters are electronic components made of ferrite employed for suppressing and dissipating high-frequency electrical noises.

  • The use of ferrite beads can minimize EMI emissions and absorption through the traces within a circuit board. 

  • Ferrite cores prohibit cables from acting as antennas, thereby preventing radiated EMI noise.

 Ferrite cores

Ferrite cores are inserted in cables as EMI suppressors

Ferrite filters are a critical part of electromagnetic interference (EMI) filtering in electronic system design. Ferrite beads, ferrite cores, and ferrite core inductors are examples of ferrite components that are often used in electronic systems as EMI filters. Ferrite beads can function as low pass filters to block high-frequency noises from entering a device, whereas ferrite cores are inserted in cables as EMI suppressors. In power electronic systems, ferrite core inductors serve as ferrite filters attenuating high-frequency harmonics. In this article, we will discuss how designers can use various ferrite components for filtering in different applications. 

What Are Ferrite Filters? 

Most power supplies and electronic devices produce electrical noise or EMI. The harmonics produced by switching power supplies and the noises in communication systems are of high frequencies. These disturbance signals interfere with signals in circuits and degrade quality.

Ferrite is a ceramic compound of an iron oxide integrated with nickel, zinc, and manganese compounds. Ferrite filters are electronic components made of ferrite used for suppressing and dissipating high-frequency electrical noises. These filters are effective at attenuating EMI in electromagnetic circuits and devices.

Ferrite beads, ferrite cores, and ferrite core inductors are all examples of ferrite filters. Ferrite beads or rings are commonly used for filtering purposes in electronic circuits. They are used as low-pass filters to block high-frequency signals from entering a circuit. Ferrite cores or ferrite sleeves are often used for suppressing EMI in cables. Similarly, ferrite core inductors are utilized as filters in switching power converters. Let's take a closer look at ferrite bead filters, ferrite cores, and ferrite core inductor filters in the upcoming sections.

Ferrite Beads 

Ferrite beads are ferrite inductors of low inductance value and minimum parasitic capacitance. They are more of a lossy inductor and are free from unwanted resonances. Ferrite beads offer low-quality factors and high self-resonant frequency. At radio-frequency (RF) or very high frequency (VHF) ranges, ferrite beads provide high impedance. In high-frequency applications, ferrite beads are more like inductors compared to coil inductors, provided the resonant frequency of a ferrite bead is greater than the system frequency. When the resonant frequency is closer around a signal or system frequency, the ferrite beads dissipate energy in the form of heat, just like a resistor. Power loss in the form of heat energy offered by ferrite beads removes EMI noises in a circuit. 

One of the major applications of ferrite beads is for EMI reduction in high-speed PCBs. At high frequencies, PCB traces behave like antennas. The use of ferrite beads can minimize EMI emissions and absorption through traces within a circuit board. 

Ferrite Cores 

Cables are susceptible to EMI noises. If not properly installed and shielded, cables act as a source of electrical noises and high voltage spikes. Without protection, voltage spikes and EMI can damage the hardware or cause malfunctioning of the connected circuits or devices. Installing ferrite core filters can shield cables from EMI. Depending on the type of ferrite core filter and when and where it is installed, the effectiveness of the ferrite filter varies. Ferrite cores prohibit cables from acting as antennas, thereby preventing them from radiating EMI noises. Ferrite cores are usually used as cable shields or cable EMI suppressors, as they offer cost-effective filtering.

Ferrite Core Inductor Filters 

The noises in switching power converters are either common-mode or differential noises. To suppress the noise or EMI generated in switching power converters, inductors and capacitors are used to form LC filters. The filter inductor design is important for achieving the desired filter attenuation characteristics of an LC filter. The magnetic or core materials used in the inductor design are significant in providing the desired filter attenuation characteristics.

When the coils used in common-mode or differential mode filter inductors are wound on a  ferrite core, they are called ferrite core inductors. Ferrite core inductors are very efficient when used as filter inductors. They are excellent in filtering out high-frequency spikes and EMI in signals, improving the quality of the converter output and its performance. The availability of ferrite cores in various shapes as well as their comparatively low cost and low losses make ferrite core inductors popular for switching power converter filtering applications.

The selection of a ferrite filter is dependent on where it is going to be installed. For suppressing noises transmitted through PCB traces, ferrite beads are the best filters. For cable EMI suppression, ferrite cores work well. For a specific filtering requirement in a given application, you may find multiple options. With Cadence’s PCB design and analysis software, you can design, simulate, and analyze your circuit with different filter options and find which one works best.

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