Inductors in EMC - Part 2: Ferrite beads

This article introduces ferrite beads. Ferrite beads are magnetic components that play a key role in suppressing high-frequency noise and preventing unwanted radiation. Learn more about the applications, use cases and different types of ferrite beads that can be found on the market.

What are ferrite beads?

Basically, ferrite beads are – simply describes – electrical conductors surrounded by a magnetic material (ferrite, a ceramic material of iron(III) oxide Fe2O3). Ferrite beads attenuate high frequency signals (typically f >1MHz) and dissipate them in the form of heat. They present a variable impedance to signals depending upon their frequency and come in different sizes and shapes. The functioning principle depending on the frequency can be described like this:

• From direct-current (DC, 0Hz) until the resonance frequency fr the ferrite bead impedance behavior is inductive.

• At the resonance frequency fr and around fr, it is purely resistive.

• For frequencies higher than fr, the ferrite bead impedance behavior is capacitive due to parasitic elements. In this last region, a ferrite will not act as a good filter anymore.

When fighting noise, the relevant frequency band is the resistive one, because along it, ferrite beads stop behaving mainly as inductors and start behaving as resistors: dissipating noise and producing heat.

Impedance characteristics of a ferrite bead [5].

Types of ferrite beads

Nowadays, different types of ferrite beads can be found on the market. In the following, two types are presented:

• Axial ferrite beads – Cable mount.

• Chip ferrite beads – PCB mount.

Axial ferrite beads

Axial ferrite beads consist of a magnetic material in a cylindric shape, whereby one or several wires are passing through the cylinder. The magnetic material confines the magnetic field and it presents high loss at high frequencies, so it is a perfect solution for filtering high-frequency noise (typical from f=1MHz up to f=1GHz).

A ferrite bead is a bead of ferrite with a lead or cable passed through it [1].

A subgroup of axial ferrite beads, known as clamp or clip ferrites, are widely used to solve interference issues in systems whose design has already been finished and there is not much room or time for modifications elsewhere (e.g. on the Printed Circuit Board (PCB)). They come in different shapes and sizes and clamp ferrites can be mounted very flexible on the design to suppress noise currents (e.g. common mode noise through cables).

Clamp ferrites are placed around emitting wires and cables [8].

Applications

Chip ferrite beads are also known as Surface Mounted Device (SMD) ferrite beads. Modern assembled PCBs (PCBAs) contain up to several hundreds and thousands of SMD components and chip ferrite beads are commonly used to protect sensitive electronic circuits against interference coming from other – more noisy – components of the same board. Chip beads come in very well-known packages such as 0603 or 1206, among many others, which makes this kind of beads helpful and suitable when performing a PCB design. Chip ferrite beads come the same dimensions as SMD capacitors or resistors (see below).

Chip ferrite bead [1].

Internal structure of chip ferrite beads [1].

Modelling of ferrite beads

Some characteristics such as inductive, resistive or capacitive behaviour of ferrite beads have already been introduced above in the section “What are ferrite beads?”. Modelling ferrite beads using an accurate model is critical for predicting their behaviour and for choosing the most suitable one for your purpose. Ferrite beads can be modelled with the following components:

• Rdc: Resistance at Direct Current.

• Rac: Resistance at Alternating Current.

• Lbead: Bead Inductance.

• Cpar: Parasitic capacitance.

This model is accurate up to the Sub-GHz band [2]

All the parameters are usually given by manufacturers at specific frequencies, such as 100MHz. Good and common practice is to simulate the filter characteristics and performance with a SPICE software tool such as LTspice [6], or TINA-TI [7].

Real-world applications of ferrite beads

There are many situations in which ferrite beads can be used. Some examples are presented here:

• Differential noise filter. In the example below, L1 and C1 act as a low-pass filter, reducing the impact of external disturbances at the input of the operational amplifier.

Example of differential noise filter (ferrite bead L1 and capacitor C1).

• Power supply line filter. A PI filter – with two capacitors and a ferrite – can be used to suppress high-frequency noise on a power line and provide a separation of noisy and sensitive circuits.

Example of a PI filter to filter a power supply line [3].

• Common mode filter for cables. When there is noise in both conductors of a circuit (common mode noise), a ferrite clamp can concentrate the magnetic field and cancel the effect of the noise of one conductor with the other one.

Example of a clamp ferrite used to filter common-mode noise at power wirings [4].

Conclusion

This article presented ferrite beads: very versatile magnetic components that can reduce high-frequency (typically for f >1MHz) interference in different situations (to an electronic circuit and from an electronic circuit). As electronics design engineer, it is important to have a good knowledge about them, both theoretically and practically. This knowledge is fundamental for a successful design of complex electronic systems and for passing electromagnetic compatibility (EMC) tests.

This blog post was contributed by Ignacio de Mendizábal, electronics engineer, Brussels. Many thanks to Ignacio for his valuable work. Reviewed and posted by Reto Keller, electronics engineer, Switzerland.

Ignacio de Mendizábal

Reto Keller

References

[1] Murata, Basics of Noise Countermeasures, Lesson 4, Chip ferrite beads. https://article.murata.com/en-global/article/basics-of-noise-countermeasures-lesson-4, accessed 09 October 2020.

[2] Jefferson Eco and Aldrick Limjoco, Devices, Ferrite Beads Demystified, Analog Devices. https://www.analog.com/en/analog-dialogue/articles/ferrite-beads-demystified.html, accessed 09 October 2020.

[3] Twilio Inc. Designing Analog Hardware. https://developer.electricimp.com/resources/designing_analog_hw, accessed 09 October 2020.

[4] Minimizing EMI, System Design Guidelines, Avoid noise in analog hardware design. https://www.unitronicsplc.com/Download/SoftwareHelp/U90LadderKnowledgebase/Minimizing_EMI__System_Design_Guidelines.htm, accessed 09 October 2020.

[5] Würth Elektronik. https://www.we-online.de/katalog/datasheet/742792621.pdf, accessed 09 October 2020.

[6] LTspice, SPICE-based simulation program. https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html, accessed 09 October 2020.

[7] TINA-TI, SPICE-based simulation program. https://www.ti.com/tool/TINA-TI, accessed 09 October 2020.

[8] Würth Elektronik. https://www.we-online.com/catalog/en/WE-STAR-TEC/, accessed 09 October 2020.