Connect with us

×

ARTICLES

DESIGN NEWS

PROJECTS

TECHNICAL ARTICLES

INDUSTRY ARTICLES Coming Soon

BY
CATEGORY

Forum

ALL FORUMS

CIRCUITS & PROJECTS

EMBEDDED & MICRO

MATH & SCIENCE

COMMUNITY

Education

Textbooks

Video Lectures

Worksheets

Webinars Coming
Soon

By
Category

IoT

ARDUINO

Power

Log In/Sign Up

Log
In

Sign
Up

Sign
in with

Member
Benefits

Log
In

Sign Up

Sign in with

Member
Benefits

Home

Technical Articles

Clean Power for Every IC, Part 3: Understanding Ferrite
Beads

Related Content

Clean Power for Every IC, Part 3:
Understanding Ferrite Beads

3 days ago by
Robert Keim

Ferrite beads used in conjunction with
bypass capacitors can provide improved power-supply filtering and
decoupling.

Articles That Provide Supporting Information

AC Capacitor Circuits

AC Inductor Circuits

Previous Articles in This Series

Clean Power for Every IC, Part 1: Understanding Bypass
Capacitors

Clean Power for Every IC, Part 2: Choosing and Using Your
Bypass Capacitors

If Capacitance Is Not Enough

We concluded the previous article with a condensed set of recommendations for
properly designing and laying out a power-supply bypass network that will store
sufficient charge for long-term power-supply deviations and present low
impedance to dominant noise frequencies. You can be confident that this approach
will be more than adequate for most designs.

Nevertheless, the success of the capacitor-only approach does not mean that
there is no room for improvement, and this is where a lesser-known decoupling
component comes into play. If you feel that any ICs in your circuit are
especially noise-sensitive (and therefore need additional filtering) or
especially noisy (and therefore need additional decoupling from the power rail),
you should explore the wonderful world of ferrite beads.

What’s in a Bead?

The simplest form of a ferrite bead is a conducting wire inserted through a
hollow piece of ceramic material known as ferrite.

The electromagnetic properties of ferrite allow the material to influence the
current flowing through the conductor. The precise nature of this influence
depends in part on the type of ferrite (e.g., manganese-zinc vs. nickel-zinc),
and the properties of a particular ferrite material can be further refined via
the manufacturing process. In many surface-mount ferrite beads, the conductor is
formed into a coil structure, with individual windings layered between ferrite
sheets. Thus, the electrical characteristics depend also on the details of the
winding construction.

Ferrite beads can be divided into two general categories: high-Q (aka
resonant) beads, and low-Q (aka nonresonant) beads. High-Q beads are intended
for applications that require high levels of resonance, such as oscillators and
specialized filters. In the context of power-supply filtering, however, we need
to minimize resonance (as discussed later in the article), so high-Q beads are
out. For the remainder of this article, you can assume that any mention of
ferrite beads refers to low-Q beads.

It’s Not an Inductor, Not a Capacitor, Not a Resistor. . .

In our attempt to understand ferrite beads, we can start by considering a
first-order equivalent circuit and then translating this equivalent circuit into
a generic plot of impedance vs. frequency.

The inductor is placed in the center as a reminder that the predominant
response of a ferrite bead is inductive, i.e., impedance increases with
frequency. However, at some point (generally somewhere between 30 and 500 MHz),
the parallel capacitance begins to dominate the inductance, and impedance then
decreases with frequency. The relatively small parallel resistance (say on the
order of 100 Ω) reduces the resonance associated with the capacitor and
inductor, such that the impedance levels off at the transition point instead of
peaking in typical high-Q fashion. This response is evident in the following
plot showing the measured impedance characteristics of a standard SMD ferrite
bead manufactured by Wurth Electronics:

The black line indicates the overall impedance, which (though not shown in
the plot) starts at Rseries, otherwise known as the bead’s DC
resistance. It then increases linearly during the inductive frequency range,
levels off at 300 MHz, and then begins to decrease before leveling off at 1.1
GHz.

The red and blue dotted lines indicate that the overall impedance is the
result of two distinct elements, namely, inductive reactance (XL) and
frequency-dependent resistance (R). This brings up an important point: the
equivalent circuit given above is designed to replicate the frequency response
of the bead—it does not convey the bead’s internal structure. The equivalent
model is helpful for understanding how a ferrite bead’s impedance changes with
frequency and for performing simulations, but it is primarily the ferrite
material itself that determines the component’s impedance properties. This is
important to understand because the equivalent circuit might distract you from
one of the defining characteristics of ferrite beads: they actually
dissipate high-frequency energy.

Noise In, Heat Out

Recall that ideal inductors and capacitors do not dissipate any energy; they
merely store energy, either in a magnetic field (inductors) or an
electric field (capacitors). A resistor, on the other hand, takes energy out of
the circuit and dissipates it as heat. Ferrite beads, unlike inductors, are
intentionally resistive at high frequencies. This is why the above
plot has the red dotted line labeled “R”—from about 100 MHz to 1 GHz, the bead
exhibits significant resistive impedance, not reactive impedance. Actually, some
ferrite beads and ferrite-core inductors are almost identical in construction,
except that the ferrite bead uses a more “lossy” ferrite material because the
manufacturer wants the bead to dissipate rather than store
high-frequency energy.

But why belabor this point? We belabor for two reasons. First, you cannot
truly understand a ferrite bead until you have adequately pondered this
fundamental distinction between an inductor and a bead. Second, this “lossy”
characteristic makes the ferrite bead especially suitable for noise suppression.
Why? Inductance can lead to resonance and ringing when high-frequency noise
energy stored in the inductor interacts with capacitance elsewhere in the
circuit. As we saw in the previous articles, ringing can become seriously
problematic even when we are dealing only with parasitic inductance. We don’t
want to exacerbate the resonance/ringing situation, and thus we opt for ferrite
beads over inductors.

Choose Carefully

The key to maximizing the noise-suppression benefits of a ferrite bead is to
ensure that the targeted noise frequencies fall within the bead’s resistive
band—i.e., that portion of the frequency response where the resistive impedance
dominates the reactive impedance. This means that you cannot simply look at the
salient specs listed in a catalog or data sheet. For example, let’s say you
expect a noise spike at 100 MHz due to a microprocessor clock signal. Digi-Key’s
description for the Wurth part discussed above is the following:

And among the specifications you see this:

You might assume, based on this information, that the bead’s resistive
band includes 100 MHz. In this case, you’d be right—as shown in the frequency
response plot, this particular bead enters its resistive state at about 80
MHz.

However, there is still significant inductive reactance at 100 MHz, and
clearly the bead would provide optimum performance at noise frequencies around
300 MHz.

Now, let’s say you were thinking about part number MMZ1608D121CTAH0 from TDK.
The description and impedance spec are the following:

If you again assume that the bead is primarily resistive at 100 MHz, you will
find yourself in a bit of trouble:

The plot reveals that 100 MHz is still very much within the inductive portion
of the bead’s impedance curve. Based on the point at which the curve starts to
level out, the resistive band begins at about 500 MHz, and the bead does not
reach its optimum-noise-frequency point until 700 MHz. So if you choose this
bead, not only is the impedance lower at the targeted noise frequency, but the
type of impedance—namely, reactive—could make your circuit more
susceptible to ringing or even severe oscillation on the power line.

Recap

Here we have explored some of the essential physical and electrical
characteristics of ferrite beads, and we have seen that these components can be
particularly effective in improving power quality when they are used to suppress
noise frequencies that fall within their band of resistive-dominant impedance.
In the next article, we will discuss specific techniques for incorporating
ferrite beads into power-supply bypass networks.

Tags:

System
Noise decoupling
Signal
and Power Integrity ferrite
bead

Share

Share

Share

Share

Recommended Articles

Special Cases in Nodal Analysis

This article describes certain special cases when
performing nodal analysis.

Tristan
Cornell

July 09, 2015

Superposition and What it Means For You

This article describes superposition and its simple
applications.

Tristan
Cornell

May 11, 2015

Five Places to Find Part Info Online

These online sources not only help you find part data
quickly, but can also help you determine availability and manage bills of
materials.

Dan
Romanchik

June 25, 2015

Resistance in Parallel Networks

Parallel circuits are one of two fundamental circuit
configurations we come across every day. Like a parallel universe, parallel
circuits and networks can give us unexpected results. Adding more...

Marie
Christiano

June 30, 2015

Intro to Magnetic Devices and the Science Behind
Them

Magnetic devices shape our world: they power our homes,
tools, toys, and they even store our data. Quantum physics provides insights
needed to understand magnetism and its relationship to the...

Marie
Christiano

July 07, 2015

Comments

1 CommentLogin

Please login before you can comment.

w2aew
2015-10-02

You may want to view my video that gives a practical review of ferrite bead
characteristics and applications:
https://www.youtube.com/watch?v=81C4IfONt3o

Reply

Please login before you can reply to this comment.

Close

Other Content

Measuring Resistance, In Circuit and Out
Technical Article

Semiconductor Mergers and Acquisitions in 2015….So
Far
News

Resistor Color Codes
Textbook

Ohm’s Law worksheet
Worksheet

← Articles Index

WHO WE ARE

More
about us

Content

News

Projects

Technical
Articles

Textbook

Downloads

Video
Lectures

Worksheets

Forum

Categories

Latest

Arduino

Power

Embedded

IoT

Sensors

Digital
ICs

Test &
Measurement

Connect with Us

Facebook

Twitter

Contact Us

YouTube

LinkedIn

Sign Up

Register

© EETech Media, LLC. All
rights reserved

Privacy
Policy · Terms of
Service · User
Agreement