Understanding Ferrite Beads and
Applications
Steve Weir
IPBLOX, LLC
sweir@ipblox.com
steve@teraspeed.com
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“CONFIDENTIAL PROPERTY OF IPBLOX LLC”
This document includes valuable trade secrets. Unauthorized
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Secrets Act.
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Ferrite Beads “Dark Magic”?
• Ferrite beads are often employed by EMC
specialists to solve noise problems.
– Beads have a reputation for magically
eliminating some EMC problems
• Ferrite beads are also often used in high
frequency analog circuits.
– Frequent application is power filtering
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Why Makes Ferrite Beads Special?
• Ferrites are highly permeable materials– They make good, dense transformers and
inductors in their linear region
• Ferrites are highly resistive
– Unlike other high permeability materials like iron,
ferrite material has a much higher resistivity
– High resistivity means low eddy current losses
up to “high” frequencies, IE they pass signals
without much loss up to high frequency
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What Makes Ferrite Beads Special?
• Ferrites are special due to high frequency
RESISTIVE losses
– Ferrites exhibit eddy current losses like any conductive
material
• Creates resistive loss
• Loss increases with frequency
• In ferrites used for EMC this does not happen until 10’s or 100’s
of MHz
• Resistive loss at high frequency makes a good EMI
trap
– Conducted noise can be turned to heat where it does no
harm
• Does not circulate through system
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Limitations of Ferrites
• All ferrites make EXCELLENT LINEAR
INDUCTORS up to at least 1MHz, often well
beyond 10MHz
• At high frequencies ferrites exhibit parasitic
capacitance that bypasses the resistive loss.
– Insertion loss falls off at 800MHz or lower
– Insertion loss no more than 10dB at 2GHz even
for the highest frequency ferrites
– The actual working frequency range depends on
the formulation
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Ferrite Bead Response Regions
• Ferrite beads exhibit three response regions:
• Inductive, resistive, and capacitive
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Ferrite Bead Inductive Region
• At low frequencies, ferrites make
EXCELLENT INDUCTORS!
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Ferrite Bead Resistive Region
• Ferrite beads are typically only resistive over
one frequency decade
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Ferrite Bead Capacitive Region
• Ferrite beads become capacitive at high
frequencies
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Ferrite Bead Response Regions
• Useful insertion loss may be realized in all
three impedance regions
• However, care must be taken combining
ferrite beads with other components that are
also reactive in either the inductive or
capacitive regions
• The inductive region is usually the most
DANGEROUS, and often overlooked
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Inductive Region Issues
• At low frequencies where X >= R, a ferrite
bead behaves as a high Q inductor.
• When building noise filters, it is important to
mind the port impedances and Q.
• A moderate Q inductor in the form of a
ferrite bead operating in its inductive region
feeding a high Q ceramic bypass
capacitor(s) results in high Q, ( lots of
peaking )
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Example S21 Responses
• The responses shown
demonstrate that for
any LP cut-off with a
high Q capacitor in the
inductive region, very
pronounced peaking
occurs.
– Amplifies any noise in
the band!
• SMPS ripple
• Digital noise
– Almost always in
passband of circuits like
PLLs.
– High Z to output
• Peaking depends on
capacitor
ESR vs.
bead
Page
13
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jwL
Example S21 Responses
• A cut-off in the
resistive region does
not peak badly
(27pF in figure)
• It filters over a
narrow range
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Example S21 Responses
• A lower frequency
cut-off peaks badly
due to high Q of
bead and capacitor
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Example S21 Responses
• Peaking near the
VRM switching
frequency can be
very bad!
• Amplifying source
noise > 10:1 is
probably not what
we want from a
filter!
Page 16
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The Need for Damping
• A low performance filter may be constructed
using a ferrite bead and a small capacitance
( 27pF in the example )
– The capacitance may be planar, discrete or a
combination
• Rule of thumb: Unperforated 4mil planes
FR4 material ≈ 225pF / sq in
– Undamped, a plane cavity would have to be <
0.12” sq to avoid peaking with a MPZ1608S221A
bead
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Damping Options
• Damping can be achieved by a number of
means.
• The most common:
–
–
–
–
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Adding series resistance
Adding shunt resistance
Adding series resistor to the capacitor
Adding a damped dominant pole
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Damping Series Resistor S21
• Preserves mid and HF
loss
• Resistor may need to
dissipate a lot of power
• Resistor may result in
unacceptable DC
voltage drop
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Damping w/ Shunt R
• Generally impractical as low value R draws
multiple amperes for modest impedances
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Damping w/ Cap w/ Series R
• Variation of shunt R
• Bypass cap acts as DC
block to resistor
• Solves peaking
• Several disadvantages
– Reduced mid band loss from
resistance
– Reduced HF loss from
resistance & ESL
• Best used w/ big cap value
allowing small R value
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Damping w/ Dominant Pole
• Further refinement of
shunt scheme, uses a
dominant pole RC
shunt for damping +
HF cap for high
insertion loss
• Low dissipation
• Good mid and HF loss
But,
• Requires more parts
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Damping w/ Capacitor Selection
• Can damp w/ a
capacitor with C and
ESR such that:
– ESR*√C >= 1.4√LBEAD
• Obviates need for
external resistor
• Requires lower Q cap
than MLCC
– Generally Al electrolytic
or tantalum with high
ESL
– Require MLCC(s) to get
low ESL for HF filtering
• Larger cap values drop
FCUTOFF & Z22
– Improves SMPS rejection
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Load-side Impedance, Z22
• S21 determines
rejection of outside
noise
• Load current, port 2,
impinges noise voltage
on the network loadside impedance, Z22
• Bypass capacitor /
plane / interconnect
inductance drive Z22
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How Beads Impact Z22
• Beads isolate power nodes into nets that are often
routed as traces by necessity
– Example: Virtex 4 FX series devices power application
notes require up to 80 power nodes EACH NODE
SEPARATELY isolated with a ferrite
– 10 instances each of 8 power supplies:
•
•
•
•
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AVCCAUXMGT
AVCCAUXRXA
AVCCAUXRXB
AVCCAUXTX
VTRXA
VTRXB
VTTXA
VTTXB
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Example Virtex4™ FX
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Interpreting Data Sheets
• Ferrite bead data sheets usually present
data in one of two forms:
– Z, X, R plots
– Scattering parameters based on 50 ohm ports
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Interpreting Data Sheet: Z, X, R Plots
• Z, X, R plots are usually presented in linear
impedance magnitude versus logarithmic
frequency.
• For simple single parallel LRC model,
– L ≈ 1.41*XPEAK / (2*∏*FXPEAK )
– R ≈ ZPEAK
• This model reasonably accurate in inductive
and resistive regions
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Interpreting Data Sheet S Params
• S parameters assume 50 ohm ports.
• 50 ohm source and load ports often misinterpreted
for power delivery
– Hides peaking that occurs in actual applications
– Real source port impedance usually very low
– Real load port impedance may be almost any value
• Effective resistance often quite high >> 50 ohms
• SPICE based lumped equivalent extraction is most
accurate
• Always evaluate with appropriate external circuit
model
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Ferrite Bead Design Checklist
• How much S21 insertion loss do I need
versus frequency?
– Can I meet this with placement and/or etch
manipulation
– Is a ferrite bead the right tool for the job?
• What Z22 requirements does my load have?
– Will isolating a voltage node(s) result in too much
PCB inductance?
• Trace instead of plane / puddle?
Page 30
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Ferrite Bead Design Checklist,
Cont’d
• What low frequency resistance can I
tolerate?
• Control peaking at FCUTOFF with proper
network design
• Insure filter is not defeated by placement /
layout
Page 31
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Summary
• Ferrite beads may be used to isolate circuits
– Reduced noise in analog power feeds
• Ultra-quiet clock power, reduces jitter
• Quiet PLL power, reduces jitter
• Quiet A/D, D/A power, improves S/N
– Reduced output / input feedback in high frequency
circuits
• Can prevent oscillations
– Reduced EMI conducted into main power rails
– Reduced susceptibility to ESD and EFT
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Summary
• Both S21 and Z22 requirements must be
considered in design
– At HF it is the load side bypass cap network doing
the noise suppression work
– Low inductance on load side critical for high
frequency circuits
• Use good layout technique & right choice of parts
• Ferrite beads are linear inductors at LF
– Some means of damping is required to prevent
transferring MORE NOISE near filter cut-off than
w/o the ferrite
• Dominant pole method provides best overall
response, but at highest cost and most parts
Page 33
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Contact Information
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eng@ipblox.com
steve@teraspeed.com
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Other Partners
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v (401) 284-1827
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Serial link development
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Beaverton, OR 97008
v (503) 430-1065
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Metrology
Measurement based IBIS models
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