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© 2009, IPBLOX LLC, All Rights ReservedPage 1
Understanding Ferrite Beads and Applications
Steve WeirIPBLOX, LLC
[email protected]@teraspeed.com
© 2009, IPBLOX LLC, All Rights ReservedPage 2
Property Rights Disclosure
“PROPERTY OF IPBLOX LLC”
Information contained in this document is not to be reproduced in any form without permission of IPBLOX LLC. Any information in this document is proprietary and may not be used or disclosed without the express permission of IPBLOX, LLC.
“CONFIDENTIAL PROPERTY OF IPBLOX LLC”
This document includes valuable trade secrets. Unauthorized disclosure of use of this document may violate the Uniform TradeSecrets Act.
© 2009, IPBLOX LLC, All Rights ReservedPage 3
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
© 2009, IPBLOX LLC, All Rights ReservedPage 4
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
© 2009, IPBLOX LLC, All Rights ReservedPage 5
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
© 2009, IPBLOX LLC, All Rights ReservedPage 6
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
© 2009, IPBLOX LLC, All Rights ReservedPage 7
Ferrite Bead Response Regions• Ferrite beads exhibit three response regions:• Inductive, resistive, and capacitive
© 2009, IPBLOX LLC, All Rights ReservedPage 8
Ferrite Bead Inductive Region• At low frequencies, ferrites make
EXCELLENT INDUCTORS!
© 2009, IPBLOX LLC, All Rights ReservedPage 9
Ferrite Bead Resistive Region• Ferrite beads are typically only resistive over
one frequency decade
© 2009, IPBLOX LLC, All Rights ReservedPage 10
Ferrite Bead Capacitive Region• Ferrite beads become capacitive at high
frequencies
© 2009, IPBLOX LLC, All Rights ReservedPage 11
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
© 2009, IPBLOX LLC, All Rights ReservedPage 12
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 )
© 2009, IPBLOX LLC, All Rights ReservedPage 13
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 jwL
© 2009, IPBLOX LLC, All Rights ReservedPage 14
Example S21 Responses
• A cut-off in the resistive region does not peak badly (27pF in figure)• It filters over a
narrow range
© 2009, IPBLOX LLC, All Rights ReservedPage 15
Example S21 Responses
• A lower frequency cut-off peaks badly due to high Q of bead and capacitor
© 2009, IPBLOX LLC, All Rights ReservedPage 16
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!
© 2009, IPBLOX LLC, All Rights ReservedPage 17
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
© 2009, IPBLOX LLC, All Rights ReservedPage 18
Damping Options
• Damping can be achieved by a number of means.• The most common:– Adding series resistance – Adding shunt resistance – Adding series resistor to the capacitor– Adding a damped dominant pole
© 2009, IPBLOX LLC, All Rights ReservedPage 19
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
© 2009, IPBLOX LLC, All Rights ReservedPage 20
Damping w/ Shunt R
• Generally impractical as low value R draws multiple amperes for modest impedances
© 2009, IPBLOX LLC, All Rights ReservedPage 21
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
© 2009, IPBLOX LLC, All Rights ReservedPage 22
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 lossBut,• Requires more parts
© 2009, IPBLOX LLC, All Rights ReservedPage 23
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
© 2009, IPBLOX LLC, All Rights ReservedPage 24
Load-side Impedance, Z22
• S21 determines rejection of outside noise• Load current, port 2,
impinges noise voltage on the network load-side impedance, Z22• Bypass capacitor /
plane / interconnect inductance drive Z22
© 2009, IPBLOX LLC, All Rights ReservedPage 25
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:• AVCCAUXMGT VTRXA• AVCCAUXRXA VTRXB• AVCCAUXRXB VTTXA• AVCCAUXTX VTTXB
© 2009, IPBLOX LLC, All Rights ReservedPage 26
Example Virtex4™ FX
© 2009, IPBLOX LLC, All Rights ReservedPage 27
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
© 2009, IPBLOX LLC, All Rights ReservedPage 28
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
© 2009, IPBLOX LLC, All Rights ReservedPage 29
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
© 2009, IPBLOX LLC, All Rights ReservedPage 30
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?
© 2009, IPBLOX LLC, All Rights ReservedPage 31
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
© 2009, IPBLOX LLC, All Rights ReservedPage 32
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
© 2009, IPBLOX LLC, All Rights ReservedPage 33
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
© 2009, IPBLOX LLC, All Rights ReservedPage 34
Contact InformationIPBLOX, LLC
150 N. Center St. #211Reno, NV 89501v (866) 675-4630f (707) [email protected]
[email protected] Delivery Solutions
Teraspeed Consulting Group, LLC121 North River DriveNarragansett, RI 02882 v (401) 284-1827 f (401) [email protected] E/M ModelingSerial link developmentJitter analysis
Z2 Consulting13610 SW Harness LaneBeaverton, OR 97008v (503) 430-1065 f (503) [email protected] based IBIS models
Teraspeed Consulting Group, LLCOther Partners