Understanding Ferrite Beads and Applications

© 2009, IPBLOX LLC, All Rights ReservedPage 1

Understanding Ferrite Beads and Applications

Steve WeirIPBLOX, LLC

[email protected]@teraspeed.com


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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


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


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


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


Ferrite Bead Response Regions• Ferrite beads exhibit three response regions:• Inductive, resistive, and capacitive


Ferrite Bead Inductive Region• At low frequencies, ferrites make

EXCELLENT INDUCTORS!


Ferrite Bead Resistive Region• Ferrite beads are typically only resistive over

one frequency decade


Ferrite Bead Capacitive Region• Ferrite beads become capacitive at high

frequencies


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


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 )


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


Example S21 Responses

• A cut-off in the resistive region does not peak badly (27pF in figure)• It filters over a

narrow range



• A lower frequency cut-off peaks badly due to high Q of bead and capacitor



• Peaking near the VRM switching frequency can be very bad!• Amplifying source

noise > 10:1 is probably not what we want from a filter!


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


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


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


Damping w/ Shunt R

• Generally impractical as low value R draws multiple amperes for modest impedances


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


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


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


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


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


Example Virtex4™ FX


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


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


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


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?


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


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


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


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Understanding Ferrite Beads and Applications