uso de ferritas para emi

8/13/2019 Uso de Ferritas Para Emi

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STEWARD- U.S.A. Telephone: 423/867-4100 Fax 423/867-4102 Internet: http://www.steward.comSCOTLAND Telephone: 44-(0)1-506-414200 Fax 44-(0)1-506-410694

SINGAPORE Telephone: (65)337-9667 Fax (65)337-9686

Properties Of FerritesIntroduction

Ferrites are a class of ceramic ferromagneticma-terials that by definition can be magnetized to

produce large magnetic flux densities in responseto small applied magnetization forces. Originally re-ferred to as magnetic insulators, ferrites were firstused as replacements for laminated and slug ironcore materials in low loss inductors intended for useabove 100 kilohertz (kHz). At these frequencies,laminated and slug iron are plagued by excessiveeddy current losses whereas the high volume re-sistivity of ferrite cores limit power loss to a fractionof other core materials. Today, Steward ferrites arethe core material of choice for modern high densityswitch mode power supply and pulse transformer

design.

Fundamental PropertiesWhile frequently nicknamed magic beads in

marketing literature, EMI suppression ferrites areactually well understood magnetic components.Ferrites intended for EMI applications above 30 MHzare mixtures of iron, nickel and zinc oxides that arecharacterized by high volume resistivity (107ohm-cm) and moderate initial permeability (100 to 1500).

Ferrites are most frequently used as two termi-nal circuit elements, or in groups of two terminal

elements. The unique high frequency noise suppres-sion performance of ferrites can be traced to theirfrequency dependent complex impedance, asshown in Figure 10. At low frequencies (below ~10MHz), a Steward type chip bead presents a small,predominately inductive impedance of less than 100ohms, as shown in Figure 11. At higher frequen-cies, the impedance of the bead increases to over

600 ohms, and becomes essentially resistiveabove100 MHz. When used as EMI filters, ferrites can

thus provide resistive loss to attenuate and dissi-pate (as minute quantities of heat) high frequencynoise while presenting negligible series impedanceto lower frequency intended signal components.When properly selected and implemented, ferritescan thus provide significant EMI reduction while re-maining transparent to normal circuit operation! Forhigh frequency applications, ferrites should beviewed as frequency dependent resistors. Sincethey are magnetic components that exhibit signifi-cant (and useful) loss over a bandwidth of over 100MHz, ferrites can be characterized as high fre-

quency, current operated, low Q series losselements. Whereas a purely reactive (i.e., com-posed only of inductors and capacitors) EMI filtermay induce circuit resonances and thus establishadditionalEMI problem frequencies, lossy ferritescannot. In fact, ferrites are often used in high fre-quency amplifier design and power supply designto prevent or significantly reduce unintended highfrequency oscillations.

FIGURE 11: Typical impedance versus frequencycharacteristics of Steward high impedance chip bead

HZ0805E601R-00Z, R, X

Lvs. Frequency

Impedance, Resistance, Inductive Reactance

Frequency (MHz)1 10 100 1000

Impedance()

0

800

600

400

200

XL

R

Z

FIGURE 10: Simple equivalent circuit model of a two terminal ferrite bead

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A Closer Look At Ferrite ImpedanceThe previously described complex impedance of

ferrites can be analyzed further if the situation con-sidered is limited to small applied magnetizationforces (i.e., small forward current, few turns of con-ductor around/through the core). In such cases, the

application of incremental increases in magnetiz-ing force Hto a ferrite will result in a correspondingincrease in magnetic flux density Bin the core. Thisoperation typically displayed graphically via a de-vices B-Hcurve, as shown in Figure 12.

With the previously mentioned restrictions, the

impedance of a given ferrite bead or core can beexpressed as:

Z = R(f) + jL(f)

The frequency dependent loss term arises fromthe loss of energy incurred as a result of oscillationof microscopic magnetic regions (called domains)within the ferrite. The loss and the ferrite impedancecan be expressed in terms of a complex permeabil-ity as:

Z = K {jo [((f) - j(f) )] }

Z = Ko(f) + jKo(f)

Z = R(f) + jL(f)

where:

(f) = the real component of the frequency

dependent series complex relativepermeability

(f) = the imaginary component of thefrequency dependent series complexrelative permeability

K = a constant corresponding to thenumber of windings and thecore dimensions

o = permeability of free space

= radian frequency = 2f

The loss tangent (tan d) of a ferrite material canbe defined as the ratio of the imaginary part to thereal part of the materials relative permeability.

tan d =(f)

(f)

Figure 13 gives a graphical representation of theloss tangent. As is true with the permeability, theloss tangent is frequency dependent. The loss tan-gent is an intrinsic property of a given ferrite materialformulation. Choosing a particular ferrite materialcorresponds to choosing a particular loss tangentand an associated impedance versus frequencycharacteristic.

FIGURE 12: Virgin B-H curve for a typical ferrite

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DC & Low Frequency AC Bias Effects in a BoardLevel Application

Steward ferrites deliver maximum series imped-ance under zero DC and low frequency AC bias;i.e., when zero net flux is induced into the deviceby circuit bias currents. Since EMI suppression

ferrites are frequently used to filter common modeEMI on conductors carrying DC or AC power, theyshould be applied so as to encircle pairs or groupsof conductors that carry equal and opposite (bal-anced) low frequency (e.g., 60 Hz) alternating anddirect currents. For example, suppose an EMC en-gineer wishes to reduce the high frequency noiseon a DC power supply output cable. The engineerproposes two solutions. The first implementation,shown in Figure 15, employs two ferrite cores, onefor the +5 volt conductors, and one for the groundor power return conductors. In this case, each fer-

rite will be subject to a large net DC bias, which willresult in a large reduction in the high frequency im-pedance of the ferrite, and a correspondingreduction in EMI suppression performance. In thesecond implementation, displayed in Figure 16,equal numbers of +5 volt and ground conductors

are passed through a single ferrite. In this instance,the ferrite sees equal and opposite DC currentsand thus zero net magnetic flux density. The ferritewill be able to provide maximum series impedance

for high frequency common mode currents and re-main unaffected by the DC operation of the encircledconductors.

Some applications may not permit a ferrite to op-erate under zero bias. While ferrites can still functionas lossy elements with non-zero DC and low fre-quency flux densities, the user must be aware thatthe impedance of the device will decrease undersuch bias. This drop in impedance can be easilycompensated by increasing the mass of the part.To aid the designer with non-zero bias applications,Steward provides impedance versus DC bias cur-

rent information for all applicable component families.Figure 17 shows the impedance versus DC biasbehavior of the Steward Part NumberHZ0805E601R-00.

FIGURE 15: Incorrect usage of cable ferrites to filter conductors

carrying large DC components

FIGURE 16: Correct usage of cable ferrites on DC carrying conductors

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Ferrites For EMI Suppression OnPCBsAttacking EMI Problems At The Source

A fundamental EMC design principle requires thatEMI be attenuated at its source on the PC board.This strategy confines noise to the small regions ofa given PC board and reduces the possibility thathigh frequency noise will couple to other circuits(often called receptor or victim circuits) that mayradiate the noise more efficiently through intercon-necting wires or openings in a products shielding.Attacking EMI at the source generally provides themost cost effective design approach, since filteringis targeted only to a few specific noise generatingcircuits, rather than to every single possible noisereceptor in the entire product. Effective source fil-tering also helps limit overall EMC design costs byreducing the need for additional shielding that wouldotherwise be necessary to confine unfiltered highfrequency noise components.

HZ0805E601R-00Z vs. Frequency

Impedance Under DC Bias

Frequency (MHz)1 10 100 1000

Impedance

()

0

800

600

400

200

Noise On The PC Board Power& Ground Distribution Network

PC board generated EMI originates from the peri-odic switching of digital circuits. A simple noise modelof a digital integrated circuit (IC) is shown in Figure18. Each time the IC output switches state, it causeshigh frequency current to flow from the PC boardpower distribution bus (Vcc and ground). This ac-tion will introduce a small differential noise voltagedrop, or sag, across the boards power bus. Sincethis process will repeat with each transition of theICs output, the noise that is induced on the PC boardpower and ground will oscillate at a frequency equalto the operating frequency of the IC. Additional ICsthat reside on the PC board will see this noise volt-age and couple it to other areas of the system. Powersupply and data cables that are connected to thePC board power and ground bus will also transportand radiate the IC switching noise throughout andoutside of the system.

FIGURE 17: Impedance versus DC bias behavior

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We can generalize this power bus noise voltage

problem by modeling the PC board power bus as alumped impedance through which active devices(integrated circuits, for example) draw high fre-quency current. An ideal board impedance wouldhave a value of zero ohms, i.e., an active devicecould draw infinite switching current yet introduceno significant differential noise voltage to the PCboard bus. This ideal situation is never achieved in

FIGURE 18: Noise voltage and current source models of anintegrated circuit on a PC board

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practice. To reduce the magnitude of the board im-pedance, circuit designers add decouplingcapacitors across the power and ground conduc-tors of the PC board in an attempt to provide a localsource of charge for each active device. This tech-nique can also be viewed as placing a high

frequency short circuit across the active devicespower and ground pins, as shown in Figure 19.

While decoupling capacitors may provide ad-equate noise filtering at frequencies up to 75 MHz,their performance at higher frequencies will be dra-matically reduced by the presence of circuitresonances. These resonances arise from the in-teraction of the decoupling capacitors with devicelead and interconnect inductance in essence, ca-pacitors become functional inductors at higherfrequencies. Many EMC engineers have observed

and solved frustrating noise problems that arise un-expectedly from unique combinations of noisefrequencies, PC board layouts and decoupling ca-pacitors.

Filtering The Power Input Pins OfActive Devices With EMI Suppression Ferrites

While the resonant behavior of decoupling capaci-tor arrangements limits their effectiveness at higherfrequencies, the performance of Steward ferritesactually improveswith increasing frequencies. Since

Steward EMI suppression ferrites present an es-sentially resistive (lossy) impedance at highfrequencies, they cannot by themselves introduceperformance limiting circuit resonances. When usedin conjunction with decoupling capacitors, ferritescan provide additional EMI source suppression byblocking and dissipating power bus noise generatedby high speed logic devices. Note that a capacitorstill must be used at the power input pin of the ac-tive device, since the ferrite by its nature will blockthe high speed switching current that the devicerequires to operate. Figure 20 shows an example of

a ferrite bead and capacitor filter that is often usedin personal computer clock oscillator circuits.

FIGURE 19: PC board noise model with board impedance,

integrated circuit, and decoupling capacitors

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Note that this application subjects the ferrite to anet DC bias current. As discussed in the previoussection, the impedance and resulting noise attenu-ation of a ferrite drops with increasing net DC or

low frequency AC bias current; therefore, theamount of attenuation obtained from a ferrite DCfilter circuit will depend upon the current require-ments of the active device and the impedanceversus forward DC current characteristic of the fer-rite. Complete information on impedance versus DCbias current characteristics is provided for the Stew-ard part families that are subject to DC bias in typicalapplications.

Filtering DC Power To Multiple& Individual PC Boards

Time-to-market design pressures have inspireda new generation of modular electronic products

whose features can be easily upgraded with cost-effective interchangeable PC boards. For successfulEMI control of such product architecture, EMC en-gineers must design in a type of configurationindependence in which any possible combinationof product features and hardware options will al-ways pass mandatory U.S. and international EMIrequirements. Since high frequency noise is oftenproduced on and conducted through a PC boardspower distribution bus, the tendency of interchange-able circuit boards to create EMI problems can besubstantially reduced by filtering the power input toeach circuit board, as shown in Figure 21.

FIGURE 20: Ferrite and decoupling capacitors forhigh frequency DC power filtering

FIGURE 21: Using ferrites to filter the DC power input

of interconnected PC boards

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This design approach can also substantially re-duce common frequency type problems where thenoise output of multiple circuit boards with identicaloperating frequencies combine at one or more fre-quencies to create large radiated emission testfailures. Examples of DC power filtering can be foundin notebook computers, where external batterypacks, AC adapters, and facsimile, printer, and othercommunication options must connect to an EMI-noisy main system module. Other applicationsinclude backplane/daughter board arrangements asfound in low cost computer network hardware,where multiple PC boards receive power and datafrom a single high frequency backplane arrange-ment.

Since the described DC filter applications willsubject the ferrite components to DC bias current,the maximum in-circuit impedance (and hence maxi-

mum noise attenuation) achieved will be less thanthat obtained under zero bias conditions. For ac-ceptable performance in power applications, thelarger, higher current Steward common mode multi-line devices and multiple aperture devices arerecommended for PC board power filtering. In ap-plications involving DC bias above 300 milliamperes,the greater cross-sectional area and higher zerobias impedance of these devices will provide betterperformance than smaller radial and surface mountdevices. A common mode choke is a superior solution, due

to its independence from DC/AC bias effect over asingle line differential mode product. For example,the CM4545Z131R-00 retains 96% of its perfor-mance from 0 to 10 amps.

Filtering Of Input/Output (I/O) Data ConductorsOne of the most common and cost effective ap-

plications of ferrites is the filtering of conductors thatmust bring signals into and out of an EMI noisy en-vironment such as the inside of a high speedpersonal computer enclosure. For example, energyradiated from a central processor (CPU) integratedcircuit (IC) may couple into the driver IC that sendsto and receives data from the systems externakeyboard and mouse, as shown in Figure 22. Thelong external cables of these devices then radiatethe noise that previously was confined to the shieldedenclosure of the computer. Steward ferrites can be

used between the driver IC and the keyboard andmouse connector to insert a large signal loss inseries with the high frequency CPU noise on thedata lines. Since the keyboard and mouse signalshave essentially zero signal energy above 1 MHz,they will pass through the ferrite filter undisturbed.As shown in Figure 23, Steward multiline suppres-sor ferrites, CM3032 series, provide a compactmeans of filtering up to 8 data lines simultaneously,thus minimizing filter part count and assembly timeas compared with that required for single data linefilters.

FIGURE 22: Noise coupling between high speed CPU IC and keyboard/mouse interface IC

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Ferrites For EMI Suppression On

CablesIntroduction

Internal and external cable assemblies in com-puter equipment often act as miniature antennas asthey transform noise voltages and noise currentsinto large sources of radiated EMI. Stewards line offerrite beads for cable assemblies provide a costeffective approach to attenuate noise currents onflat and round cable bundles before they can beconverted into radiated EMI.

Unshielded cable assemblies will radiate EMI dueto the common mode noise that is present on their

copper conductors. This noise is characterized byequal in phase high frequency currents that flow inthe same direction along all the wires in the cables,as shown in Figure 24. These currents induce a netmagnetic field with a specific magnitude and direc-tion. Stewards cable ferrites attenuate the noisecurrents by capturing the magnetic field and con-verting a portion of its energy into heat. In terms oftwo terminal electrical device behavior, the ferrite issaid to present a large lossy impedance to the com-mon mode current. A Steward core used around a

group of wires is common mode choke.

Internal Cable AssembliesBy reducing the EMI generated by cables inside

the equipment, Steward ferrites can reduce the costand amount of overall shielding required to confineEMI within a products enclosure. Steward cableferrites can be applied on internal power cables thatcarry direct current (DC), alternating current (AC),or analog and digital signals. Often, an upgrade willuse a Steward common mode arrays (CM3032 se-ries) as a replacement for an internal ribbon cablewhen it is desired to switch to a board mount solu-tion.

External Cable AssembliesOriginal equipment manufacturers (OEMs) use

Steward ferrites to suppress EMI on external powerand data cables for central processor units (CPUs),monitors, keyboards, printers, and other peripheralequipment. The long external power and data cablesof these devices act as efficient antennas to trans-mit internally generated noise outside to theequipments enclosure. By suppressing EMI onthese cables, Steward ferrites can often reduce ex-ternal cable shielding requirements, permitting theuse of lower cost cables in many applications.

FIGURE 23: Filtering of keyboard and mouse data lineswith Steward multiline ferrite

FIGURE 24: Ferrite on cable with common mode EMI

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Selecting Cable Ferrites For OptimumPerformance

Precision electronic components such as Stew-ard EMI suppression ferrites should be selected withconsideration of the intended application. In general,a cable ferrite should be selected to yield the high-est in-circuit series impedance for the noisefrequencies of greatest concern. For Steward type25, 28 and 29 materials, this highest impedance willcorrespond to a maximum in-circuit loss and maxi-mum EMI suppression.

Core Size and VolumeOnce the ferrite material and approximate part

dimensions are selected for a given application, in-circuit impedance and noise suppressionperformance can be optimized by:

1) increasing the length of the portion of theconductor surrounded by the ferrite

2) increasing the cross sectional area of theferrite (especially for power applications)

3) selecting a ferrite with an inner diametermost closely matching the outer diameter ofthe wire or wire bundle to be filtered

In general, the best ferrite for a particular appli-cation is the longest, thickest device that can beaccommodated and whose inner aperture is closelymatched to the outer dimensions of the cable to betreated. When installed on flexible cable harnesses,

ferrite cores of significant mass should be encap-sulated by heat shrink tubing or otherwise protectedand secured in place.

Number Of TurnsThe series impedance of a high frequency ferrite

device can be increased by running two or moreturns of the treated conductor through the ferritescore. Magnetic theory predicts that the impedanceof the device will increase with the square of thenumber of turns. However, due to the lossy and non-linear nature of EMI suppression ferrites, a ferrite

bead with two turns will yield somewhat less thanfour times the impedance of an identical part woundwith only one turn of the conductor. Sinceinterwinding capacitance will increase along with thenumber of added turns, Steward recommends thata maximum of two conductor turns be wound on asingle part. Increasing the number of turns beyondtwo will tend to degrade performance at higher fre-quencies where interwinding capacitancedominates the characteristics of the device.

Placement At The EMI Source Location OrAT I/O boundaries

In most filter applications, the ferrite should beplaced as close to the source as possible. This wilprevent the noise source from coupling to othestructures where filtering may be less effective ordifficult to implement. For input / output (I/O) circuitshowever, where conductors may enter and exit ashielded enclosure, the ferrite should generally beplaced as close as possible to the shield penetra-tion. This implementation prevents noice fromcoupling to the conductor at a physical location inthe enclosure after the filter. Figure 25 illustratesboth filter placement techniques.

Choice Of MaterialFor EMI suppression applications, Steward offers

four unique ferrite materials with three distinguishing parameters:

Operating frequency of maximum impedance

Frequency selectivity or broadness of maximumimpedance versus frequency

Volume resistivity

EMI filtering applications are seldom narrowbandapplications usually require noise attenuation andhence a large lossy impedance over a broad rangeof frequencies. Type 28 material is generally the beschoice for the filtering of cables or single line appli-

cations with wideband EMI problems above 30 MHzFor applications where maximum attenuation and se-ries impedance above 200 MHz is critical and lowersignal frequency impedance is required (as in 100Base-T applications), Stewards type 25 material isthe optimum choice. Finally, type 29 material is spe-cifically formulated for multi-line packages to achievea high volume resistivity of 108ohms or more. Thismaterial is designed for use in circuits with un-insu-lated conductors and where stringent leakagecurrent and / or breakdown voltage requirementsexist.

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SINGAPORE T l h (65)337 9667 F (65)337 9686

Protection From Shock & Vibration

Brittleness and vulnerability to physical shock areinherent characteristics of ferrites and other ceramicmaterials. Many customer applicatiions utilize EMIsuppression ferrites in equipment that is subject tothe shock and vibration of shipping, handling, and in-stallation processes. When installed on flexible cableharnesses, ferrite cores of significant mass shouldbe encapsulated by heat shrink tubing or otherwiseprotected and secured in place. Small surface nicksand chipping will not significantly degrade a ferritesperformance.

FIGURE 25: Placement of cable EMI suppression at the

EMI source and at an I/O boundary

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