The use of soft ferrites for interference suppression. Application Note

Transcription

1 The use of soft ferrites for interference suppression pplication Note Y G E O O M P N Y

2 The Use of Soft Ferrites for Interference Suppression ontents Introuction 1 General principles of EM 2 EM regulations 2 Material specifi cations 7 EMI-suppression prouct lines 12 pplications 14 esign consierations 18 Impeance concept 18 iterature, software an sample boxes 19

3 1. Introuction In the fi el of electromagnetic compatibility several trens attribute to a growing necessity of EM engineering. In signal processing : hange from analog to igital (steep pulse eges, overshoot, ringing). Increase of clock frequencies. In power conversion : hange from linear to switchemoe supplies (high switching frequency, harmonics). Increase of switching frequencies. These trens, irecte to functional upgraing or reucing cost, inevitably also contribute to an increasing level of electromagnetic interference (EMI) emissions. Together with the increasing use of electronics this leas to a general EM egraation. s a consequence, EM legislation is getting worl-wie more strict. The most important regulations are the European Norms (EN) which are applicable in all European Union (EU) an European Free Trae ssociate (EFT) countries, F in Unite States an VI in Japan. The uniform legislation in the European Union is along the lines of the EM irective 89/336/EE. For every pro uct to which no specifi c European norm applies, a general regulation is manatory. These are the so calle Generic Requirements (resiential, commercial an light inustry: EN for emissions an EN for immunity). This inclues all electric an electronic proucts, no matter how trivial they seem to be! Of course the fi rst step to avoi interference problems is a goo esign practice, to tackle the problem right from the start. This can be insuffi cient if the interference is irectly relate to the inherent operating principle an too late if the interference is etecte not earlier than in the fi nal esign phase. In such cases extra suppression components are necessary, like ferrites, capacitors or shieling elements. Ferrites provie a solution to many problems of conucte an (inirectly) raiate interference. They can be applie almost anywhere : Shifte on wire or cable as beas, tubes or cable shiels. Mounte on P as beas-on-wire, wieban chokes, SM inuctors, multilayer suppressors or integrate inuctive components. Ring cores or U cores in mains fi lters, in the circuit, in a separate box or moule in a connector. Wieban chokes or coile ro inuctors in electrical appliances or motors. No groun connections are necessary as ferrites are connecte in series with the interfering circuit an not in parallel as in the case of a capacitor. The wieban, lossy impeance makes ferrites well-suite as RF suppressor component. 1

4 2. General principles of EM 2.a. Regulations Historically, all EMI regulations state emission limits only. These efi ne the maximum level of interference allowe as a function of frequency. In case of conucte interference it applies to the voltage on all inputs an outputs of the equipment, in case of raiate interference it applies to the fi el strength at a certain istance. Often two levels are state: lass for commercial an inustrial areas. lass for omestic an resiential areas. lass is always stricter than class. lso immunity is becoming subject of regulation. Taking into account the severity of the EM problem, equipment must also be able to operate without functional egraation in a minimum EMI ambient. The ifference between the actual level of emissions or susceptibility an the EM limits is the require attenuation by fi ltering or shieling. 2.b. Sources an propagation The source etermines whether the interference is a transient or ranom variation in time (commutation motors, broacast transmitters etc.) or a perioic signal (e.g. switchemoe power supplies). The frequency spectrum will be continuous in the fi rst case an a line spectrum in the secon. In practice, the minimum an maximum frequency involve are much more relevant an both types of sources can be broaban. Ranom variations are broaban if they are very fast, harmonic isturbances if the basic frequency is high or if the eviation from a sine wave is consierable. Interferences can propagate as an electromagnetic wave in free space. Suppression then requires shieling with conuctive materials. lso propagation occurs via conuctive paths such as the mains network, to which the majority of electrical equipment is connecte. elow 30 MHz this is the main propagation moe. Suppression is one with a high impeance in series (inuctor), a low impeance in parallel (capacitor) or a combination of both (fi lter). Propagation via the mains can take place in two ifferent moes : common an ifferential moe. part from phase an null which carry the supply current, there is the safety earth connection, which is generally taken as a reference. Z G Z G Z S Z Z E o Fig. 2 Insertion loss of an inuctor. ommon-moe : Phase an null interference voltages are equal. This is likely to occur if phase an null are close together an interference is coupling in from an external fi el (raiation or crosstalk). ifferential-moe : Phase an null interference voltages have opposite phase angle but equal magnitue. This is likely to occur in case of switching equipment connecte to the mains. In general a combination of both types can be present. 2.c. Suppression with ferrites t RF frequencies a ferrite inuctor shows a high impeance which suppresses unwante interference. The resulting voltage over the loa impeance will be lower than without suppression component, the ratio of the two is the insertion loss, see Fig. 2. E 2

5 level in /V µv quasi peak average f (MHz) Fig. 1a European generic emission norm (resiential, commercial, light inustry). level in /V µv quasi peak average f (MHz) Fig. 1b European generic emission norm (inustrial environment). 3

6 The insertion loss is expresse as : I = 20. log 10 (E o /E) [] Z G +Z +Z S = 20. log 10 [] Z G +Z The ecibel () as a unit is practical because interference levels are also expresse in. However insertion loss e pens on source an loa impeance, so it is not a pure prouct parameter like impeance (Z). In the application, source an loa impeance generally are not 50 Ω resistive. They might be reactive, frequency epenent an quite ifferent from 50 Ω. onclusion : insertion loss is a stanarize parameter for comparison, but it will not preict irectly the attenuation in the application. t low frequency, a ferrite inuctor is a low-loss, constant self-inuctance. Interferences occur at elevate frequencies an there the picture changes. osses start to increase an at a certain frequency, the ferrimagnetic resonant frequency, permeability rops rapily an the impeance becomes almost completely resistive. t higher frequencies it even behaves like a lossy capacitor. While for most applications the operating frequency shoul stay well below this resonance, effective interference suppression is achieve up to much higher frequencies. The impeance peaks at the resonant frequency an the ferrite is effective in a wie frequency ban aroun it. The material choice follows from the critical interference frequencies; ieally they shoul coincie with the ferrimagnetic resonance frequency, the top of the impeance curve. ccoring to Snoek s law, this resonant frequency is inversely proportional to the initial permeability, which gives us a guie for material choice. The higher the interference frequency, the lower the material permeability shoul be. The whole RF spectrum can be covere with a few materials if the right permeability steps are chosen. t the resonant frequency an above, the impeance is largely resistive, which is a favourable characteristic of ferrites. Firstly, a low-loss inuctance can resonate with a capacitance in series (positive an negative reactance), leaing to almost zero impeance an interference amplification! resistor cannot resonate an is reliable inepenent of source an loa impeances. Seconly, a resistance issipates interfering signals rather than refl ecting them to the source. Small oscillations at high frequency can amage semiconuctors or negatively affect circuit operation an therefore it is better to absorb them. Thirly, the shape of the impeance curve changes with the material losses. lossy material will show a smooth variation of impeance with frequency an a real wieban attenuation. Interferences often have a wieban spectrum to suppress. 4

7 2.. urrent-compensation Ferrite inuctors inserte separately in both lines suppress both common an ifferential moe interference. However, saturation by the supply current can be a problem. Remeies are a low permeability material, a gappe or open circuit core type. isavantage is the larger number of turns require to achieve the same inuctance, leaing to higher copper losses. ll this can be overcome with current-compensation. Phase an null supply currents are opposite an have equal magnitue. If both conuctors pass through the same holes in the ferrite core, the net current is theoretically zero an no saturation occurs. In other wors, these currents generate opposite fl uxes of equal magnitue that cancel out. In practice, some stray fl ux will occur. The stray fl ux paths will not coincie an these fl uxes o not cancel out. Examples of current-compensate inuctors : ring core with two winings with equal number of turns. The wining irections are such that the incoming current through one wining an the equally large outgoing current through the other generate opposite fl uxes of equal magnitue. urrentcompensation woul be almost ieal with both winings along the total circumference, one over the other. ut in practical cases each wining is place on one half of the ring core because of insulation requirements. twiste wire inuctor, which is woun with the twiste wire pair as if it were a single wire. tube or roun cable shiel shifte on a coaxial cable. fl at cable shiel, shifte on a fl at cable. Here the net current of all inuctors together is zero. In case of an I/O cable, such as coax or fl at cable, the problem will not be saturation by high current. The reason for the current-compensation is now that the actual signal is also of RF frequency an it woul be suppresse together with the interference. The current-compensate inuctor has one limitation: it is only active against common-moe interference. However the small leakage inuctance will also suppress some ifferential-moe interference. 5

8 ssortment of EMI-suppression ferrite proucts 6

9 3. Material specifications There are ifferent material categories : Manganese-zinc ferrites (MnZn) These ferrites have a high permeability but also a low resistivity an are most effective at low frequencies. The ferrites 3S3 an 3S4 have a higher resistivity an are real wieban materials as well. 3S5 has been esigne for high c bias at high temperature. Nickel-zinc ferrites (NiZn) These materials usually have a lower permeability but much higher electrical resistivity than the manganese-zinc ferrites an are effective up to 1000 MHz. 4S60 has the highest permeability an 4S3 was ae for HF suppression. Iron power Permeability of this material is also low but banwith is less than for nickel-zinc ferrites because of their low resistivity. Their main avantage is a saturation fl ux ensity which is much higher than for ferrites, so they are suitable for very high bias currents. The main material parameters are given in the table while the typical impeance curves are given in Fig. 5. For manganese zinc ferrites the frequency at which the impeance peaks, is given in Fig. 6. Main material parameters. The impeance peak frequency versus permeability curve clearly confi rms Snoek s law. For the nickel zinc ferrites the same law is vali, but at high frequency the picture is more complex. part from resonant losses, ey current losses will play an important role. They reuce the impeance at high frequencies for manganese zinc ferrites. For nickel zinc ferrites they are not very important below 100 MHz ue to the much higher resistivity. The 415 curve in Fig. 5 peaks at 100 MHz although permeability is higher than that of 31. secon complicating factor is parasitic coil capacitance. The 41 an 465 curves (measure on the same ring size an with equal number of turns for comparison) are limite by coil capacitance, whereas the 4S2 an 4S3 curves of Fig. 9 an 10 were measure on a bea (N=1) an peaks at higher frequency. Type Material µ i sat (mt) Manganese Zinc Nickel Zinc 3E8 3E7 3E6 3E5 3E26 3E S1 3S S4 31 3S3 4S S2 4S Iron Power Table 1 : 2P90 Main material parameters * Maximum 140 * operating temperature low T c ( ) ρ (Ωm)

10 New materials Some materials have been ae in recent years : Manganese-zinc ferrite 3S5 In orer to meet the EMI regulations in the frequency range from 150 khz up to 30 MHz, FERROXUE has introuce its new 3S5 EMI suppression material. lthough several ferrites are available for this frequency range, harly any material can keep its absolute value of complex permeability (efi ning the inuctor s impeance) when operating on a bias fi el ( current) at high temperature. With the introuction of 3S5, FERROXUE is fi lling this gap. pplying 3S5 in an inuctor gives EMI suppression over the full frequency range an has the major benefi t of suffi cient permeability even when high bias currents together with high temperature are applie. Preferre applications With the ever increasing eman of interference suppression, 3S5 can be applie in those applications where both high operating temperatures (140 º) an high currents are involve e.g. power lines in inustrial, but especially automotive environments. Suppressing of interference signals along these lines can be achieve by inserting 3S5-base inuctors. Suitable core shapes are those that are generally use for EMI suppression. Nickel-zinc ferrite 4S60 New EMI material 4S60 is the high permeability NiZn ferrite (µ i = 2000) with high resistivity for EMI applications in the frequency range aroun 30 MHz. ue to its high permeability, 4S60 allows reucing size, if the upgoing slope of the impeance curve is important. eing 4S60 recommene when wieban impeance is neee for noise fi lters, preferre applications are: - ine attenuation - urrent compensate chokes - ommon moe coils Manganese-zinc ferrite 3S3 FERROXUE introuces also the high frequency EMI suppression material capable to attenuate unwante interference up to 1 GHz, the 4S3 material. With the ever increasing eman of EMI suppression materials for higher frequencies, the material 4S3 completes actual FX EMI range materials proviing esigners the capability of suppressing interference up to 1GHz. eyon broaban impeance material 4S2, the 4S3 offer excellent impeance for higher frequencies being the attenuation optimum between 250 MHz an 1GHz. Fig. 3 Impeance curves at 100, measure on a toroi Ø14 x Ø9 x 5 mm with 5 turns Fig. 4 Impeance curves at 25, measure on an SM bea 3 x 3 x 4.6 mm with 5 turns 8

11 Fig. 5 Impeance versus frequency for several ferrite materials. (measure on TN14/9/5 ring cores with 5 turns) E6 permeability E E F3 3S F35 3F45 4S S S2 4S f (MHz) Fig. 6 Frequency of impeance peak for some ferrite materials. 9

12 Z(Ω) f (MHz) Fig. 7 Effect of bias current on the impeance of a 3S1 SM bea. ( measure on S3/3/4.6 beas) Z(Ω) f (MHz) Fig. 8 Effect of bias current on the impeance of a 3S5 SM bea. ( measure on S3/3/4.6 beas) 10

13 Z(Ω) f (MHz) Fig. 9 Effect of bias current on the impeance of a 4S2 SM bea. ( measure on S3/3/4.6 beas) Z(Ω) f (MHz) Fig. 10 Effect of bias current on the impeance of a 4S3 SM bea. ( measure on S3/3/4.6 beas) 11

14 4. EMI suppression prouct lines variety of shapes is use for EMI suppression (see the table below). For most of these prouct types have efi ne a stanar range with balance size istribution an logical material selection. part from the stanar range, proucts can be custom esigne to fi t aspecifi c application. Solerability an taping are in accorance with accepte IE an EI norms. thorough quality control is maintaine in all stages of the prouction process : raw materials inspection, power batch control, statistical process control (SP) an prouction batch control as fi nal inspection. Our prouction facilities are certifi e to ISO 9001 an ISO For etaile information on prouct lines, ask for the appropriate prouct brochure, see at the back. Sample boxes are available to support the esigner. Type Shape Main applications magnetically close cores magnetically open cores inuctors ferrite ring cores iron power rings tubes beas multihole cores cable shiels plate with holes ros bobbin cores beas-on-wire SM beas & chokes wieban chokes multilayer suppressors integrate inuctive components Table 2 : Prouct shapes with their main applications mains filters lamp immers roun cable shieling wire & component lea filtering wire filtering (multi-turn) roun & flat cable shieling flat cable connector shieling commutation motors in cars power line chokes P supply line / RF filtering P supply line / RF filtering omestic appliances, various P supply line / RF filtering P supply line / RF filtering Ferroxfoil absorber flexible sheet where ever raiation occurs Range of SM beas an chokes 12

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16 5. EMI suppression applications Whereas the material choice is erive from the EMI frequency ban, the core shape an way of wining are largely etermine by practical consierations an possible saturation by the loa current. ccoring to the last criterion, three application groups can be istinguishe : small signal, intermeiate an power. 5.a. Small signal applications oaxial cable shieling (roun cable shiel, tubes, ring cores) Flat cable shieling (rectangular cable shiel) If the cable carries an information signal, either analog or igital, saturation will be no issue. This is typically the case with cable shieling. Insie iameter is fi xe by the cable imensions an impeance ajuste mainly by the length an / or number of shiels. Impeance epens linearly on length an only logarithmically on the outsie imensions. The prouct can be in one piece for mounting uring manufacturing or split for retrofi t solution. split prouct uses special clamps to prevent a parasitic air gap with loss of impeance. very simple (temporal) retrofi t solution for fl exible cable is wining a few turns on a ring core of large iameter. The large inner iameter an short length (small impeance) are compensate by using more than one turn. The suppression is only common moe. able connector shieling (plate with holes) built-in suppression for the connector of a fl at cable is a ferrite plate with holes fi tting over the separate pins. The material must be nickel-zinc to prevent shortcircuit. ecause the holes are close together, this confi guration approximates the common moe confi guration of the above mentione cable shiels. 5.b. Intermeiate applications omponent lea filtering (beas) eas are small tubes especially esigne for suppression. If a specific known component is the source, e.g. a ioe causing overshoot oscillations when entering the nonconuctive state, then the bea is shifte irectly over the leas of this component. P inuctors (beas-on-wire, SM beas & chokes, gappe SM beas, multilayer suppressors, integrate inuctive components) If the source is not known, but the propagation path can be ientifie, e.g. the power supply lines or a fast igital clock line, then this line shoul be blocke. The bea has two equivalents : for through-hole mounting a beaon-wire (bea glue on a wire, axially tape an reele). for surface mounting an SM bea (bea with flat wire, blister tape an reele). larger impeance can be achieve with a multi-turn choke. For even higher attenuation either a multilayer suppressor or a complete filter can be mae by aing capacitors. SM ceramic multi-layer capacitors (M) are best suite for this purpose because of their very small lea inuctance an excellent highfrequency characteristics. 14

17 a b c SM bea () SM wieban choke (WS) SM common moe choke (MS2) EMI-suppression bea () 40 ± ± max Wieban choke (W) l ea on wire (W) SM common moe choke (MS4) a c H b Multihole core (MH6) Multihole core (MHR2) I plate (PT) H H Multihole core (MH2) H Multihole core (MHR6) Multihole core (MH2) E E Tubular cable shiel (ST) Flat cable shiel (SF) isecte flat cable shiel (SU) isecte arcae shape cable shiel (S) E E E E isecte tubular cable shiel with plastic case (S-EN) isecte flat cable shiel with plastic case (SU-EN) isecte arcae shape cable shiel with plastic case (S-EN) E a c b Multilayer suppressor (MS, MP, MN) Multilayer inuctor (MI, MH) F I G Integrate Inuctive omponent (II) H Ferroxfoil absorber sheet (FXF) Fig. 11 Overview of small signal suppression proucts 15

18 P Mains N P = phase N = null E = earth x Wire filtering (beas, two-hole cores) If only the printe circuit boar that generates the interference is known, then the wires connecting it with other system boars shoul be filtere. Wires can be filtere with a bea like component leas. To achieve more impeance, multihole cores are a goo solution. The wire is simply rawn through several holes until sufficient impeance is achieve. The system parts are not necessarily boars. In an electric shaver for instance you will fin a filter between mains plug an motor consisting often of a bea on either lea, combine with 3 capacitors. y y Fig. 12 Typical mains filter configuration. P E N oa Wieban chokes Wieban chokes are mounte on ifferent places, often not on circuit boars. Their main avantage is a combination of high impeance an large banwith. The wires are woun through holes in the core, thus separating them physically an reucing parasitic coil capacitance. Several insulate types are available to prevent short-circuit between wire bens or of wire bens with other metallic parts. 5.c. Power applications urrent-compensate chokes in mains filters (ferrite ring cores) Most equipment nowaays has switche-moe power supplies to reuce volume an weight. Electronic circuits have been miniaturise constantly an the remaining subsystems set the size limits. television set is not much more than a picture tube an a power supply. For EM purposes, a mains filter is necessary. The same hols for the electronic ballast of energy-saving fluorescent lamps. Mains filters are also manufacture as separate components. The following components can be foun in mains filters : two inuctors on the same core for low-frequency attenuation (harmonics of the switching frequency) two y capacitors for aitional common-moe attenuation (at higher frequencies) a x capacitor for ifferentialmoe attenuation H Ring core (T, TN, TX, T) Tube (TU) Ro (RO) Impeer core (IMP) s a b obbin core () c F E E core (E) E U core (U) Fig. 13 Some proucts use in power applications. 16

19 The choke has to fulfil contraicting requirements : high inuctance as well as high rate current. To prevent an unpractical choke size, current-compensation is applie to a ring core in a high-permeability material (see also section 2..). Many variations exist accoring to the specific equipment type, e.g. the compensate choke alone can be moule in the plug of TV supply cables. amp immers (iron power ring cores) Fluorescent lamps cannot be imme like incanescent lamps simply by ecreasing voltage, because below their threshol they turn off. Electronic immers use a variable part of the supply voltage perio by means of elaye thyristor ignition. The harmonics of the mains frequency require iron power i.s.o. laminate silicon iron to reuce ey current losses. On the ignition instant a parasitic ringing can be observe, of which the frequency (a few MHz) is etermine by parasitic inuctances an capacitances in the circuit. t MHz frequencies the losses of iron power are large an the ringing is issipate in a few perios. Ferrites have much less losses an woul reflect a large part of the ringing energy, which coul amage the semiconuctors of the control circuitry. Power line chokes (bobbin cores) If chokes operate on separate power lines an current-compensation is not possible, then an open core type must be chosen. To reach a high inuctance, hunres of turns can be necessary an a bobbin core is the appropriate shape. Range of multilayer suppressors in stanar EI sizes 0402 to 1812 Electric commutation motors in cars (ros) In a moern car, many electric commutation motors are applie. There are a starter motor, a fuel pump, small ventilators, screen wiper motors, winow lift motors, sun roof motors etc. The commutation is accompanie by high-frequency sparks which cause RF interference. This will be picke up by the FM raio, but if motor functions are regulate electronically, also safety is at stake. arge currents are involve, starter motor current can be as high as 40. ue to the frequency (FM ban aroun 100 MHz) the inuctance oes not have to be very large an a ro with a single layer wining is the right choice. Motor temperatures can reach 150, so the urie temperature of the ferrite shoul be well over 200, in combination with goo HF impeance behaviour. The low permeability is no problem in a ro shape. 3S3 is the ieal material for this application. 5.. Raiation suppression applications I plates an absorber sheets Thin fi lm technology an I plates provie electromagnetic shieling for multiple applications. I plates base on an ultra thin ferrite sintere into the form of a plate have been esigne specially to be attache on a PU, or any integrate circuit which requires EMI shieling to assure perfect operation. Ferroxfoil thin fi lm sheets base on an absorptive electromagnetic shieling material consist of magnetic material an resin. They suppress noise raiate from electronic equipment over a wie range of frequencies, offer fl exibility in fabrication an yiel excellent performance for many frequency ranges, being its avantages even relevant for RFI HF ban application. Other examples of its use are mobile evices incluing notebook Ps, igital cameras an cell phones, computer main boar, imaging chip. 17

20 6. esign consierations Even without any trials or calculations, a lot of problems can be avoie beforehan by goo esign practices. In orer of priority they are : avoi generating interference (minimize clock rate, smoothen pulse shape), keep it far away (separate power components an circuits from the rest) impee its propagation (minimize conuctor path length an component lea length), suppress with ferrites an capacitors. The following points shoul be consiere while taking EMI-suppression measures : The insertion of ferrite components lowers equally emission an susceptibility, the essence is blocking the propagation path. The ferrite shoul always be locate as close to the source as possible. ll intermeiate circuitry an cable length acts as antenna an prouces raiate interference. The same hols for capacitors or any type of suppression component. The ferrite an the conuctor shoul be close together. eas, tubes an cable shiels shoul fit close aroun the wire or cable an other core shapes shoul be woun tightly. If not, then stray flux is present, which converts into mutual inuctance if other circuit parts are close enough to be in the stray fiel. Especially for open core types like ros an bobbin cores, the stray flux can be a problem. obbin cores are better than ros. part from keeping istance to other circuit parts, the positioning is important. For long thin ros a horizontal position is the best. The core axis is horizontal, so the magnetic fiel is almost parallel to the P an the inuce electric fiel almost perpenicular. This results in only low inuce voltages in P tracks. For inuctors with many turns, the wining metho influences the parasitic coil capacitance. Too much capacitance causes early frequency roll-off of the impeance. Ways to reuce parasitic capacitance are multi-chamber wining (separation of turns in groups), an 90 egree crosswining (electrical ecoupling of ajacent turns). apacitors shoul always be connecte with leas as short as possible, because the leas have parasitic inuctance (in the orer of 10 nh/cm) which causes early frequency roll-off in the attenuation curve. In general filters shoul be laye-out as compact as possible. ppenix. Impeance concept.1. Material The impeance curve can be translate to a pure material curve, the so-calle complex permeability curve. s impeance consists of a reactive an a resistive part, permeability shoul have two parts too to represent this. The real part correspons to the reactance, positive for an inuctance, negative for a capacitance, an the imaginary part to the losses. Z = jω. (µ -jµ ). o = ω. µ. o + jω. µ. o Z = R + jx R = ω. µ. o, X = ω. µ. o (ω = 2. π. f) Z = (R 2 + X 2 ) = ω. o. (µ 2 + µ 2 ) Where o is the inuctance if initial permeability were equal to 1 : o = µ o. n2. e / l e (µ o = 4 π x 10-7 = x 10-6 [H/m]) For the calculation of effective magnetic imensions e an l e, see next paragraph. µ' µ'' Z µ' µ" Z fr fr frequency frequency Fig. 14 omplex permeability an impeance. 18

21 .2. ore size The choice of a suppression prouct is mae in two steps. First the material choice corresponing to the interference frequencies occurring an afterwars the right core size an turns for the impeance level require. The simplest way of calculation is taking the impeance curve of a reference core of the same material. alculation from complex permeability is another possibility, but it s more bothersome. Two factors have to be correcte : effective magnetic imensions an turns. Z :: N 2. e / l e Z = Z o. (N 2 / N o 2 ). ( e / eo ). (l eo / l e ) The parameters with inex o correspon to the reference core. The number of turns N is always an integer number. Half a turn geometrically is 1 turn magnetically. For a bea with a single wire going through, N = 1 turn. The effective magnetic imensions e (area) an l e (length) are calculate from geometric imensions accoring to IE 205. For com plicate geometries this involves complex formulas. Therefore the suppliers usually specify these ata in their hanbooks. For a cylinrical geometry (ring core, tube, bea, bea-on-wire) a simple formula applies :.3. ias current Often a supply or mains current is passing through the inuctor to allow normal operation of the connecte equipment. This current inuces a high fi el strength in the ferrite core, which can lea to saturation. Impeance then ecreases along with permeability, especially for low frequencies. The infl uence of a bias current can be calculate. The inuce fiel strength is irectly proportional to the current : H = n. I / l e Whether this fi el causes a signifi cant saturation or not, can be seen in the curve of permeability versus bias fi el. However, this only inicates the ecrease of inuctance at low frequency. The impeance iterature, Software an Sample oxes General catalogues & Software ata Hanbook : Soft ferrites an ccessories Soft Ferrites an ccessories esign Tools isk at high frequency ecreases less. gain, impeance can be calculate from reference curves if they show impeance versus frequency with bias current as a parameter. First, bias current is translate to the current that woul inuce the same fi el strength in the reference core, which means the same state of core saturation : I o = I. (n/n o ). (l eo /l e ) For a ring core, tube or bea the effective length is l e = π. ln(o/i) / (1/I-1/O) Now the relative impeance ecrease will be the same : Z bias = Z. (Z o bias / Z o ) Specific brochures Ferroxfoil fl exible sheet EMI absorber SM eas an hokes Gappe SM beas for power inuctors 3S5 the new meium frequency EMI ferrite for high bias current conitions SM wieban choke with extra metallization Wieban hokes able Shieling Power Inuctors Multilayer Suppressors an Inuctors II Integrate Inuctive omponents 3S4 a new Soft Ferrite for EMI suppression 3S3 a new Soft Ferrite for EMI suppression e / l e = h / (2. π). ln(o/i) O = outer iameter I = inner iameter h = height Sample boxes SMPEOX9 SMPEOX10 SMPEOX11 SMPEOX12 SMPEOX13 SMPEOX14 SMPEOX14 SM eas an hokes able Shieling EMI-suppression Proucts Multilayer suppressors Multilayer inuctors II II emo boar 19

22 If you require impeance graphs or other etaile prouct ata, which are not presente in this brochure, please visit our website at : 20