Comparison of different structures of ferriteEMI suppressors

Mirjana Damnjanovic, Goran Stojanovic, Ljiljana Zivanov and Vladan Desnica

Faculty of Technical Sciences, University of Novi Sad, Novi Sad, Serbia

AbstractPurpose – Present 3D electromagnetic simulators have high accuracy, but they are time and memory expensive. Because of that, fast and simpleexpression for impedance is also necessary for initial inductor design. In this paper new efficient method for total impedance calculation of ferriteelectromagnetic interference (EMI) suppressor is given. By using an algorithm, it is possible to predict correctly all variations of electrical characteristicsintroduced by varying geometry parameters of EMI suppressor.Design/methodology/approach – The starting point for calculation of electrical characteristics of EMI suppressor is Greenhouse theory. Greenhousedecomposed inductor into its constituent segments. Basically, all segments of conductive layer are divided into parallel filaments having small,rectangular cross sections. The self- and mutual-inductance were calculated using the concept of partial inductance. Total impedance of EMI suppressoris calculated taking care of dimension of chip size, material that are used and geometry of conductive layer.Findings – The Simulator for Planar Inductive Structures (SPISe) simulates effects of ferrite materials and geometrical parameters of planar inductivestructures. With proposed software tool, designers can predict performance parameters quickly and easily before costly prototypes are built. SPISesoftware offers substantially reduced time to market, and increases device performance. The computed impedances, given by our software tool arecompared with measured data and very good agreement was found.Practical implications – Applied flexible efficient methods for impedance calculation of EMI suppressor are able to significantly increase the speeddesign of multilayer suppressors for universal series bus, low-voltage differential signaling and in other high-speed digital interfaces incorporated innotebooks and personal computers, digital cameras and scanners. Also, ferrite suppressors have been successfully employed for attenuating EMI inswitching power supplies, electronic ignition systems, garage door openers, etc.Originality/value – The paper presents realized structures of ferrite EMI suppressors. New geometries of conductive layer are proposed. In addition,using simple model of inductor, the paper develops a CAD simulation tool SPISe for calculation of electrical characteristics of EMI suppressors withdifferent geometry of conductive layer.

Keywords Electromagnetism, Attenuators

Paper type Research paper

Introduction

Electromagnetic interference (EMI) is a degradation in

performance of an electronic system caused by an

electromagnetic disturbance. With the increasing amount of

electronic equipment being used together in areas where they

can affect each other, the probability of EMI becomes higher.Most noise emitted from electronic equipment is at

frequencies higher than circuit signals. Therefore, low-pass

filters, which only pass signals with frequencies lower than a

specified frequency and attenuate signals with frequencies

higher than this frequency, are generally used as EMI filters

(Murata Manufacturing Co., Ltd, 1998).Multilayer suppressors have a wide range of applications,

such as EMI suppression in universal series bus, low-voltage

differential signaling and in other high-speed digital interfaces

incorporated in notebooks and personal computers, digital

cameras and scanners. Also, ferrite suppressors have been

successfully employed for attenuating EMI in switching power

supplies, electronic ignition systems, garage door openers, etc.

They suppress harmonic noise in products with higher clock

frequencies and minimize operation errors caused by local

oscillators in mobile products.The size, performance and reliability of multilayer

components make them very suitable for high density

mounting. They are made in Electronic Industries

Association EIA standard sizes: 0402, 0603, 0805, 1206

and 1210 (Figure 1) and they have impedance between 6 and

2000V at 100MHz (Ferroxcube Products Corp., 2001).

Ferrite EMI suppressors

Ferrite components have been used for reducing or

eliminating conducted EMI on printed circuit boards in

wiring and cables for decades (Naishadham, 1999;

Mardiguian, 2000; Keenan, 2003; Amemiya et al., 2002;

Kumar, 1999).In recent years, miniaturization of electronic devices, dense

mounting of components and higher clock frequencies,The current issue and full text archive of this journal is available at

www.emeraldinsight.com/1356-5362.htm

Microelectronics International

23/3 (2006) 42–48

q Emerald Group Publishing Limited [ISSN 1356-5362]

[DOI 10.1108/13565360610680758]

The present work was partially supported by the Ministry of Science,Technology and Development, Republic of Serbia, on Project NumberMNTR-006116B.

The authors would like to thank Littelfuse Ireland Limited, Dundalk,Co. Louth, for manufacturing ferrite EMI suppressors.

42

especially in the area of communication, computing and

information technology lead to a need for very small

suppression devices. Therefore, miniature chip components

have been developed for such situation.These multilayer components consist of highly conductive

layer embedded in ferrite monolithic structure, which

provides them a good magnetic shielding. These

components have been fabricated using the ceramic

coprocessing technology. Electrical characteristics of these

components depend on the geometry of conductive material

and core, and of the permeability of the core material m.The purpose of this paper is to explore design, modeling

and characterization of different structures of coils embedded

in the soft ferrites. In order to achieve that goal, a compact

computer program Simulator for Planar Inductive Structures

(SPISe) is developed as CAD tool for calculation of electrical

properties of different structures of ferrite multilayer

suppressors.The SPISe simulates effects of ferrite materials and

geometrical parameters of planar inductive structures. With

proposed software tool, designers can predict performance

parameters quickly and easily before costly prototypes are

built. SPISe software offers substantially reduced time to

market, and increases device performance.

Influence of ferrite material

Two different soft ferrite materials are used for realization of

these ferrite EMI suppressors (MMG Neosid Ltd, 2001):1 low permeability NiZn ferrite material, denoted LP

(which is used for suppression at high frequencies above

200MHz); and2 high permeability NiZn ferrite material denoted HP

(which is used for suppression over a wide range of

frequencies from 20 to 200MHz).

The permeability of the ferrite material is a complex

parameter consisting of a real mr0 and imaginary part mr

00;both parts of the permeability are frequency dependent. They

intersect at ferrimagnetic resonant frequency, as it can be seen

in Figure 2.Owing to the presence of ferrite core, multilayer suppressor

behaves as a frequency dependent resistor. At low

frequencies, losses in inductor are low. Losses start to

increase as frequency increase; at ferrimagnetic resonant

frequency, the inductor behaves as a frequency-dependent

resistor and no longer as a true inductance. This is very

important in elimination of conducted EMI.Along with the desired insertion loss of the inductor,

parasitic effects such as inter-turn and inter-winding

capacitance, self-resonance, dielectric and magnetic losses of

the core, etc. play an important role in the design of such

component (Naishadham, 1999).

Influence of conductive paste

Outside the range of frequencies where conducted EMI has to

be eliminated, the inductor must have low losses. To obtain

that goal, for conductive layer has to be chosen material with

high conductivity.If the conductivity of conductive paste is higher, direct

resistance RDC of EMI suppressor is smaller. As the frequency

increases, magnetic losses dominate and the total impedance

of suppressor is determined by the characteristics of ferrite

material. However, the ratio of maximal total impedance

ZMAX and resistance RDC of ferrite EMI suppressor will be

bigger if conductivity of conductive paste is higher (i.e. better

EMI suppression will be achieved).The influence of three different conductive pastes: silver Ag,

platinum Pt and silver-palladium PdAg (Du Pont, 2001)

on the total impedance is observed in Damnjanovic et al.

(2004a,b).Platinum conductive paste is used for realization of

proposed EMI suppressors.

Influence of structure of conductive layer

Besides the characteristics of materials, the geometry of

conductive layer determines the total impedance, also.

Because of that, it is very important to choose appropriate

structure of suppressor.Configuration of conductive layer of simple structures of

EMI suppressors are shown in Figure 3 (before upper ferrite

layer is put on):. narrow line (width w ¼ 450mm, length l ¼ 2.01mm and

thickness t ¼ 10mm);. wide line (w ¼ 950mm, l ¼ 2.01mm, t ¼ 10mm);. zig-zag (w ¼ 50mm, length of segments l ¼ 1mm,

t ¼ 10mm and angle between segments a ¼ 758); and. meander (w ¼ 150mm, t ¼ 10mm, length of segments

l ¼ 1mm, pitch between two neighboring segments

p ¼ 450mm).

Mechanical dimensions of realized EMI suppressor are

standard EIA size 0805, as it is shown in Figure 4

Figure 1 Range of multilayer suppressors in standard EIA sizes0402-1812

Figure 2 Complex permeability vs frequency for high permeability NiZnferrite material (MMG Neosid Ltd)

Comparison of different structures of ferrite EMI suppressors

Mirjana Damnjanovic et al.

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(D ¼ 1.1mm, E ¼ 0.25mm ^ 0.075, L ¼ 2.01mm ^ 0.2,

W ¼ 1.25mm ^ 0.20).The size of suppressor determines its electrical

characteristics. The influence of dimensions of suppressor is

observed in Aharoni (1998). The calculation of

demagnetizing factor is built in simulation tool SPISe.

Calculation of electrical characteristics of EMIsuppressors

To obtain the optimal design of inductor it is much more

convenient to use some simulation tool, than make a specific

test component. Because of that, the simulation tool SPISe

for calculation of impedance of ferrite EMI suppressor is

developed.

Model of EMI suppressor

Simulation tool SPISe is built upon simple equivalent circuit

model of an inductor, which is shown in Figure 5 (Yu and

Holmes, 2002; Kim et al., 2002). The equivalent series

resistance of the inductor R is caused by losses in the

conductive material and magnetic losses. Usually, for ferrite-

core inductors the core losses dominates at RF:

R ¼ vm00

r L0; ð1Þ

where L0 is inductance of the coil with the core removed

(Reggiani et al., 2002; Bartoli et al., 1994). Inductance L isequivalent series inductance of the inductor:

L ¼ m0

rL0: ð2Þ

The equivalent lumped capacitance of the inductor Crepresents the parasitic effects of the conductive material.Using the equivalent circuit of an inductor the total

impedance Z can be determined as:

Zð jvÞ ¼ R þ j · ðvLð12 v2LCÞ2 vR2CÞð12 v2LCÞ2 þ v2R2C2

: ð3Þ

From the expressions (1)-(3), it can be shown that the totalimpedance of the inductor Z can be increased by increasingL0 (e.g. by optimal design of the geometry of the conductive

layer), or by choosing the appropriate ferrite material.

Simulation tool SPISe

The calculation of impedance of zig-zag or meander structure

of inductor is very complex. Therefore, the segments ofmeander and zig-zag structure are divided into parallelfilaments having small, rectangular cross sections. Taking care

of the current flow, to every filament is assigned vector, as itcan be seen in Figure 6. The self- and mutual-inductancewere calculated using the concept of partial inductance

(Ruehli, 1972; Ruehli et al., 1995; Greenhouse, 1974).For straight conductor of rectangular cross section self-

inductance is:

Li ¼ 2 · l · ðln 2l

w þ tþ 0:25049þ w þ t

3lþ m

4T Þ ðnHÞ; ð4Þ

where Li is the inductance in (nH); l, w and t are length, widthand thickness of the conductor in (cm), respectively, m is the

Figure 3 Realized structures of EMI suppressor

Figure 4 Mechanical dimensions of realized EMI suppressor –standard EIA size 0805

Figure 5 Equivalent circuit model of an inductor

Figure 6 Calculation of total impedance of meander coil by using theconcept of partial inductance

Comparison of different structures of ferrite EMI suppressors

Mirjana Damnjanovic et al.

Microelectronics International

Volume 23 · Number 3 · 2006 · 42–48

44

conductor’s permeability, and T is a frequency-correction

parameter.The mutual inductance between two straight parallel

conductors of rectangular cross section can be calculated as:

Lij ¼ 2l lnl

GMDþ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ l

GMD

� �2s0

@1A

0@

2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ GMD

l

� �2s

þGMD

l

1A;

ð5Þ

where l is the conductors’ length, GMD is geometric mean

distance between the conductors:

lnGMD ¼ ln d 21

12

d

w

� �2

þ 1

60

d

w

� �4

þ 1

168

d

w

� �6

þ 1

360

d

w

� �8

þ 1

660

d

w

� �10

þ. . .

!:

ð6Þ

Assume that a conductive layer of suppressor is divided into n

filaments with rectangular cross sections. Then the total

inductance of structure L can be calculated as the sum of

partial self-inductances of all elementary filaments and the

sum of all mutual inductances between all elementary

filaments:

L ¼Xn

i¼1

Li þXn

i¼1

Xn

j¼1

Lij ; where i – j: ð7Þ

Using these formulae the simulation tool SPISe was

developed. Computation concept of electrical characteristics

of ferrite EMI suppressor is described in Stojanovic et al.

(2006), where is explained with more details calculation of its

resistance and capacitance.Some basic steps of the algorithm of simulation tool SPISe

are shown in Figure 7. Firstly, designer has to choose chip

size, ferrite material, conductive paste and structure of EMI

suppressor. Then, user has to set desired values for

dimensions of selected structure. Based on all initial values

of input data, the simulation tool SPISe will make test if the

values of input parameters are correct (in allowed range).

Then, if everything is correct, SPISe will determine the

maximal number of turns Nmax for meander and zig-zag

structure. User can choose number of turns N not greater

than Nmax.Subroutine KOORD will determine set of vectors, which

will describe selected structure of EMI suppressor (as it can

be seen for meander structure in Figure 6). After that,

calculation of self- and mutual-inductance are conducted

(subroutines LS and M, respectively), and of all other

electrical parameters of EMI suppressor.The simulation tool SPISe offers many possibilities to

design the EMI suppressor with best performance. This

simulation tool can be used for calculation of electrical

characteristics of more complex structures (Damnjanovic

et al., 2004a,b; Raghavendra et al., 2004).

Results of simulation and measurement

In order to design EMI suppressors with the best

performance, with taking care of range of unwanted

frequencies that can occur, designer have to selectappropriate conductive paste and ferrite material. In this

paper, simulation results obtained for four different EMI

suppressor structures and two different ferrite materials willbe compared with measurement results.Realized EMI suppressors were electrically tested in the

range of frequencies from 1MHz to 3GHz using Agilent4287A RF LCR meter.Comparison of measured and calculated values of total

impedance Z for narrow line in low permeability LP ferritematerial is shown in Figure 8. At low frequencies (up to

3MHz), losses in suppressor are low. As frequency increases,

series resistance RS dominates and suppressor behaves as afrequency-dependent resistor. This suppressor can be used for

suppression of unwanted frequencies above 200MHz.

Figure 7 Algorithm of simulation tool SPISe

Comparison of different structures of ferrite EMI suppressors

Mirjana Damnjanovic et al.

Microelectronics International

Volume 23 · Number 3 · 2006 · 42–48

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If the conductor’s line is wider, for the same ferrite materialLP, inductance L and dc resistance RDC decrease. Because ofthat, series resistance RS and reluctance XL decrease, andhence, total impedance Z will be smaller.On the other hand, if the conductive line is longer, than

inductance L0 is bigger (see zig-zag and meander), andinductance L and impedance Z of structure will be bigger. InFigure 9 and Figure 10 simulation and measurement results

of inductance L and total impedance Z for all four structuresin LP soft ferrite material are compared, except for meander,because the measurement was not successful.As it can be seen from these figures, very good agreement

between measured (dot line) and calculated values (solid line)of inductance L and total impedance Z were found for lowpermeability LP ferrite material. Difference was less than

5 percent.Note that dependence of total impedance vs frequency

Z ¼ f(freq) for low permeability soft ferrite material have the

same shape for all structures (narrow and wide line, zig-zagand meander). At high frequencies the shape of dependenceZ ¼ f(freq) is determined by the characteristics of ferritematerial (equations (1) and (2)):

Z ¼ R þ jvL < jvL0ðm0

r 2 jm00

r Þ: ð8Þ

The same conclusion can be made for dependence ofinductance vs frequency L ¼ f(freq) in LP ferrite (Figure 9).

The shape of dependence L ¼ f(freq) is determined by the

characteristics of ferrite material (equation (2)).In Figure 11 and Figure 12 simulation and measurement

results for HP soft ferrite material are presented, except for

narrow line, because the measurement was not successful.

This nickel-zinc ferrite has low loss factor at low frequencies

(up to 1MHz), while high suppression impedance over

100MHz.

Figure 8 Simulated values of RS, XL and Z and measured values of Z fornarrow line in LP ferrite

Figure 9 Measured and calculated inductance for all structures in LPferrite material

Figure 10 Measured and calculated total impedance for all structuresin LP ferrite material

Figure 11 Measured and calculated inductance for all structures in HPferrite material

Figure 12 Measured and calculated total impedance for all structuresin HP ferrite material

Comparison of different structures of ferrite EMI suppressors

Mirjana Damnjanovic et al.

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As it can be seen from this figures, dependence of total

impedance vs frequency Z ¼ f(freq) for high permeability

ferrite material have the same shape for all structures. This

dependence Z ¼ f(freq) is determined by the characteristics of

HP ferrite material. Good agreements between measured and

calculated values were found. Difference between measured

and calculated values was less than 10 percent.In Table I, comparison of measured and simulated values of

RDC, ZMAX, ZMAX/RDC for simple EMI suppressor structures

is presented.If calculated values of total impedance Z for the same

structure of conductive layer and two different ferrite

materials are compared (e.g. wide line), it can be noticed

that LP material has better ratio of ZMAX and RDC (RDC is

smaller and ZMAX is bigger).

Conclusion

Ferrite EMI suppressors consist of highly conductive layer

embedded in a ferrite monolithic structure. The total

impedance of the suppressor depends of permeability of

core material m and of geometry of the conductive material

and core.The influence of two ferrite materials (low LP and high HP

permeability material) and four structures of conductive layer

(narrow and wide line, zig-zag and meander) on electrical

characteristics of EMI suppressors is presented in this paper.If the conductor’s line is wider, inductance L and dc

resistance RDC decrease. Because of that, series resistance RS

and reluctance XL decrease, and hence, total impedance Z

will be smaller. On the other hand, if conductive line is longer

(like for zig-zag and meander structure), than inductance L0 is

bigger, and inductance L and impedance Z of structure will be

bigger. The biggest inductance and impedance has meander,

then zig-zag, narrow line and wide line.In order to determine electrical characteristics of EMI

suppressors simulation tool SPISe is developed. With

proposed software tool, designers can predict performance

parameters quickly and easily before costly prototypes are

built. SPISe software offers substantially reduced time to

market, and increases device performance.

In addition, realized EMI suppressors were electricallytested. Results of measurements and calculation of electricalcharacteristics of EMI suppressors have been compared. Agood agreement was found.These results will be very useful for construction of the

ferrite EMI suppressors with optimal performance.

References

Aharoni, A. (1998), “Demagnetizing factors for rectangularferromagnetic prism”, Journal of Applied Physics, Vol. 83,pp. 3432-4.

Amemiya, F., Takagi, K., Kuwebara, N., Hamada, S. andIwamoto, Y. (2002), “Developing a common-mode chokecoil with high-permeability core used high-speedtelecommunications port for UTP cable”, IEEEInternational Symposium on EMC, Vol. 1, pp. 314-9.

Bartoli, M., Reatti, A. and Kazimierczuk, M.K. (1994),“High frequency models of ferrite core inductors”, IEEETransactions on Magnetics, pp. 1670-5.

Damnjanovic, M., Zivanov, L. and Stojanovic, G. (2004a),“Influence of geometry of conductive layers and differentferrites on impedance of EMI suppressor”, paper presentedat the 16th International Conference on ElectricalMachines – ICEM 2004, PS6-28.

Damnjanovic, M., Zivanov, L., Stojanovic, G. and Desnica,V. (2004b), “Modeling and simulation of impedance offerrite EMI suppressor with two conductive layers”, IEEE24th International Conference on Microelectronics MIEL 2004,Vol. 1, pp. 245-8.

DuPont (2001), technical information available at: www.dupont.com

Ferroxcube Products Corp. (2001), “Multilayer suppressorsand inductors”, available at: www.ferroxcube.com

Greenhouse, H.M. (1974), “Design of planar rectangularmicro-electronic inductors”, IEEE Transactions on Parts,Hybrids and Packages, Vol. PHP 10, pp. 101-9.

Keenan, A. (2003), “Board level EMI suppression usingferrite components”, IEEE International Symposium onEMC, Vol. 2, pp. 1252-4.

Kim, T.H., Lee, J., Kim, H. and Kim, J. (2002), “3GHz widefrequency model ferrite bead for power/ground noisesimulation of high-speed PCB”, paper presented at IEEEConference on Electrical Performance of ElectronicPackaging, October 21-23, pp. 217-20.

Kumar, K. (1999), “Leadless devices for EMI suppression”,Proceedings International Conference on Electro-magneticInterference and Compatibility, pp. 343-56.

Mardiguian, M. (2000), EMI Troubleshooting Techniques,McGraw-Hill, New York, NY.

MMG Neosid Ltd (2001), “Soft ferrite materials”, availableat: www.mmg-neosid.com

Murata Manufacturing Co., Ltd (1998), “Basics of EMIfilters”, available at: www.murata.com

Naishadham, K. (1999), “A rigorous experimentalcharacterization of ferrite inductors for RF noisesuppression”, paper presented at IEEE Radio andWireless Conference, RAWCON 99, August 1-4, pp. 271-3.

Raghavendra, R., Bellew, P., Mcloughlin, N., Stojanovic, G.,Damnjanovic, M., Desnica, V. and Zivanov, L. (2004),“Characterization of novel varistor þ inductor integratedpassive devices”, IEEE Electron Devices Letters, Vol. 25No. 12, pp. 778-80.

Table I Measured and calculated values of RDC, ZMAX, ZMAX/RDC andonly calculated for narrow line in HP and meander in LP material

Measurement Calculation

RDC (V) ZMAX (V)

ZMAX/

RDC RDC (V) ZMAX (V)

ZMAX/

RDC

LP ferrite at 1.65 GHz at 1.62 GHz

Narrow line 1.15 45.7 41.54 1.08 46.44 43.00

Wide line 0.40 29.5 74.31 0.38 29.98 78.89

Zig-zag 4.87 88.9 18.25 5.31 90.58 17.06

Meandera – – – 7.99 120.22 15.05

HP ferrite at 0.81 GHz at 0.81 GHz

Narrow linea – – – 2.06 33.8 16.42

Wide line 1.40 24.3 17.36 1.34 25.46 19.00

Zig-zag 7.23 76.4 10.51 6.72 71.11 10.58

Meander 10.70 101.0 9.44 9.89 90.48 9.14

Note: aFor narrow line in HP material measurement was not successful

Comparison of different structures of ferrite EMI suppressors

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Reggiani, U., Grandi, G., Sancineto, G., Kazimierczuk, M.K.and Massarini, A. (2002), “High-frequency behavior oflaminated iron-core inductors for filtering applications”,IEEE Transactions on Magnetics, pp. 654-60.

Ruehli, A.E. (1972), “Inductance calculations in a complexintegrated circuit environment”, IBM Journal Res. Develop.,Vol. 16, pp. 470-82.

Ruehli, A., Paul, C. and Garret, J. (1995), “Inductancecalculations using partial inductances and macromodels”,IEEE Proceedings International Symposium on EMC, August14-18, pp. 23-7.

Stojanovic, G., Damnjanovic, M., Desnica, V., Zivanov, L.,Raghavendra, R., Bellew, P. and Mcloughlin, N. (2006),“High performance zig-zag and meander inductors

embedded in ferrite material”, Journal of Magnetism and

Magnetic Materials, Vol. 297 No. 2, pp. 76-83.Yu, Q. and Holmes, T.W. (2002), “RF circuit modeling of

ferrite-core inductors and characterization of core

materials”, IEEE Transactions on Electromagnetic

Compatibility, Vol. 44 No. 1, pp. 258-63.

Corresponding author

Mirjana Damnjanovic can be contacted at: mirad@uns.ns.

ac.yu

Comparison of different structures of ferrite EMI suppressors

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