The use of soft ferrites for EMI suppression

The use of soft ferrites
for interference suppression
Application Note
A
Y A G E O
C O M P A N Y
The Use of Soft Ferrites
for
Interference Suppression
Contents
Introduction
1
General principles of EMC
2
EMC regulations
2
Material specifications
7
EMI-suppression product lines
12
Applications
14
Design considerations
18
Impedance concept
18
Literature, software and sample boxes
19
The most important regulations
are the European Norms (EN)
which are applicable in all European
In the field of electromagnetic
compatibility several trends attribute Union (EU) and European Free
Trade Associated (EFTA) countries,
to a growing necessity of EMC
FCC in United States and VCCI in
engineering.
Japan. The uniform legislation in the
European Union is along the lines of
In signal processing :
the EMC directive 89/336/EEC. For
• Change from analog to digital
every product to which no specific
(steep pulse edges, overshoot,
European norm applies, a general
ringing).
regulation is mandatory. These are
• Increase of clock frequencies.
the so called Generic Requirements
(residential, commercial and light
In power conversion :
industry: EN 61000-6-3 for emissions
• Change from linear to switchedand EN 61000-6-1 for immunity).
mode supplies (high switching
This includes all electric and
frequency, harmonics).
electronic products, no matter how
• Increase of switching frequencies.
trivial they seem to be !
1. Introduction
These trends, directed to functional
upgrading or reducing cost, inevitably
also contribute to an increasing level
of electromagnetic interference
(EMI) emissions. Together with the
increasing use of electronics this
leads to a general EMC degradation.
As a consequence, EMC legislation is
getting world-wide more strict.
Of course the first step to avoid
interference problems is a good
design practice, to tackle the
problem right from the start. This
can be insufficient if the interference
is directly related to the inherent
operating principle and too late if the
interference is detected not earlier
1
Ferroxcube
than in the final design phase. In such
cases extra suppression components
are necessary, like ferrites, capacitors
or shielding elements.
Ferrites provide a solution to
many problems of conducted and
(indirectly) radiated interference.
They can be applied almost
anywhere :
• Shifted on wire or cable as beads,
tubes or cable shields.
• Mounted on PCB as beads-on-wire,
wideband chokes, SMD inductors,
multilayer suppressors or
integrated inductive components.
• Ring cores or U cores in mains
filters, in the circuit, in a separate
box or moulded in a connector.
• Wideband chokes or coiled rod
inductors in electrical appliances or
motors.
No ground connections are
necessary as ferrites are connected
in series with the interfering circuit
and not in parallel as in the case
of a capacitor. The wideband, lossy
impedance makes ferrites well-suited
as RF suppressor component.
2. General principles
of EMC
they are very fast, harmonic
disturbances if the basic frequency is
high or if the deviation from a sine
wave is considerable.
2.a. Regulations
Historically, all EMI regulations stated
emission limits only. These define
the maximum level of interference
allowed as a function of frequency.
In case of conducted interference it
applies to the voltage on all inputs
and outputs of the equipment, in
case of radiated interference it
applies to the field strength at a
certain distance. Often two levels are
stated:
• Class A for commercial and industrial areas.
• Class B for domestic and residential areas.
Class B is always stricter than
class A. Also immunity is becoming
subject of regulation. Taking into
account the severity of the EMC
problem, equipment must also be
able to operate without functional
degradation in a minimum EMI
ambient. The difference between
the actual level of emissions or
susceptibility and the EMC limits is
the required attenuation by filtering
or shielding.
Common-mode :
Phase and null interference voltages
are equal. This is likely to occur if
phase and null are close together
and interference is coupling in from
an external field (radiation or crosstalk).
Differential-mode :
Phase and null interference voltages
have opposite phase angle but equal
magnitude. This is likely to occur
in case of switching equipment
connected to the mains. In general
a combination of both types can be
present.
Interferences can propagate as an
electromagnetic wave in free space.
Suppression then requires shielding
with conductive materials. Also
propagation occurs via conductive
paths such as the mains network,
to which the majority of electrical
equipment is connected.
Below 30 MHz this is the main
propagation mode. Suppression
is done with a high impedance in
series (inductor), a low impedance in 2.c. Suppression with ferrites
parallel (capacitor) or a combination At RF frequencies a ferrite inductor
of both (filter).
shows a high impedance which
suppresses unwanted interference.
Propagation via the mains can
The resulting voltage over the
take place in two different modes :
load impedance will be lower than
common and differential mode. Apart without suppression component, the
from phase and null which carry the ratio of the two is the insertion loss,
supply current, there is the safety
see Fig. 2.
earth connection, which is generally
taken as a reference.
ZG
ZS
ZL
2.b. Sources and propagation
The source determines whether the
interference is a transient or random
variation in time (commutation
motors, broadcast transmitters etc.)
or a periodic signal (e.g. switchedmode power supplies). The frequency
spectrum will be continuous in the
first case and a line spectrum in the
second. In practice, the minimum
and maximum frequency involved
are much more relevant and both
types of sources can be broadband.
Random variations are broadband if
ZG
ZL
Eo
Fig. 2 Insertion loss of an inductor.
2
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E
dBµV
dB
level in dB/V
80
quasi peak
75
average
70
65
60
55
50
45
40
0
1
10
f (MHz)
Fig. 1a European generic emission norm 61000-6-3 (residential, commercial, light industry).
dBµV
dB
level in dB/V
80
quasi peak
75
average
70
65
60
55
50
45
40
0
1
10
f (MHz)
Fig. 1b European generic emission norm 61000-6-1 (industrial environment).
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Ferroxcube
The insertion loss is expressed as :
critical interference frequencies;
impedance and interference
ideally they should coincide with
amplification! A resistor cannot
the
ferrimagnetic
resonance
resonate
and is reliable independent
IL = 20 . log10 (Eo/E) [dB]
frequency, the top of the impedance
of source and load impedances.
curve. According to Snoek’s
• Secondly, a resistance dissipates
|ZG+ZL+ZS|
law, this resonant frequency is
.
interfering signals rather than
= 20 log10
[dB] inversely proportional to the initial
reflecting them to the source.
permeability, which gives us a guide
|ZG+ZL|
Small oscillations at high frequency
for material choice. The higher the
The decibel (dB) as a unit is practical interference frequency, the lower the can damage semiconductors or
because interference levels are also
negatively affect circuit operation
material permeability should be. The
expressed in dB. However insertion whole RF spectrum can be covered
and therefore it is better to absorb
loss depends on source and load
them.
with a few materials if the right
impedance, so it is not a pure
permeability steps are chosen.
product parameter like impedance
At the resonant frequency and above, • Thirdly, the shape of the impedance
(Z). In the application, source and
curve changes with the material
the impedance is largely resistive,
load impedance generally are not 50 which is a favourable characteristic
losses. A lossy material will show a
Ω resistive. They might be reactive,
smooth variation of impedance
of ferrites.
frequency dependent and quite
with frequency and a real wideband
• Firstly, a low-loss inductance can
different from 50 Ω.
attenuation. Interferences often
resonate with a capacitance in
Conclusion : insertion loss is
have a wideband spectrum to
series (positive and negative
a standardized parameter for
suppress.
reactance), leading to almost zero
comparison, but it will not predict
directly the attenuation in the
application.
At low frequency, a ferrite
inductor is a low-loss, constant
self-inductance. Interferences
occur at elevated frequencies and
there the picture changes. Losses
start to increase and at a certain
frequency, the ferrimagnetic resonant
frequency, permeability drops
rapidly and the impedance becomes
almost completely resistive. At
higher frequencies it even behaves
like a lossy capacitor. While for
most applications the operating
frequency should stay well below
this resonance, effective interference
suppression is achieved up to much
higher frequencies. The impedance
peaks at the resonant frequency
and the ferrite is effective in a
wide frequency band around it. The
material choice follows from the
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Ferroxcube
2.d. Current-compensation
Ferrite inductors inserted separately
in both lines suppress both common
and differential mode interference.
However, saturation by the supply
current can be a problem. Remedies
are a low permeability material, a
gapped or open circuit core type.
Disadvantage is the larger number of
turns required to achieve the same
inductance, leading to higher copper
losses. All this can be overcome with
current-compensation. Phase and
null supply currents are opposite
and have equal magnitude. If both
conductors pass through the same
holes in the ferrite core, the net
current is theoretically zero and no
saturation occurs. In other words,
these currents generate opposite
fluxes of equal magnitude that cancel
• A tube or round cable shield shifted
out.
In practice, some stray flux will occur. on a coaxial cable.
The stray flux paths will not coincide
• A flat cable shield, shifted on a flat
and these fluxes do not cancel out.
cable. Here the net current of all
inductors together is zero.
Examples of current-compensated inductors :
• A ring core with two windings with
equal number of turns. The winding
directions are such that the
incoming current through one
winding and the equally large
outgoing current through the
other generate opposite fluxes
of equal magnitude. Currentcompensation would be almost
ideal with both windings along the
total circumference, one over the
other. But in practical cases each
winding is placed on one half of
the ring core because of insulation
requirements.
In case of an I/O cable, such as coax
or flat 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 and it would be
suppressed together with the interference. The current-compensated
inductor has one limitation: it is only
active against common-mode interference. However the small leakage
inductance will also suppress some
differential-mode interference.
• A twisted wire inductor, which is
wound with the twisted wire pair
as if it were a single wire.
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Ferroxcube
Assortment of EMI-suppression ferrite products
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Ferroxcube
3. Material specifications
There are different material categories :
• Manganese-zinc ferrites (MnZn)
These ferrites have a high
permeability but also a low
resistivity and are most effective at
low frequencies. The ferrites 3S3
and 3S4 have a higher resistivity
and are real wideband materials
as well. 3S5 has been designed for
high dc 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 and are
effective up to 1000 MHz. 4S60 has
the highest permeability and 4S3
was added for HF suppression.
• Iron powder
Permeability of this material is also
low but bandwidth is less than for
nickel-zinc ferrites because of their
low resistivity. Their main advantage
is a saturation flux density 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
impedance curves are given in Fig.
5. For manganese zinc ferrites the
frequency at which the impedance
peaks, is given in Fig. 6.
Main material parameters.
The impedance peak frequency
versus permeability curve clearly
confirms Snoek’s law. For the nickel
zinc ferrites the same law is valid,
but at high frequency the picture is
more complex. Apart from resonant
losses, eddy current losses will play
an important role. They reduce the
impedance at high frequencies for
manganese zinc ferrites. For nickel
zinc ferrites they are not very
important below 100 MHz due to
the much higher resistivity. The 4A15
curve in Fig. 5 peaks at 100 MHz
although permeability is higher than
that of 3B1. A second complicating
factor is parasitic coil capacitance.
The 4B1 and 4C65 curves (measured
on the same ring size and with equal
number of turns for comparison) are
limited by coil capacitance, whereas
the 4S2 and 4S3 curves of Fig. 9 and
10 were measured on a bead (N=1)
and peaks at higher frequency.
Type
Material
µi
Bsat
(mT)
Tc
(°C)
ρ
(Ωm)
Manganese
Zinc
3E8
3E7
3E6
3E5
3E26
3E27
3C11
3S1
3S5
3C90
3S4
3B1
3S3
18000
15000
12000
10000
7000
6000
4300
4000
3800
2300
1700
900
250
350
400
400
400
450
400
400
400
545
450
350
400
350
100
130
130
120
155
150
125
125
255
220
110
150
200
0.1
0.1
0.1
0.5
0.5
0.5
1
1
10
5
103
0.2
104
Nickel
Zinc
4S60
4A15
4S2
4S3
4C65
2000
1200
700
250
125
260
350
350
350
350
100
125
125
250
350
105
105
105
105
105
Iron PowderTable 1 : 2P90
90
Main material parameters.
1600
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Ferroxcube
140 * operating temperature
low
* Maximum
New materials
Preferred applications
With the ever increasing demand
of interference suppression, 3S5
can be applied in those applications
• Manganese-zinc ferrite 3S5
where both high operating temperaIn order to meet the EMI regulations tures (140 ºC) and high currents
in the frequency range from 150
are involved e.g. power lines in
kHz up to 30 MHz, FERROXCUBE
industrial, but especially automotive
has introduced its new 3S5 EMI sup- environments. Suppressing of interpression material. Although several
ference signals along these lines can
ferrites are available for this frequen- be achieved by inserting 3S5-based
cy range, hardly any material can
inductors. Suitable core shapes are
keep its absolute value of complex
those that are generally used for EMI
permeability (defining the inductor’s suppression.
impedance) when operating on a
bias field (DC current) at high tem• Nickel-zinc ferrite 4S60
perature. With the introduction of
New EMI material 4S60 is the high
3S5, FERROXCUBE is filling this gap. permeability NiZn ferrite (µi = 2000)
with high resistivity for EMI applicaApplying 3S5 in an inductor gives
tions in the frequency range around
EMI suppression over the full frequency range and has the major ben- 30 MHz. Due to its high permeability, 4S60 allows reducing size, if
efit of sufficient permeability even
the upgoing slope of the impedance
when high bias currents together
curve is important.
with high temperature are applied.
Some materials have been added in
recent years :
Fig. 3 Impedance curves at 100 °C,
measured on a toroid Ø14 x Ø9 x 5 mm
with 5 turns
Being 4S60 recommended when
wideband impedance is needed for
noise filters, preferred applications
are:
- Line attenuation
- Current compensated chokes
- Common mode coils
• Manganese-zinc ferrite 3S3
FERROXCUBE introduces also
the high frequency EMI suppression material capable to attenuate
unwanted interference up to 1 GHz,
the 4S3 material.
With the ever increasing demand of
EMI suppression materials for higher
frequencies, the material 4S3 completes actual FXC EMI range materials providing designers the capability
of suppressing interference up to
1GHz. Beyond broadband impedance
material 4S2, the 4S3 offer excellent impedance for higher frequencies being the attenuation optimum
between 250 MHz and 1GHz.
Fig. 4 Impedance curves at 25 °C,
measured on an SMD bead 3 x 3 x 4.6 mm
with 5 turns
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Ferroxcube
Fig. 5 Impedance versus frequency for several ferrite materials. (measured on TN14/9/5 ring cores with 5 turns)
permeability
10000
3E6
9000
3E5
8000
7000
6000
3E25
5000
3C11
4000
3S5
3000
4S60
3C90
2000
3C92
1000
3F3
3F35
4A15
3F45
4S2 4S3
3B1
0
1
3S4
10
100
4B1
f (MHz)
Fig. 6 Frequency of impedance peak for some ferrite materials.
9
Ferroxcube
4C65
1000
Z(Ω)
50
45
0.0 A
0.5 A
1.0 A
2.0 A
3.0 A
40
35
30
25
20
15
10
5
0
10
1
100
f (MHz) 1000
Fig. 7 Effect of bias current on the impedance of a 3S1 SMD bead. ( measured on BDS3/3/4.6 beads)
Z(Ω)
50
45
0.0 A
0.2 A
0.3 A
0.5 A
1.0 A
2.0 A
3.0 A
40
35
30
25
20
15
10
5
0
1
10
100
f (MHz) 1000
Fig. 8 Effect of bias current on the impedance of a 3S5 SMD bead. ( measured on BDS3/3/4.6 beads)
10
Ferroxcube
Z(Ω)
60
0.0 A
0.2 A
0.3 A
0.5 A
1.0 A
2.0 A
3.0 A
50
40
30
20
10
0
10
1
100
f (MHz) 1000
Fig. 9 Effect of bias current on the impedance of a 4S2 SMD bead. ( measured on BDS3/3/4.6 beads)
Z(Ω)
90
80
0.0 A
0.5 A
1.0 A
2.0 A
3.0 A
70
60
50
40
30
20
10
0
1
10
100
f (MHz) 1000
Fig. 10 Effect of bias current on the impedance of a 4S3 SMD bead. ( measured on BDS3/3/4.6 beads)
11
Ferroxcube
can be custom designed to fit
aspecific application. Solderability
and taping are in accordance with
accepted IEC and EIA norms.
A thorough quality control is
maintained in all stages of the
production process : raw materials
inspection, powder batch control,
statistical process control (SPC) and
production batch control as final
inspection. Our production facilities
are certified to ISO 9001 and ISO
14001. For detailed information
on product lines, ask for the
appropriate product brochure,
see at the back. Sample boxes are
available to support the designer.
Type
Shape
Main applications
magnetically closed cores
ferrite ring cores
iron powder rings
tubes
beads
multihole cores
cable shields
plate with holes
rods
bobbin cores
beads-on-wire
SMD beads & chokes
wideband chokes
multilayer suppressors
integrated inductive components
flexible sheet
mains filters
lamp dimmers
round cable shielding
wire & component lead filtering
wire filtering (multi-turn)
round & flat cable shielding
flat cable connector shielding
commutation motors in cars
power line chokes
PCB supply line / RF filtering
PCB supply line / RF filtering
domestic appliances, various
PCB supply line / RF filtering
PCB supply line / RF filtering
where ever radiation occurs
4. EMI suppression
product lines
A variety of shapes is used for EMI
suppression (see the table below).
For most of these product types
Ferroxcube have defined a standard
range with balanced size distribution
and logical material selection. Apart
from the standard range, products
magnetically open cores
inductors
Ferroxfoil absorber
Table 2 : Product shapes with their main applications
Range of SMD beads and chokes
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Ferroxcube
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Ferroxcube
5. EMI suppression
applications
Whereas the material choice is
derived from the EMI frequency
band, the core shape and way of
winding are largely determined
by practical considerations and
possible saturation by the load
current. According to the last
criterion, three application groups
can be distinguished : small signal,
intermediate and power.
5.a. Small signal applications
• Coaxial cable shielding
(round cable shield, tubes, ring
cores)
• Flat cable shielding
(rectangular cable shield)
If the cable carries an information
signal, either analog or digital,
saturation will be no issue. This
is typically the case with cable
shielding. Inside diameter is fixed by
the cable dimensions and impedance
adjusted mainly by the length
and / or number of shields.
Impedance depends linearly on
length and only logarithmically on
the outside dimensions. The product
can be in one piece for mounting
during manufacturing or split for
retrofit solution. A split product uses
special clamps to prevent a parasitic
air gap with loss of impedance. A
very simple (temporal) retrofit
solution for flexible cable is winding
a few turns on a ring core of large
diameter. The large inner diameter
and short length (small impedance)
are compensated by using more than
one turn. The suppression is only
common mode.
• Cable connector shielding
(plate with holes)
A built-in suppression for the
connector of a flat cable is a ferrite
plate with holes fitting over the
separate pins. The material must
be nickel-zinc to prevent shortcircuit. Because the holes are
close together, this configuration
approximates the common mode
configuration of the above
mentioned cable shields.
5.b. Intermediate applications
• Component lead filtering
(beads)
Beads are small tubes especially
designed for suppression. If a specific
known component is the source,
e.g. a diode causing overshoot
oscillations when entering the nonconductive state, then the bead is
shifted directly over the leads of this
component.
• PCB inductors
(beads-on-wire, SMD beads
& chokes, gapped SMD beads,
multilayer suppressors, integrated
inductive components)
If the source is not known, but the
propagation path can be identified,
e.g. the DC power supply lines or a
fast digital clock line, then this line
should be blocked. The bead has two
equivalents :
• for through-hole mounting a beadon-wire (bead glued on a wire,
axially taped and reeled).
• for surface mounting an SMD bead
(bead with flat wire, blister taped
and reeled).
A larger impedance can be achieved
with a multi-turn choke. For even
higher attenuation either a multilayer
suppressor or a complete filter
can be made by adding capacitors.
SMD ceramic multi-layer capacitors
(CMC) are best suited for this
purpose because of their very small
lead inductance and excellent highfrequency characteristics.
14
Ferroxcube
A
a
b
D
L
A
B
c
C
C
B
SMD bead (BD)
40
±5
EMI-suppression bead (BD)
D
40
±5
10
d
SMD common mode choke (CMS2)
SMD wideband choke (WBS)
A
d
6
C
L
∅0.6
B
l
14
max
SMD common mode choke (CMS4)
Bead on wire (BDW)
Wideband choke (WBC)
c
a
d
H
d
L
b
D
L
Multihole core (MHC6)
D
Multihole core (MHR2)
IC plate (PLT)
d
d
d
H
D
L
H
L
D
H
L
Multihole core (MHR6)
Multihole core (MHB2)
D
Multihole core (MHC2)
E
B
C
D
L
D
B
E
A
B
A
D
A
d
Tubular cable shield (CST)
C
Bisected arcade shaped cable shield (CSA)
Flat cable shield (CSF)
C
Bisected flat cable shield (CSU)
E
E
B
C E
B
B
A
C
A
D
A
E
B
B
D
B
C
A
C
D
C
D
C
A
A
D
Bisected flat cable shield
with plastic case (CSU-EN)
Bisected tubular cable shield with
plastic case (CSC-EN)
Bisected arcade shaped cable shield
with plastic case (CSA-EN)
a
B
c
A
C
D
E
B
C
D
b
A
F
Multilayer suppressor (MLS, MLP, MLN)
Multilayer inductor (MLI, MLH)
I
G
H
Integrated Inductive Component (IIC)
Fig. 11 Overview of small signal suppression products
15
Ferroxcube
Ferroxfoil absorber sheet (FXF)
5.c. Power applications
L
P
P
Cy
Mains
• Current-compensated chokes
in mains filters
(ferrite ring cores)
Most equipment nowadays has
switched-mode power supplies
to reduce volume and weight.
Electronic circuits have been
miniaturised constantly and the
remaining subsystems set the size
limits. A television set is not much
more than a picture tube and a
power supply. For EMC purposes, a
mains filter is necessary. The same
holds for the electronic ballast of
energy-saving fluorescent lamps.
Mains filters are also manufactured
as separate components.
Cx
E Load
Cy
N
N
L
P = phase
N = null
E = earth
Fig. 12 Typical mains filter configuration.
• Wire filtering
(beads, two-hole cores)
If only the printed circuit board
that generates the interference is
known, then the wires connecting it
with other system boards should be
filtered. Wires can be filtered with
a bead like component leads. To
achieve more impedance, multihole
cores are a good solution. The wire
is simply drawn through several
holes until sufficient impedance is
achieved. The system parts are not
necessarily boards. In an electric
shaver for instance you will find a
filter between mains plug and motor
consisting often of a bead on either
lead, combined with 3 capacitors.
D
• Wideband chokes
Wideband chokes are mounted on
different places, often not on circuit
boards. Their main advantage is
a combination of high impedance
and large bandwidth. The wires are
wound through holes in the core,
thus separating them physically and
reducing parasitic coil capacitance.
Several insulated types are available
to prevent short-circuit between
wire bends or of wire bends with
other metallic parts.
The following components can be
found in mains filters :
• two inductors L on the same core
for low-frequency attenuation
(harmonics of the switching
frequency)
• two Cy capacitors for additional
common-mode attenuation (at
higher frequencies)
• a Cx capacitor for differentialmode attenuation
H
D
d
L
Rod (ROD)
Tube (TUB)
d
D
d
L
Ring core (T, TN, TX, TC)
B
D
L
D
d
a
s
Impeder core (IMP)
c
D
d
b
B
F
E
A
Bobbin core (BC)
A
Fig. 13 Some products used in power applications.
16
E
C
E core (E)
Ferroxcube
C
U core (U)
The choke has to fulfil contradicting
requirements : high inductance
as well as high rated current. To
prevent an unpractical choke size,
current-compensation is applied to
a ring core in a high-permeability
material (see also section 2.d.).
Many variations exist according to
the specific equipment type, e.g. the
compensated choke alone can be
moulded in the plug of TV supply
cables.
• Lamp dimmers
(iron powder ring cores)
Fluorescent lamps cannot be
dimmed like incandescent lamps
simply by decreasing voltage,
because below their threshold they
turn off. Electronic dimmers use a
variable part of the supply voltage
period by means of delayed thyristor
ignition. The harmonics of the mains
frequency require iron powder i.s.o.
laminated silicon iron to reduce
eddy current losses. On the ignition
instant a parasitic ringing can be
observed, of which the frequency (a
few MHz) is determined by parasitic
inductances and capacitances in the
circuit. At MHz frequencies the
losses of iron powder are large and
the ringing is dissipated in a few
periods. Ferrites have much less
losses and would reflect a large part
of the ringing energy, which could
damage the semiconductors of the
control circuitry.
• Power line chokes
(bobbin cores)
If chokes operate on separate power
lines and current-compensation is
not possible, then an open core type
must be chosen. To reach a high
inductance, hundreds of turns can be
necessary and a bobbin core is the
appropriate shape.
is accompanied by high-frequency
sparks which cause RF interference.
This will be picked up by the FM
radio, but if motor functions are
regulated electronically, also safety is
at stake. Large currents are involved,
starter motor current can be as high
as 40 A. Due to the frequency (FM
band around 100 MHz) the inductance does not have to be very large
and a rod with a single layer winding
is the right choice. Motor temperatures can reach 150 °C, so the Curie
temperature of the ferrite should
be well over 200 °C, in combination
with good HF impedance behaviour.
The low permeability is no problem
in a rod shape. 3S3 is the ideal material for this application.
5.d. Radiation suppression
applications
Range of multilayer suppressors
in standard EIA sizes 0402 to 1812
• Electric commutation motors
in cars (rods)
In a modern car, many electric commutation motors are applied. There
are a starter motor, a fuel pump,
small ventilators, screen wiper
motors, window lift motors, sun
roof motors etc. The commutation
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Ferroxcube
• IC plates and absorber sheets
Thin film technology and IC plates
provide electromagnetic shielding for
multiple applications. IC plates based
on an ultra thin ferrite sintered
into the form of a plate have been
designed specially to be attached
on a CPU, or any integrated circuit
which requires EMI shielding to
assure perfect operation. Ferroxfoil
thin film sheets based on an absorptive electromagnetic shielding material consist of magnetic material and
resin. They suppress noise radiated
from electronic equipment over a
wide range of frequencies, offer flexibility in fabrication and yield excellent performance for many frequency
ranges, being its advantages even
relevant for RFID HF band application. Other examples of its use are
mobile devices including notebook
PCs, digital cameras and cell phones,
computer main board, imaging chip.
6. Design
considerations
rods and bobbin cores, the stray
flux can be a problem. Bobbin cores
are better than rods. Apart from
keeping distance to other circuit
Even without any trials or
parts, the positioning is important.
calculations, a lot of problems can be
For long thin rods a horizontal
avoided beforehand by good design
position is the best. The core axis
practices. In order of priority they
is horizontal, so the magnetic field
are :
is almost parallel to the PCB and
the induced electric field almost
• avoid generating interference
perpendicular. This results in only
(minimize clock rate, smoothen
low induced voltages in PCB tracks.
pulse shape),
• For inductors with many turns, the
• keep it far away (separate power
winding method influences the
components and circuits from the
parasitic coil capacitance. Too
rest)
much capacitance causes early
• impede its propagation (minimize
frequency roll-off of the impedance.
conductor path length and
Ways to reduce parasitic
component lead length),
capacitance are multi-chamber
• suppress with ferrites and
winding (separation of turns in
capacitors.
groups), and 90 degree crosswinding (electrical decoupling of
The following points should be conadjacent turns).
sidered while taking EMI-suppression • Capacitors should always be
measures :
connected with leads as short as
possible, because the leads have
• The insertion of ferrite
parasitic inductance (in the order
components lowers equally
of 10 nH/cm) which causes early
emission and susceptibility, the
frequency roll-off in the attenuation
essence is blocking the propagation curve. In general filters should be
path. The ferrite should always be
layed-out as compact as possible.
located as close to the source as
possible. All intermediate circuitry
Appendix A.
and cable length acts as antenna
and produces radiated interference. Impedance concept
The same holds for capacitors
or any type of suppression
A.1. Material
component.
The impedance curve can be
• The ferrite and the conductor
translated to a pure material curve,
should be close together.
the so-called complex permeability
Beads, tubes and cable shields
curve. As impedance consists of
should fit close around the wire or
a reactive and a resistive part,
cable and other core shapes should
permeability should have two parts
be wound tightly. If not, then stray
too to represent this. The real
flux is present, which converts into
part corresponds to the reactance,
mutual inductance if other circuit
positive for an inductance, negative
parts are close enough to be in the
for a capacitance, and the imaginary
stray field.
• Especially for open core types like
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Ferroxcube
part to the losses.
Z = jω . (µ’-jµ”) . Lo
= ω . µ” . Lo + jω . µ’ . Lo
Z = R + jX →
R = ω . µ” . Lo ,
X = ω . µ’ . Lo
(ω = 2 . π . f)
|Z| = √(R2 + X2)
= ω . Lo . √(µ’2 + µ”2)
Where Lo is the inductance if initial
permeability were equal to 1 :
Lo = µo . n2 . Ae / le
(µo = 4 π x 10-7 = 1.2566 x 10-6
[H/m])
For the calculation of effective magnetic dimensions Ae and le, see next
paragraph.
µ' µ''
µ'
µ"
fr
frequency
Z
|Z|
fr
frequency
Fig. 14 Complex permeability
and impedance.
A.2. Core size
A.3. Bias current
The choice of a suppression product
is made in two steps. First the
material choice corresponding to the
interference frequencies occurring
and afterwards the right core size
and turns for the impedance level
required.
The simplest way of calculation
is taking the impedance curve
of a reference core of the same
material. Calculation from complex
permeability is another possibility,
but it’s more bothersome. Two
factors have to be corrected :
effective magnetic dimensions and
turns.
Often a DC supply or AC mains
current is passing through the
inductor to allow normal operation
of the connected equipment. This
current induces a high field strength
in the ferrite core, which can lead
to saturation. Impedance then
decreases along with permeability,
especially for low frequencies. The
influence of a bias current can be
calculated. The induced field strength
is directly proportional to the
current :
Z :: N2 . Ae / le →
Z = Zo . (N2 / No2) . (Ae/Aeo) .
(leo / le)
The parameters with index o
correspond to the reference core.
The number of turns N is always
an integer number. Half a turn
geometrically is 1 turn magnetically.
For a bead with a single wire going
through, N = 1 turn. The effective
magnetic dimensions Ae (area)
and le (length) are calculated from
geometric dimensions according
to IEC 205. For complicated
geometries this involves complex
formulas. Therefore the suppliers
usually specify these data in their
handbooks. For a cylindrical
geometry (ring core, tube, bead,
bead-on-wire) a simple formula
applies :
Ae / le = h / (2 . π) . ln(OD/ID)
OD = outer diameter
ID = inner diameter
h = height
H = n . I / le
Whether this field causes a
significant saturation or not, can be
seen in the curve of permeability
versus bias field. However, this only
indicates the decrease of inductance
at low frequency. The impedance
at high frequency decreases less.
Again, impedance can be calculated
from reference curves if they show
impedance versus frequency with
bias current as a parameter. First,
bias current is translated to the
current that would induce the same
field strength in the reference core,
which means the same state of core
saturation :
Io = I . (n/no) . (leo/le)
For a ring core, tube or bead the
effective length is
le = π . ln(OD/ID) / (1/ID-1/OD)
Now the relative impedance
decrease will be the same :
Zbias = Z . (Zo bias / Zo)
Literature, Software and Sample Boxes
General catalogues & Software
Data Handbook : Soft ferrites and Accessories
Soft Ferrites and Accessories Design Tools Disk
Specific brochures
Ferroxfoil flexible sheet EMI absorber
SMD Beads and Chokes
Gapped SMD beads for power inductors
3S5 the new medium frequency EMI ferrite for high bias current conditions
SMD wideband choke with extra metallization
Wideband Chokes
Cable Shielding
Power Inductors
Multilayer Suppressors and Inductors
IIC Integrated Inductive Components
3S4 a new Soft Ferrite for EMI suppression
3S3 a new Soft Ferrite for EMI suppression
Sample boxes
SAMPLEBOX9
SAMPLEBOX10
SAMPLEBOX11
SAMPLEBOX12
SAMPLEBOX13
SAMPLEBOX14
SAMPLEBOX14A
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Ferroxcube
SMD Beads and Chokes
Cable Shielding
EMI-suppression Products
Multilayer suppressors
Multilayer inductors
IIC
IIC demo board
If you require impedance graphs or other detailed product data,
which are not presented in this brochure,
please visit our website at :
www.ferroxcube.com
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Ferroxcube