Engineering Note ILB, ILBB Ferrite Beads

Engineering Note ILB, ILBB Ferrite Beads
www.vishay.com
Vishay Dale
Electro-Magnetic Interference and Electro-Magnetic Compatibility
(EMI/EMC)
INTRODUCTION
Manufacturers of electrical and electronic equipment
regularly submit their products for EMI/EMC testing to
ensure regulations on electromagnetic compatibility are
met. Inevitably, some equipment will fail, as the interference
transmitted on cables connected to the equipment exceeds
regulated limits, resulting in radiated emissions failure.
Additional problems can occur when connected equipment
causes interference problems with the equipment under test
resulting in component malfunction.
There are many ways to reduce the level of conducted and
radiated interference, especially during the initial design of
the circuit board.
These techniques include proper routing of tracks, proper
use of ground planes, power supply impedance matching,
and reducing logic frequency to a minimum.
Even with the most diligent employment of good EMI/EMC
circuit design practices, not all interference or compatibility
issues can be eliminated. At this point, additional
components can be added, allowing the circuit to comply
with design and regulation limits for EMI/EMC.
This engineering note will review both initial circuit board
design practices and identify some after design
components that can be used to solve EMI/EMC problems.
CIRCUIT DESIGN TIPS TO REDUCE EMI/EMC
PROBLEMS
There are several areas where good circuit design practices
are critical to the reduction or elimination of EMI/EMC
problems. How the PCB layout is approached - not simply
in the design but also the choice of components - directly
affects the degree of EMI/EMC interference. Another area of
concern is the circuit design of the power supply.
PCB Design Tips
• Avoid slit apertures in PCB layout, particularly in ground
planes or near current paths
• Areas of high impedance give rise to high EMI, so use
wide tracks for power lines on the trace sides
• Make signal tracks stripline and include ground plane and
power plane whenever possible
• Keep HF and RF tracks as short as possible, and lay out
the HF tracks first (Fig. 1)
Avoid Track Stubs
Fig. 2
• On sensitive components and terminations, use
surrounding guard ring and ground fill where possible
• A guard ring around trace layers reduces emission out of
the board; also, connect to ground only at a single point
and make no other use of the guard ring (Fig. 3)
Ground
Fill on
Trace
Side
Guard
Ring
Guard
Ring on
Trace
Side
Use Guard Ring and Ground Fill
on Terminations and Sensitive Components
Fig. 3
• When you have separate power planes, keep them over a
common ground to reduce system noise and power
coupling (Fig. 4)
Vcc
Vdd
Vcc
Vdd
Avoid Overlapping Power Planes
Fig. 4
• The power plane conductivity should be high, so avoid
localized concentrations of via and through hole pads
(surface mount is preferred mounting method)
• Track mitering (beveling of edges and corners) reduces
field concentration
• If possible, make tracks run orthogonally between
adjacent layers (Fig. 5)
Keep HF Tracks Short
Fig. 1
Make Tracking Run Orthogonally
Between Layers
• Avoid track stubs, as they cause reflections and
harmonics (Fig. 2)
Fig. 5
Revision: 20-Sep-13
Document Number: 34097
1
For technical questions, contact: [email protected]
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Engineering Note ILB, ILBB Ferrite Beads
www.vishay.com
Vishay Dale
• Do not loop tracks, even between layers, as this forms a
receiving or radiating antenna.
• Do not leave floating conductor areas, as they act as EMI
radiators; if possible connect to ground plane (often, these
sections are placed for thermal dissipation, so polarity
should not be a consideration, but verify with component
data sheet). (Fig. 6)
Vcc
DC
DC
CCT1
DC
DC
CCT2
GND
Isolated Individual Systems
Fig. 10
Component Considerations
Do Not Leave Floating Conducting Areas
Fig. 6
Power Supply Considerations
• Eliminate loops in the supply lines. (Fig. 7)
PSU
CCT1
CCT2
Vcc
GND
PSU
CCT1
CCT2
Vcc
GND
• Locate biasing and pull up/down components close to
driver/bias points.
• Minimize output drive from clock circuits.
• Use common mode chokes (Vishay Dale series LPT4545
or LPT3535 or the LPE series of surface mount
transformers) between current carrying and signal lines to
increase coupling and cancel stray fields. (Fig. 11)
Signal
Input
Receiving
Circuit
Use Common Mode Choke Between Signal Lines
Fig. 11
Eliminate Loops in Supply Lines
• Decouple close to chip supply lines, to reduce component
noise and power line transients. (Fig. 12)
Fig. 7
• Decouple supply lines at local boundaries. (Fig. 8)
GND
Vcc
c
Vcc
CCT1
CCT1
Decouple Close to IC Supply Lines
Fig. 12
GND
Decouple Supply Lines at Local Boundaries
Fig. 8
• Place high speed circuits close to Power Supply Unit
(PSU) and slowest sections furthest away to reduce power
plane transients. (Fig. 9)
PSU
High
Speed
Circuit
(Micro)
Medium
Speed
Circuit
(Display)
Low
Speed
(Interface)
DC
Circuit
(Analog)
Place High Speed Circuits Close to PSU
Fig. 9
• Isolate individual systems where possible (especially
analog and digital systems) on both power supply and
signal lines. (Fig. 10)
Revision: 20-Sep-13
• Use low impedance capacitors for decoupling and
bypassing (ceramic multilayer capacitors, like those
offered by Vishay Vitramon are preferred, offering high
resonant frequencies and stability).
• Use discrete components for filters where possible
(surface mount is preferable due to lower parasitic and
aerial effects of termination’s compared to through hole
components).
• Ensure filtering of cables and overvoltage protection at the
terminations (this is especially true of cabling that is
external to the system, if possible all external cabling
should be isolated at the equipment boundary).
• Minimize capacitive loading on digital output by
minimizing fanout, especially on CMOS ICs (this reduces
current loading and surge per IC).
If available, use shielding on fast switching circuits, main
power supply components and low power circuitry
(shielding is expensive and should be considered a “last
resort” option).
Document Number: 34097
2
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Engineering Note ILB, ILBB Ferrite Beads
www.vishay.com
MAGNETIC COMPONENTS FOR ELECTRO
MAGNETIC INTERFERENCE REDUCTION AND
ELECTRO MAGNETIC COMPATIBILITY
Products that use magnetics to reduce electro-magnetic
interference and improve electro-magnetic compatibility
within the circuit can be classified into several categories:
inductors, chokes, transformers, ferrite beads, capacitors,
and integrated passive devices that can incorporate any or
all of the above devices. When considering any of these
EMI/EMC components, it is necessary to identify circuit
paths or areas likely to conduct or radiate noise.
Inductors
The most common magnetic EMI filter is the inductor or
choke. Inductors are used for both line filtering and energy
storage. If a circuit is suspected of being a source for EMI,
often, selection of the right inductor can help eliminate the
problem. For radiated interference, the choice of a shielded
or toroidal inductor can often eliminate (or at least greatly
reduce) the offending frequency. In fact, toroidal inductors
like Vishay Dale’s LPT-4545 and LPT-3535 surface mount,
or Vishay Dale’s TE, TD, or TJ series of leaded toroids
virtually eliminate radiated fields because of the toroid’s
unique ability to contain the magnetic flux within its core.
LPT 4545
Toroidal
Inductor
The toroid is also less susceptible to induced noise from
other components as the applied magnetic field would
induce equal and opposite currents inside the toroid, thus
canceling the induced interference.
Chokes
Common mode and differential mode chokes are used to
eliminate noise on a pair of conductors. Common mode
noise is defined as noise that is present or “common” to
both conductors, and can be the result of induced noise
caused by the “antenna” effect of a conductor or PC trace.
Common mode noise is typically “in phase” within the
conductors, while differential noise is present on only one
conductor or present in opposite phase in both conductors.
Common mode chokes use the properties of two closely
coupled magnetic fields to eliminate the interference
problem by canceling the noise within the magnetic fields.
They are best employed to eliminate noise or EMI on cables
or signal tracks. The choke should be located as close to the
driver or receiver circuit as possible, or at the signal entry
point of the circuit board. The proper selection of inductive
component can also help in matching line impedance and
can act as a bandwidth filter for the circuit. Vishay Dale’s
LPT and LPE series products can be configured in the
common or differential mode depending on your
application.
Transformers
The main benefit of using a transformer for EMI/EMC is that
it can provide an isolation barrier between a signal line and
the signal processing circuit (particularly where the signal
line exits the board or system). This is true of signals being
Revision: 20-Sep-13
Vishay Dale
driven or received, since isolating the line reduces common
mode noise and eliminates ground (or signal return)
potential differences between systems.
One particular area where high noise immunity is essential is
in thyristor/triac driving circuits. Here the transformer
provides an isolation between the driven load and a logic
based controller. The isolating pulse transistor provides
much better noise immunity than an insulated gate bi-polar
transistor (IGBT) due to inherently lower coupling
capacitance (typically 10’s of pF for a pulse transformer
compared to nF for a power IGBT device). The lower
coupling capacitance improves the circuit’s immunity from
noise from the main power supply or from power switching
devices. Vishay Dale’s LPE and PT transformers can be
used to meet your transformer needs. Many more EMI/EMC
configurations can be provided through our custom
magnetic design department.
LPE Series
Transformer
Surface Mount Ferrite Beads
Chip impeders, also called ferrite chip beads, perform the
function of removing RF energy that exists within a
transmission line structure (printed circuit board trace). To
remove unwanted RF energy, chip beads are used as high
frequency resistors (attenuators) that allow DC to pass while
absorbing the RF energy and dissipating that energy in the
form of heat.
ILBB-0603
ILB-1206
ILBB/ILB
Ferrite
Beads
ILBB-0805
Surface mount ferrite beads have many advantages:
• Small and light weight
• Inexpensive
• High impedance values removes broad range of RF
energy
• Closed magnetic circuit eliminates cross talk
• Beads are inherently shielded
• Low DCR ratings minimizes desired signal degradation
• Excellent current carrying capacity compared to
alternatives
• Outstanding performance at removing RF energy
• Spurious circuit oscillations or resonances are reduced
because of the bead’s resistive characteristics at RF
frequencies
• Broad impedance ranges (several Ω to 2000 Ω)
• Operates effectively from several MHz to 1 GHz
Document Number: 34097
3
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Engineering Note ILB, ILBB Ferrite Beads
www.vishay.com
EMI/EMC Component Selection
Before incorporating EMI/EMC components, it is necessary
to identify the circuit paths and circuit areas most likely to
conduct noise, and to identify circuit areas likely to act as
antennas and radiate noise. At this point the most
appropriate location for the chosen components can be
determined.
The actual components chosen are determined by the
frequency and signal level of the noise to be eliminated.
Consideration should also be given for the frequencies that
are to remain intact.
For attenuation less than 5dB inductive, EMI components
are generally the best choice. For attenuation less than 5 dB,
circuit type must first be considered.
Working with a high speed signal circuit, your best choice is
a complex filter consisting of inductive and capacitive
components (such as an LCR Filter). If your circuit is a
general signal type (i.e., not a high speed circuit) grounding
stability must first be determined. For stable grounds,
capacitive EMI components are an excellent choice.
However, if the circuit has an unstable ground, high
impedance inductive components should be considered for
EMI suppression needs.
Designing equipment and choosing components is not an
easy process. Often, the only measure of design success is
the overall radiation level from your equipment. Trial and
error is a long tedious process that can take several months
to complete, and choosing the wrong component can waste
time.
Here are three suggestions for more effective design:
• Always place EMI/EMC components as close as possible
to the noise source.
• Select EMI/EMC components that match the impedance
of the noise conduction path, not necessarily that of the
circuit path. Remember that common mode noise often
travels a different path than the circuit current.
• Start with EMI/EMC components that offer sufficient
performance to meet your design standards. Component
costs can be reduced once you have a working design.
Revision: 20-Sep-13
VISHAY COMPONENTS FOR EMI/EMC
COMPLIANCE
Surface Mount Ferrite Beads
ILB-1206, ILBB-0402 to ILBB-1812
Surface Mount, High Current Ferrite Beads
ILHB-0603 to IHLB-1812
Surface Mount Bead Arrays
ILAS-1206
Surface Mount Ferrite Inductors and Chokes
LPT-4545, LPT-3535
Surface Mount Transformers
LPE Series
Surface Mount Ceramic and Tantalum Capacitors
Ferrite Beads for EMI/EMC Compliance
One of the simplest and most effective ways to reduce
EMI is through the use of ferrite beads. Initially, EMI
suppression consisted of a small bead-shaped ferrite
(hence the name bead) with a hole through the middle.
The ferrite bead was slipped over the suspected “noisy”
wire or component lead and EMI was reduced.
Today, beads are available in a variety of styles including
the original through-hole model, multiple apertures and
surface mount configurations.
How Ferrite Beads Work
The best way to conceptualize a bead is as a frequency
dependent resistor. An equivalent circuit for a bead
consists of a resistor and inductor in series. The resulting
change (of impedance over frequency) is directly
associated with the frequency dependent complex
impedance of the ferrite material.
At low frequencies (below 10 MHz) the inductive
impedance is 10 Ω or less, as shown below. At higher
frequencies, the impedance of the bead increases to
over 100 Ω, and becomes mostly resistive above
100 MHz.
600 Ω ± 25 %
800
Z
600
Z, R, X (Ω)
To chose the proper bead, you should consider the
following:
1. What is the range of unwanted frequencies?
2. What is the source of the EMI?
3. How much attenuation is required?
4. What are the environmental and electrical conditions for
the circuit (temperature DC voltage, DC bias currents,
maximum operating currents, field strengths, etc...)?
5. What is the maximum allowable profile and board real
estate for using this component?
Selection of the right bead for your particular frequencies is
not a simple process. In most cases, since beads are only
rated for impedance at 100 MHz, you will need to look at
several graphs to determine the best bead for your
frequency if it is different than 100 MHz.
This is a time consuming but necessary process to select
the correct bead value since the highest impedance bead
at 100 MHz is not necessarily the highest impedance bead
at higher or lower frequencies. DC bias will also lower the
effective impedance of the device.
Vishay Dale
400
R
200
X
0
1
10
100
Frequency (MHz)
1000
Since the bead’s impedance is essentially resistive to
high frequency circuits, the problem of resonance
experienced by other EMI filtering choices like
capacitors and inductors is eliminated. Often the bead is
the only practical solution to an EMI problem.
When used as a high frequency filter, ferrite beads
provide a resistive loss that attenuates the unwanted
frequencies through minute heating of the bead’s ferrite
material due to eddy currents. At the same time, the
bead presents minimal series impedance to the lower
frequency or direct currents of the circuit.
Document Number: 34097
4
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Engineering Note ILB, ILBB Ferrite Beads
www.vishay.com
Vishay Dale
USEFUL TABLES FOR EMI/EMC DESIGNS
DECIBELS
DB (1)
POWER RATIO
VOLTAGE CURRENT RATIO
DB (2)
POWER RATIO
0
1.0
1.0
0
1.0
1.0
3
2.0
1.4
-3
0.50
0.71
6
4.0
2.0
-6
0.25
0.50
10
10.0
3.2
-10
0.10
0.32
12
16.0
4.0
-12
0.05
0.25
14
25.0
5.0
-14
0.04
0.20
20
102
10
-20
10-2
0.10
30
103
32
-30
10-3
0.03
40
104
102
-40
10-4
10-2
60
106
103
-60
10-6
10-3
80
108
104
-80
10-8
10-4
100
1010
105
-100
10-10
10-5
120
1012
106
-120
10-12
10-6
140
1014
107
-140
10-14
10-7
VOLTAGE CURRENT RATIO
Notes
(1)
(2)
P1
dB = 10 log 10 -----P2
V1
I1
dB = 20 log 10 ------ = 20 log 10 ---V2
I2
ELECTRIC FIELD LEVELS
W
1m
10 m
100 m
1 km
10 km
1
5.5 V/m
0.55 V/m
0.05 V/m
5.5 V/m
0.55 mV/m
10
17.4 V/m
1.7 V/m
0.17 V/m
17 V/m
1.7 mV/m
100
55 V/m
5.5 V/m
0.55 V/m
55 V/m
5.5 mV/m
1K
174 V/m
17.4 V/m
1.74 V/m
170 V/m
17 mV/m
10K
550 V/m
55 V/m
5.5 V/m
550 V/m
55 mV/m
100K
1740 V/m
174 V/m
17.4 V/m
1.74 V/m
174 mV/m
Notes
• Table assumes an antenna gain of one
5.5 PA
• E = --------------------d
• P = Power at antenna in W
d = Distance from antenna in m (valid when d > λ/2π)
E = Electric field in V/m
A = Antenna gain (1 for table)
CISPR 22 LIMITS
FREQUENCY (MHz)
CLASS A
CLASS B
30 to 230
40 dB πV/m
30 dB V/m
230 to 1000
47 dB πV/m
37 dB V/m
0.15 to 0.50
66 dB πV/m
56 dB πV/m to 46 dB πV/m
0.50 to 5
60 dB πV/m
46 dB πV/m
5 to 30
60 dB πV/m
50 dB πV/m
RADIATED
Quasi-peak, antenna at 10 m
CONDUCTED
Average
Revision: 20-Sep-13
Document Number: 34097
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For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
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Engineering Note ILB, ILBB Ferrite Beads
www.vishay.com
Vishay Dale
FREQUENCY VS. WAVELENGTH
f
λ
λ/2
λ/20
10 Hz
30 000 km
4800 km
1500 km
60 Hz
5000 km
800 km
250 km
100 Hz
3000 km
480 km
150 km
400 Hz
750 km
120 km
37 km
1 kHz
300 km
48 km
15 km
10 kHz
30 km
4.8 km
1.5 km
100 kHz
3 km
480 m
150 m
1 MHz
300 m
48 m
15 m
10 MHz
30 m
4.8 m
1.5 m
100 MHz
3m
0.48 m
15 cm
1 GHz
30 cm
4.8 cm
1.5 cm
10 GHz
3 cm
4.8 mm
1.5 mm
Note
• f = Frequency
λ = Wavelength
λ/2π = Near field to far field distance
λ/20 = Antenna effects of wires and slots
CAPACITOR SELF RESONANCE
TOTAL LEAD LENGTH
FARADS
1/4"
1/2"
1"
500 pF
100 MHz
72 MHz
50 MHz
1000 pF
72
51
36
0.01 F
23
16
11
0.1 F
7.2
5.1
3.6
0.3 F
4.2
2.9
2.1
0.5 F
3.2
2.3
1.6
Note
1
• f = -------------------; L = 20 nH/inch
2π LC
RISE TIMES - FREQUENCY - LENGTH
tr
feq
Lcross
Lcross/2
Lterm
1 ns
318 MHz
1.0 ft.
6"
3"
3 ns
95 MHz
3.0 ft.
1.5 ft.
9"
10 ns
32 MHz
10 ft.
5 ft.
2.5 ft.
30 ns
9.5 MHz
30 ft.
15 ft.
7.5 ft.
100 ns
3.2 MHz
100 ft.
50 ft.
25 ft.
300 ns
950 kHz
300 ft.
150 ft.
75 ft.
1 μs
320 kHz
1000 ft.
500 ft.
250 ft.
Note
• tr =Rise time
feq=Equivalent frequency I/πtr
Lcross=Length of one rise time in free space
Lcross/2=Typical length of rise time on cable or printed circuit
board (crosstalk)
Lterm=Length of terminate on cable or printed circuit board
Revision: 20-Sep-13
Document Number: 34097
6
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
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