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Electromagnetic
Compatibility Considerations
for Switching Power Supplies
Characterization of the EMI problem
requires understanding the interference source
Electromagnetic Compatibility Considerations
for Switching Power Supplies
Switching power supplies generate Electromagnetic Interference (EMI) by
virtue of their inherent design characteristics. Internal switching power supply
circuits that generate undesirable emissions that are rich in harmonics can cause
electrical interference both internally to the circuit in which the power supply is
installed and to other electronic equipment in the vicinity of the emission source.
INTRODUCTION
UNITED STATES STANDARDS
This application note examines the rules and
regulations governing control of EMI, discusses
types of noise generated by switching power
supplies, and provides basic guidance for EMI
mitigation, whether the power supply is installed
in other equipment as part of a larger system or
designed for stand-alone applications.
In the United States the government agency
responsible for regulating communications is
the Federal Communications Commission (FCC).
Control of electromagnetic interference is outlined
in Part 15 of the FCC rules and regulations. FCC
rules decree that any spurious signal greater than
10 KHz be subject to these regulations. The FCC
further specifies the frequency bands in which
these spurious emissions must be controlled
according to the type of emission. Radiated
emissions, i.e., those radiated and coupled through
the air, must be controlled between 30 MHz and
1000 MHz. Conducted emissions, i.e., those RF
signals contained within the ac power bus, must be
controlled in the frequency band between 0.45 MHz
and 30 MHz.
LAWS, REGULATION,
AND INTERNATIONAL
COOPERATION
The electromagnetic spectrum has been widely used
for broadcasting, telecom and data communications
through intentional emissions of electromagnetic
fields. There have also been unintentional emissions
from many electrical and electronic equipment, such
as arc welding machines, household appliances
and computer equipment. In order to protect
the electromagnetic spectrum and to ensure
compatibility of collocated electrical and electronic
systems from trouble free operations, regulatory
bodies both within the United States and throughout
the world community have established standards
to control conducted and radiated electromagnetic
interference in electronic equipment. This
discussion mainly focuses on unintentional
electromagnetic compatibility in systems that utilize
switching power supplies.
page 2
The FCC further categorizes digital electronic
equipment into Class A (designated for use in a
commercial, industrial, or business environment
excluding residential use or use by the general
public) and Class B (designated for use in a
residential environment notwithstanding use in
commercial, business and industrial environments).
Examples of Class B devices are personal
computers, calculators, and similar devices for use
by the general public. Emission standards are more
restrictive for Class B devices since they are more
likely to be located close to other electronic devices
used in the home.
Electromagnetic Compatibility Considerations
for Switching Power Supplies
FCC Class A Conducted EMI Limit
Frequency of Emission (MHz}
Conducted Limit (μV)
0.45 ~ 1.6
1000
1.6 ~ 30.0
3000
FCC Class B Conducted EMI Limit
Frequency of Emission (MHz}
Conducted Limit (μV)
0.455 ~ 1.6
250
1.6 ~ 30.0
250
FCC Class A 30-Meter Radiated EMI Limit
Frequency of Emission (MHz}
Field Strength Limit (μV/m)
CISPR Class A Conducted EMI Limit
Frequency of Emission (MHz}
0.15 ~ 0.50
0.50 ~ 30.0
Conducted Limit (dBμV)
Quasi-peak
Average
79
66
73
60
CISPR Class B Conducted EMI Limit
Frequency of Emission (MHz}
Conducted Limit (dBμV)
Quasi-peak
Average
0.15 ~ 0.50
66 ~ 56*
56 ~ 46*
30 ~ 88
30
0.50 ~ 5.00
56
46
88 ~ 216
50
5.00 ~ 30.0
60
50
216 - 1000
70
above 1000
70
FCC Class B 3-Meter Radiated EMI Limit
Frequency of Emission (MHz}
Field Strength Limit (μV/m)
30 ~ 88
100
88 ~ 216
150
216 ~ 1000
200
above 1000
200
CISPR Class A 10-Meter Radiated EMI Limit
Frequency of Emission (MHz}
Field Strength Limit (dBμV/m)
30 ~ 88
39
88 ~ 216
43.5
216 ~ 960
46.5
above 960
49.5
CISPR Class B 3-Meter Radiated EMI Limit
Frequency of Emission (MHz}
Field Strength Limit (dBμV/m)
Figure 1: FCC field strength limits for conducted and
radiated emissions.
30 ~ 88
40.0
88 ~ 216
43.5
INTERNATIONAL STANDARDS
216 ~ 960
46.0
above 960
54.0
A standard widely used in the European Community
is the Third Edition of the International Special
Committee on Radio Interference (CISPR), Pub.
22, “Information Technology Equipment—Radio
Disturbance Characteristics—Limits and Methods
of Measurement,” issued in 1997. This standard is
better known as simply CISPR 22. Unlike the FCC
which regulates electromagnetic interference in the
United States, CISPR is a standards organization
without regulatory authority. However, CISPR
standards have been adopted for use by most
members of the European Community.
CISPR 22 also differentiates between Class A and
Class B devices and establishes conducted and
radiated emissions for each class. In addition, CISPR
22 requires certification over the frequency range of
0.15 MHz to 30 MHz for conducted emissions
(Recall that the FCC range starts at 0.45 MHz).
page 3
Figure 2: CISPR field strength limits for conducted
and radiated emissions.
*Decreases with the logarithm of the frequency
STANDARDS
HARMONIZATION
The FCC Part 15 rules and the requirements of CISPR
22 have been harmonized and either standard, with
minor exceptions, can be used to certify digital
electronic equipment. Harmonization requires that
the same standard be used for both conducted and
radiated emissions. Measurements made above
1000 MHz must be made in accordance with FCC
rules and limits since CISPR 22 has no specified
limits for frequencies above 1000 MHz. Conducted
and radiated emission limits specified in FCC Part
15 and CISPR 22 are within a few dB of each other
Electromagnetic Compatibility Considerations
for Switching Power Supplies
over the prescribed frequencies, so using either
set of limits does not compromise accuracy of the
measurement and certification process. FCC limits
are given in μV and CISPR limits are given in dBμV,
so conversion of the units for one set of limits is
necessary for direct comparison.
SWITCHING POWER
SUPPLIES AND EMC
STANDARDS
“Switching power supply” is a generic term that
describes a power source that uses a circuit to
convert a dc voltage to an ac voltage that can be
further processed to become another dc voltage.
Switching power supplies can be further categorized
as ac-dc power supplies (ac input) and dc-dc
converters (dc input) since both incorporate dc to
ac conversion for voltage change. By virtue of their
inherent design characteristics, switching power
supplies generate electromagnetic interference
composed of signals of multiple frequencies. The
dc-dc converter converts the input dc voltage to
an ac voltage that can be stepped up or down via a
transformer. Ac-dc power supplies also utilize high
frequency circuits for voltage conversion.
However, the internal ac voltage in either case is not
a pure sine wave but frequently a square wave which
can be represented by a Fourier series that consists of the algebraic sum of many sine waves with
harmonically-related frequencies. These multiplefrequency signals are the source of conducted and
radiated emissions which can cause interference to
both the equipment in which the switching power
supply is installed and to nearby equipment which
may be susceptible to these frequencies.
Switching power supplies generate EMI which is
subject to FCC and CISPR regulations. Since Class
A electronic equipment is marketed for use in a
page 4
commercial, industrial, or business environment,
and Class B electronic equipment is marketed for
use in a residential environment, emission limits for
Class B equipment, which is likely to be located in
close proximity to radio and television receivers, are
therefore more restrictive than Class A. In general
Class B limits are more restrictive than Class A by a
factor of 3 (~10 dB). FCC conducted emission limits
are specified for frequency ranges of 0.45-1.6 MHz
and 1.6-30 MHz. FCC radiated emission limits are
specified for frequency ranges of 30-88 MHz, 88216 MHz, and 216-1000 MHz at a fixed measuring
distance of 3 meters. These limits apply to both
systems with embedded power supplies installed and
in stand-alone applications where switching power
supplies are utilized.
EMC TESTING AND
COMPLIANCE
EMC testing and compliance is performed according
to the test procedure defined in ANSI C63.4-2009
“Methods of Measurement of Radio-Noise Emissions
from Low-Voltage Electrical and Electronic
Equipment in the Range of 9 kHz to 40 GHz”. This
ANSI Standard does not include either generic or
specific product-related limits on conducted and
radiated emissions. These limits are specified in
the FCC and CISPR documents discussed above. It
is worth noting that testing is done with the entire
system, not just the power module, especially with
embedded power modules. With external power
supplies (as in standalone power adapters), the
entire system needs to be tested, even if the power
adapter is in compliance with the regulations.
Electromagnetic Compatibility Considerations
for Switching Power Supplies
EMI/EMC FUNDAMENTALS,
SOURCES AND ASSOCIATED
FREQUENCIES
EMI cases generally include a source of interference,
a path that couples the EMI to other circuits, and a
target referred to as the “victim” whose performance
is degraded by the source EMI. The damaging effects
of EMI pose unacceptable risks in many different
technologies, thus making it necessary to control
EMI at its source or reduce the risk of exposure to
EMI to acceptable levels at the victim.
EMI can first be categorized as continuous
interference as opposed to transient interference.
Continuous interference occurs when the source
emits an uninterrupted signal composed of the
source’s fundamental frequency and associated
harmonics. Continuous interference can be further
subdivided by frequency band. Frequencies from
a few Hz up to 20 KHz are classified as audio.
Sources of audio interference include power supply
hum and associated wiring, transmission lines and
substations, audio processing equipment such
as audio power amplifiers and loudspeakers, and
demodulation of high frequency carrier waves such
as those seen in FM radio transmission.
Radio Frequency Interference (RFI) occurs in
a frequency band from 20 kHz to a constantly
increasing limit defined by advancing technology.
Sources of RFI include wireless and radio frequency
transmissions, television and radio receivers,
industrial, scientific and medical equipment, and
high frequency circuit signals such as those in
microprocessors, microcontrollers, and other high
speed digital equipment.
Broadband noise, consisting of signals of multiple
frequencies, may be spread across parts of both
frequency ranges. Sources of broadband noise include
page 5
solar activity, continuously operating spark gaps such
as arc welders, and CDMA mobile telephony.
Transient EMI arises when the source emits a short
duration pulse of energy rather than a continuous
signal. Sources include switching electrical circuitry,
e.g., inductive loads such as relays, solenoids and
electric motors. Other sources are electrostatic
discharge (ESD), lightning, nuclear and nonnuclear
electromagnetic pulse weapons, and power line
surges. Repetitive transient EMI can be caused by
electric motors, gasoline engine ignition systems
and continuous digital circuit switching.
EMI COUPLING
Coupling can occur through conduction via an
unwanted path (a so-called “sneak circuit”), through
induction (as in a transformer), and radiated or
through-the-air coupling.
Conductive coupling occurs when the coupling path
between the source and the receptor is formed by
direct contact. Direct contact may be caused by a
transmission line, wire, cable, PCB trace or metal
enclosure. Conducted noise can appear in a common
or differential mode on two conductors.
Differential mode noise results from a differential
mode current in a two wire pair. The differential
mode current is the expected current on the two
wire pair, i.e., current leaves at the source end of the
line and comes back on the return side of the line.
The noise is measured on each line with respect
to a designated reference point. The resultant
measurement would be the difference in the noise
on the two lines. Differential mode currents flow
between the switching supply and its source or
load via the power leads and these currents are
independent of ground. Consequently no differential
mode current flows through ground.
Electromagnetic Compatibility Considerations
for Switching Power Supplies
Common mode noise is caused by a common mode
current. In this case noise current flows along
both the outgoing lines in the same direction and
returns by some parasitic path through system
ground that is not part of the design, the so-called
“sneak circuit” discussed earlier. In many cases,
common mode noise is conducted through parasitic
capacitance in the circuit. Common mode currents
flow in the same direction in or out of the switching
supply via the power leads and return to their source
through ground. Common mode currents will also
flow through the capacitance formed between the
case and ground.
Common-mode signal
Vulnerable
Application
Return Path via
Earth Ground
Differential-mode signal
Vulnerable
Application
Figure 3: Definition of differential and common mode current.
Conducted EMI emissions are measured up to 30
MHz. Currents at frequencies below 5 MHz are
mostly differential mode, while those above 5 MHz
are usually common mode.
Inductive coupling occurs where the source and
receptor are separated by a short distance. Inductive
coupling can be due to electrical induction or
magnetic induction. Electrical induction results
from capacitive coupling while magnetic induction
is caused by inductive coupling. Capacitive coupling
occurs when a varying electric field exists between
page 6
two adjacent conductors, inducing a change in
voltage across the gap. Magnetic coupling occurs
when a varying magnetic field exists between two
parallel conductors, inducing a change in voltage
along the receiving conductor. Inductive coupling is
rare relative to conductive or radiated coupling.
Radiated coupling occurs when source and receptor
(victim) act as radio antennas. The source radiates
an electromagnetic wave which propagates across
the open space between the source and the victim
and is received by the victim.
Characterization of the EMI problem requires
understanding the interference source and signal,
the coupling path to the victim and the nature of
the victim, both electrically and in terms of the
significance of the malfunction. The risk posed by
the threat is usually statistical in nature; so much of
the work in threat characterization and standards
setting is based on reducing the probability of
disruptive EMI to an acceptable level rather than its
assured elimination.
EMI requirements, both radiated and conductive,
apply to an overall electronic system. Power modules
are one of many components within a system. Since
the EMI requirements apply to the overall system,
significant effort must be expended on system
design to limit noise. Most electronic equipment
has only one interface with the power source,
which is through the power supply. If adequate
EMI filters are inserted between the power supply
and the power source, conducted emissions from
the power module can be sufficiently suppressed
to meet the FCC or CISPR limits without any of
the power modules meeting the EMC standard
as a standalone component. However, it should
be noted that switching power supplies in standalone applications, typically in the form of external
power adapters, are required to operate below the
conducted EMI limits.
Electromagnetic Compatibility Considerations
for Switching Power Supplies
L
V
PRACTICAL
MEASURES
FOR EMI
MITIGATION
IN
G Power
Supply
POWER
SYSTEMS
N
V
o+
o-
In systems and circuits that are powered by
switching power supplies, good practices should
be observed in order to minimize EMI problems
and ensure agency compliance. Suppression of
EMI to levels below that specified by regulatory
bodies requires an understanding of the design of
the power supply and the application in which it is
incorporated. It is important to note that even an
application with a properly filtered switching power
supply may not achieve compliance if the application
is not designed to minimize EMI. Cautions must be
taken to use the power supply/converter properly as
intended, to prevent power supply generated noise
from radiating or reaching the source, minimize
noise pick up from the power supply, minimize
system noise generation, and prevent system
generated noise from reaching the power supply.
MITIGATION TECHNIQUES
FOR CONDUCTED EMI
To effectively mitigate conducted emissions, it is
imperative to address the differential mode noise
and common mode noise separately because the
remediation solution is different for each type of
noise. Solutions for differential mode noise will not
eliminate common mode noise present in the circuit.
The same is true for common mode noise solutions
as applied differential mode noise.
Differential mode noise can usually be suppressed by
connecting bypass capacitors directly between the
power and return lines of the power supply.
page 7
L
Vo+
Power Supply
N
Vo-
Figure 4: Differential mode noise can usually be suppressed
by connecting bypass capacitors directly between the power
and return lines of the power supply.
The power lines that require filtering may be those
located at the input or the output of the power
supply. The bypass capacitors on these lines need
to be physically located adjacent to the terminals of
the noise generating source to be most effective. The
actual location of the bypass capacitor is critical for
efficient attenuation of differential mode currents at
high frequencies. Attenuation at lower frequencies of
differential mode currents around the fundamental
switching frequency of the noise generating source
may dictate that a much higher value of bypass
capacitance be required that cannot be attained with
a ceramic style capacitor. Ceramic capacitors up to
22 μF may be suitable for differential mode filtering
across the lower voltage outputs of switching power
supplies but not suitable for inputs where 100 volt
surges can be experienced. For these applications
electrolytic capacitors are employed because of their
high capacitance and voltage ratings.
Differential mode input filters usually consist of a
combination of electrolytic and ceramic capacitors
to suitably attenuate differential mode current both
at the lower fundamental switching frequency as
well as at the higher harmonic frequencies. Further
suppression of differential mode currents can be
achieved by adding an inductor in series with the main
power feed to form a single stage L-C differential
mode low pass filter with the bypass capacitor.
Electromagnetic Compatibility Considerations
for Switching Power Supplies
Common mode conducted currents are effectively
suppressed by connecting bypass capacitors
between each power line of the supply and ground.
These power lines may be at the input and/or at the
output of the power supply. Further suppression of
common mode currents can be achieved by adding
a pair of coupled choke inductors in series with
each main power feed. The high impedance of the
coupled choke inductors to exiting common mode
currents forces those currents through the bypass
capacitors.
L
G
N
A ground plane located on the outer surfaces of the
printed circuit board, particularly if located directly
below the noise generating source, suppresses
radiated EMI significantly.
Power Supply
Power Supply
Large
Enclosed Loop
Vo+
Vo+
L
Power Supply
Power Supply
Vo-
N
Load
Figure 5: Common mode conducted currents are effectively
suppressed by connecting bypass capacitors between each
power line of the power supply and ground.
MITIGATION TECHNIQUES
FOR RADIATED EMI
Radiated EMI can be suppressed by reducing RF
impedance and reducing the antenna loop area
which is done by minimizing the enclosed loop
area formed by the power line and its return path.
The inductance of a printed circuit board track
can be minimized by making it as wide as possible
and routing it parallel to its return path. Similarly,
because the impedance of a wire loop is proportional
to its area, reducing the area between the power line
and its return path will further reduce its impedance.
Within printed circuit boards this area can be best
reduced by placing the power line and return path
one above the other on adjacent printed circuit
board layers. Recall that reducing the loop area
between a power line and its return path not only
reduces the RF impedance, but also reduces the
effectiveness of the antenna because the smaller
loop area produces a reduced electromagnetic field.
page 8
Small
Enclosed Loop
Vo-
Load
Figure 6: Reduced antenna loop area
to reduce radiated emissions.
To further reduce radiated noise, metal shielding
can be utilized to contain radiation. This is achieved
by placing the noise generating source within a
grounded conductive housing. Interface to the clean
outside environment is via in-line filters. Common
mode bypass capacitors would also need to be
returned to ground on the conductive housing.
ADDITIONAL MITIGATION
CONSIDERATIONS FOR
SWITCHING POWER SUPPLIES
Reliable wiring connections should be implemented
to and from the power supply. Wiring must be of
suitable size and be kept as short as possible, and
wiring loops should be minimized. Avoid running
input or output wirings near power devices to
prevent noise pick up.
Electromagnetic Compatibility Considerations
for Switching Power Supplies
Ensure all grounding connections are made and
properly secured. Earth ground wires should be
kept as short as possible. If the circuit or system
operations induce current transients, it is very
important to have local decoupling capacitors
to supply the pulsed current locally, instead of
letting the pulsed current propagate upstream to
the supply. These capacitors should include high
frequency ceramic caps and bulk capacitors. If the
operation allows, slow down the clock, or rising/
falling edges. Circuits with higher clock rates/fast
switching times should be located close to the
power line input to reduce power transients. It is
recommended that both analog and digital circuits
should be individually physically isolated on both
power supply and signal lines.
Not optimized to reduce EMI
Vcc
Circuit
Power
Supply
Optimized to reduce EMI
Vcc
Circuit
Power
Supply
Figure 7: Eliminate loops in supply lines.
Care must be taken to prevent ground loops in
the system, especially when the system becomes
complex. This can be achieved by using a single
point ground or a ground plane. An example is
highlighted in figure 7.
page 9
If there are multiple circuits in a system, decouple
the circuits from each other by running separate
supply lines, and/or place inductance in the supply
lines as highlighted in figure 8 below.
Vcc
Circuit 1
Circuit 2
Circuit 3
Figure 8: Decoupled supply lines at
the local Boundaries.
If needed, ferrite beads can be placed on the dc
supply lines to isolate the system and the supply.
This can be effective to prevent power switching
harmonics from disrupting the system’s operation,
or to prevent system generated noise from reaching
the power supply. On the input side, if the built-in
EMI filter is insufficient for a specific application,
additional EMI filtering can be applied before the
power supply. A bead can also be placed on the earth
ground wire between the ac inlet and the supply.
Although many of the mitigation techniques
highlighted above are applicable to the implementation
of both ac-dc and dc-dc converters within a system,
there are specific considerations that must be
addressed for dc-dc converters. The switching action
in most dc-dc converters demand a pulsed input
current, which is best supplied by local capacitors
close to the switching devices.
As many dc-dc converters are compact in size, they
generally do not contain sufficient capacitance.
The system designer will need to place additional
capacitance at the input to reduce differential mode
noise. For even better filtering performance, a PI
filter can be employed. The additional capacitors are
used to reduce common mode noise.
Electromagnetic Compatibility Considerations
for Switching Power Supplies
ADDITIONAL SYSTEM-LEVEL
MITIGATION TECHNIQUES
As mentioned above, although most switching
power supplies are designed to meet applicable
EMI standards as stand-alone modules, the system
itself needs be designed to generate a minimum EMI
profile to meet regulatory standards. Specific areas
in the system design that are candidates for EMI
mitigation practices include the signal lines, printed
circuit boards (PCB), and solid state components.
Signal line considerations include the use of low pass
filters on signal lines to reduce allowable bandwidth
on the line to the minimum that will still allow the
signal to pass un-attenuated. Feed and return loops
should be kept close on wide bandwidth signal
lines to minimize radiated emissions. Additionally,
signal lines carrying RF or near-RF signals should
be properly terminated to reduce reflection at the
termination. Ringing and overshoot on these lines
can also be minimized as a result of using the
appropriate termination.
crosstalk. Floating conductor areas can act as a
source of radiated emissions, so their use should be
avoided except for overriding thermal considerations.
Additionally, solid state components on the PCB
should be decoupled close to chip supply lines to
reduce component noise and power line transients.
SUMMARY
Switching power supplies can generate EMI because
of their inherent design. Domestic and international
regulatory bodies regulate these emissions through
promulgation of rules and standards such as the
FCC Part 15 rules and the CISPR 22 standard.
Noise has been discussed with respect to type, how
the noise is transmitted, frequency of the noise, and
noise modes in circuits. A basic design guideline for
suppression of noise has been provided, including
input/output filter circuits and reduction of antenna
loop area. Best practices for EMI mitigation as regards power supply, signal line, printed circuit board
(PCB) and components have also been discussed.
PCB high impedance runs that contribute to
EMI can be mitigated by using wide PCB metal
stripes to decrease the impedance of power lines.
Where possible, signal tracks should be designed
considering their propagation delay vs. signal rise/
fall time and include a ground and a power plane. Slit
apertures in PCB layout should be strictly avoided,
particularly in ground planes or near current paths
to reduce unwanted antenna effects. Board metal
stripes should be kept as short as is practical,
and metal stubs which can cause reflection and
harmonics should be avoided.
Also, avoid overlapping power planes to reduce
system noise and power coupling. Reduce or
eliminate sharp bends in metal stripes (also
known as beveling or track mitering) to reduce
field concentration and run conducting stripes
orthogonally between adjacent layers to reduce
page 10
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20050 SW 112th Ave.
Tualatin, Oregon 97062
©CUI Inc 2013. All rights reserved. 09/2013
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