AN-1161 [ADI]
EMC-Compliant RS-485 Communication Networks; EMC兼容RS - 485通信网络
| 型号： | AN-1161 |
| 厂家： | 
ADI |
| 描述： | EMC-Compliant RS-485 Communication Networks |
| 文件： | 总16页 (文件大小：239K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
AN-1161
APPLICATION NOTE
One Technology Way • P. O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
EMC-Compliant RS-485 Communication Networks
by James Scanlon
INTRODUCTION
In real industrial and instrumentation (I&I) applications, RS-485
communication links must work in harsh electromagnetic
three different EMC-compliant solutions for three different
cost/protection levels for RS-485 communication ports. These
different solutions are illustrated in Figure 1.
environments. Large transient voltages caused by lightning strikes,
electrostatic discharge, and other electromagnetic phenomenon
can cause damage to communication ports. These data ports must
meet certain electromagnetic compatibility (EMC) regulations to
ensure that they can survive in their final installation environments.
Analog Devices, Inc., and Bourns, Inc., have partnered to
extend their offering of system oriented solutions by co-
developing the industry’s first EMC-compliant RS-485 interface
customer design tool.
Within these requirements, there are three transient immunity
standards: electrostatic discharge, electrical fast transients, and
surge. Leaving EMC considerations to the end of the design
cycle leads to penalties, such as engineering budget and
schedule overruns. Many EMC problems are not simple or
obvious and must be considered at the start of product design.
This tool provides, up to and including, Level 4 protection levels
for IEC 61000-4-2 ESD, IEC 61000-4-4 EFT, and IEC 61000-4-5
surge. It gives designers the design options depending upon the
level of protection required and available budgets. These design
tools allow designers to reduce risk of project slippage due to
EMC problems by considering them at the start of the design cycle.
This application note describes each of these transients,
presents the design solution methodology, and demonstrates
V
V
CC
CC
ADM3485E
ADM3485E
B
A
B
A
RO
RE
RO
RE
TBU
DI
DI
TISP
DE
DE
TVS
TVS
PROTECTION SCHEME 1. TVS
PROTECTION SCHEME 2. TVS/TBU/TISP
V
CC
ADM3485E
B
A
RO
RE
GDT
TBU
DI
DE
TVS
PROTECTION SCHEME 3. TVS/TBU/GDT
Figure 1. Three EMC Compliant Solutions
Rev. 0 | Page 1 of 16
AN-1161
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
Theory of Protection.........................................................................9
RS-485 Transient Suppression Networks .................................... 10
Protection Scheme 1.................................................................. 10
Protection Scheme 2.................................................................. 11
Protection Scheme 3.................................................................. 12
Conclusion....................................................................................... 13
References........................................................................................ 14
Revision History ............................................................................... 2
RS-485 Standard ............................................................................... 3
Electromagnetic Compatibility....................................................... 4
Electrostatic Discharge ................................................................ 4
Electrical Fast Transients............................................................. 5
Surge............................................................................................... 7
Pass/Fail Criteria .......................................................................... 8
REVISION HISTORY
2/13—Revision 0: Initial Version
Rev. 0 | Page 2 of 16
Application Note
AN-1161
RS-485 STANDARD
I&I applications require the transmission of data between
multiple systems, often over very long distances. The RS-485
bus standard is one of the most widely used physical layer bus
designs in I&I applications. Applications for RS-485 include
process control networks; industrial automation; remote
terminals; building automation, such as heating, ventilation,
and air conditioning (HVAC); security systems; motor control;
and motion control.
•
•
Wide common-mode range (−7 V to +12 V) allows for
differences in ground potential between the driver and
receiver.
TIA/EIA-485-A allow for data rates of tens of Mbps.
TIA/EIA-485-A, the telecommunication industry’s most widely
used transmission line standard, describes the physical layer of
the RS-485 interface and is normally used with a higher level
protocol, such as Profibus, Interbus, Modbus, or BACnet. This
allows for robust data transmission over relatively long distances.
The key features of RS-485 that make it ideal for use in I&I
communications applications are
In real applications, however, lightning strikes, power induction
and direct contact, power source fluctuations, inductive
switching, and electrostatic discharge can cause damage to
RS-485 transceivers by generating large transient voltages.
Designers must ensure that equipment does not just work in
ideal conditions, but that it must also work in real world
situations. In order to ensure that these designs can survive in
electrically harsh environments, various government agencies
and regulatory bodies have imposed EMC regulations.
Compliance with these regulations assures the end user that
designs will operate as desired in these harsh electromagnetic
environments.
•
•
Long distance links—up to 4000 feet.
Bidirectional communications possible over a single pair of
twisted cables.
•
•
Differential transmission increases noise immunity and
decreases noise emissions.
Multiple drivers and receivers can be connected on the
same bus.
Rev. 0 | Page 3 of 16
AN-1161
Application Note
ELECTROMAGNETIC COMPATIBILITY
EMC is the ability of an electronic system to function satis-
factorily in its intended electromagnetic environment without
introducing intolerable electromagnetic disturbances to that
environment. An electromagnetic environment is composed of
both radiated and conducted energy. Therefore, EMC has two
aspects, emission and susceptibility.
object. It can also occur as a result of triboelectric charging,
which is the generation of static electricity caused by rubbing
two substances together. Alternatively, an object can be charged
as a result of induction charging. In this case, there is no
physical contact with the charged object yet charging can occur
if it is within the electric field of the charged object.
Emission is the unwanted generation of electromagnetic energy
by a product. It is often desirable to control emission in order to
create an electromagnetically-compatible environment.
The primary purpose of the IEC 61000-4-2 test is to determine
the immunity of systems to external ESD events outside the
system during operation. IEC 61000-4-2 specifies testing using
two coupling methods, contact discharge and air-gap discharge.
Contact discharge implies the discharge gun is placed in direct
connection with the unit under tested. Air gap discharge uses a
higher test voltage, but does not make direct contact with the unit
under test.
Susceptibility is a measure of the ability of electronic products
to tolerate the influence of electromagnetic energy radiated or
conducted from other electronic products or electromagnetic
influences. Immunity is the opposite of susceptibility.
Equipment that has high susceptibility has low immunity.
During air discharge testing, the charged electrode of the
discharge gun is moved toward the unit under test until a
discharge occurs as an arc across the air gap. The discharge
gun does not make direct contact with the unit under test. A
number of factors affect the results and repeatability of the air
discharge test, including humidity, temperature, barometric
pressure, distance, and rate of approach to the unit under test.
This method is a better representation of an actual ESD event,
but is not as repeatable. Therefore, contact discharge is the
preferred test method.
The international electrotechnical commission (IEC) is the
world’s leading organization that prepares and publishes
international standards for all electrical, electronic, and related
technologies. Since 1996, all electronic equipment sold to or
within the European community must meet EMC levels as
defined in specification IEC 61000-4-x.
The IEC 61000 specifications define the set of EMC immunity
requirements that apply to electrical and electronic equipment
intended for use in residential, commercial, and light industrial
environments. Within this set of specifications, there are three
types of high voltage transients that electronic designers need to
be concerned about for data communication lines. These are
IEC 61000-4-2 specifies voltage test levels for different environ-
mental conditions along with a current waveform. Table 1
shows the relationship between the environment and the test
voltage. The test levels should be selected in accordance with
the most realistic installation and environment conditions the
final product will be subjected to.
•
•
•
IEC 61000-4-2 Electrostatic Discharge (ESD)
IEC 61000-4-4 Electrical Fast Transients (EFT)
IEC 61000-4-5 Surge Immunity
Level 1 is the least severe and Level 4 the most severe. Level 1
and Level 2 are for products installed in controlled environ-
ments that have antistatic material. Level 3 and Level 4 are for
products installed in more severe environments where ESD
events with higher voltages are more common.
This application note deals with increasing the protection level
of RS-485 ports to protect against the these three main EMC
transients.
Each of these specifications defines a test method to assess
the immunity of electronic and electrical equipment against
the defined phenomenon. The following sections provide a
summary of each of these tests.
Figure 2 shows the 8 kV contact discharge current waveform
as described in the specification. Some of the key waveform
parameters to note are fast rise times of less than 1 ns and short
pulse widths of approximately 60 ns. This equates to a pulse
with total energy in the range of tens of mJ.
ELECTROSTATIC DISCHARGE
ESD is the sudden transfer of electrostatic charge between
bodies at different potentials caused by near contact or induced
by an electric field. It has the characteristics of high current in a
short time period.
The test is performed with single discharges. The test point is
subjected to at least 10 positive and 10 negative discharges. A 1 s
interval between discharges is recommended.
An object can become charged due to a number of mechanisms.
A charge can occur by simple contact with another charged
Rev. 0 | Page 4 of 16
Application Note
AN-1161
I
PEAK
30A
90%
I
I
16A
30ns
60ns
8A
10%
t
tR = 0.7ns TO 1ns
30ns
60ns
Figure 2. IEC 61000-4-2 ESD Waveform (8 kV)
Table 1. IEC 61000-4-2 Test Levels and Installation Classes
Relative Humidity
As Low as %
Antistatic
Material
Synthetic
Material
Contact Discharge
Test Voltage (kV)
Air Discharge
Test Voltage (kV)
Level/Class
1
2
3
4
35
10
50
10
X
X
2
4
6
8
2
4
8
15
X
X
ELECTRICAL FAST TRANSIENTS
Electrical fast transient testing involves coupling a number
of extremely fast transient impulses onto the signal lines to
represent transient disturbances associated with external
switching circuits that are capacitively coupled onto the
communication ports. This may include relay and switch
contact bounce or transients originating from the switching
of inductive or capacitive loads—all of which are common in
industrial environments. The EFT test defined in IEC 61000-4-4
attempts to simulate the interference resulting from these types
of events.
contact. This also reduces the loading caused by the low output
impedance of the EFT generator. The coupling capacitance
between the clamp and cable depends on cable diameter,
shielding, and insulation on the cable.
IEC 61000-4-4 specifies voltage test levels for different environ-
mental conditions. Table 2 shows the test voltage and pulse
repetition rates for the different test levels. The test levels should
be selected according to the most realistic installation and
environmental conditions the final product will be subjected to.
Traditionally, 5 kHz repetition rates are used, however this rate
is generally dependent on the end manufacturers specification.
Figure 3 shows the EFT 50 Ω load waveform. The EFT
waveform is described in terms of a voltage across 50 Ω
impedance from a generator with 50 Ω output impedance.
The output waveform consists of a 15 ms burst 5 kHz high
voltage transients repeated at 300 ms intervals. Each individual
pulse has a rise time of 5 ns and pulse duration of 50 ns,
measured between the 50% point on the rising and falling edges
of the waveform. Similar to the ESD transient, the EFT pulse
has the characteristics of fast rise time and short pulse width.
The total energy in a single pulse is similar to that of an ESD
pulse. Voltages applied to the data ports can be as high as 2 kV.
•
•
•
•
Level 1
Level 2
Level 3
Level 4
well protected
protected environments
typical industrial environment
severe industrial environment
Table 2. IEC 61000-4-4 Test Levels
Data Port Test Voltages and Repetition Rates
Level
Voltage Peak (kV)
Repetition Rate (kHz)
5 or 100
5 or 100
5 or 100
5 or 100
1
2
3
4
0.25
0.5
1
These fast burst transients are coupled onto the communication
lines using a capacitive clamp. The EFT is capacitively coupled
onto the communication lines by the clamp rather than direct
2
Rev. 0 | Page 5 of 16
AN-1161
Application Note
V
PEAK
100%
90%
tR = 5ns ± 30%
tD = 50ns ± 30%
SINGLE
PULSE
50%
tD
tR
10%
t
(ns)
15ms
BURST
OF PULSES
t
(ms)
V
PEAK
REPETITIVE
BURSTS
300ms
t
(ms)
Figure 3. IEC 61000-4-4 EFT 50 Ω Waveform
Rev. 0 | Page 6 of 16
Application Note
AN-1161
SURGE
Surge transients are caused by overvoltages from switching or
lightning transients. Switching transients can result from power
system switching, load changes in power distribution systems,
or various system faults, such as short circuits and arching faults
to the grounding system of the installation. Lightning transients
can be a result of high currents and voltages injected into the
circuit from nearby lightning strikes. IEC 61000-4-5 defines
waveforms, test methods, and test levels for evaluating the
immunity of electrical and electronic equipment when
subjected to these surges.
distance signal connections. For RS-485 ports, the 1.2 µs/50 µs
waveform is predominantly used. The waveform generator has
an effective output impedance of 2 Ω, thus the surge transient
has high currents associated with it.
Figure 4 shows the 1.2 µs /50 µs surge transient waveform. ESD
and EFT have similar rise times, pulse widths, and energy levels.
With surge, the rise time of the pulse is 1.25 µs and the pulse
width is 50 µs. The surge pulse energy can have energy levels
that are three to four orders of magnitude larger than the energy
in an ESD or EFT pulse. Therefore, the surge transient is
The waveforms are specified as the outputs of a waveform
generator in terms of open-circuit voltage and short-circuit
current. Two waveforms are described. The 10 µs/700 µs
combination waveform is used to test ports intended for
connection to symmetrical communication lines, for example
telephone exchange lines. The 1.2 µs/50 µs combination
waveform generator is used in all other cases, in particular short
considered the most severe of the EMC transient specs. Due to
the similarities between ESD and EFT, the design of the circuit
protection can be similar, however, due to its high energy, surge
must be dealt with differently. This is one of the main issues in
developing protection circuitry that improves the immunity of
data ports to all three transients while remaining cost effective.
V
PEAK
100%
90%
t1 = 1.2µs ± 30%
t2 = 50µs ± 20%
50%
10%
t2
t (µs)
30% MAX
t1
Figure 4. IEC 61000-4-5 Surge 1.2 µs/50 µs Waveform
Rev. 0 | Page 7 of 16
AN-1161
Application Note
Resistors couple the surge transient onto the communication
line. Figure 5 shows the coupling network for a half-duplex
RS-485 device. The total parallel sum of the resistance is 40 Ω.
For the half-duplex device, each resistor is 80 Ω.
A summary of the installation classes and the surge voltage for
each class is shown in Table 4. Table 4 shows the test voltages
associated with each class for line to ground coupling for
symmetric and unsymmetrical lines. It is important that the
final environment class is known to ensure the product is
immune to the threat level.
V
CC
ADM3485E
CDN
Table 4. IEC 61000-4-5 Installation Classes
B
A
RO
RE
80Ω
80Ω
Installation
Class
Unsymetrical Lines
Test levels
Symetrical Lines
Test Levels
PROTECTION
COMPONENTS
0
1
2
3
4
5
NA
NA
DI
0.5 kV
1 kV
2 kV
4 kV
4 kV
0.5 kV
1 kV
2 kV
2 kV
4 kV
DE
Figure 5. Surge Coupling Network for a Half-Duplex RS-485 Device
Table 3. IEC 61000-4-5 Test Levels
During the surge test, five positive, and five negative pulses are
applied to the data ports with a maximum time interval of
1 minute between each pulse. The standard states that the
device should be set up in normal operating conditions for the
duration of the test.
Level
Open-Circuit Test Voltage
1
2
3
4
X
0.5 kV
1 kV
2 kV
4 kV
Special
PASS/FAIL CRITERIA
When transients are applied to the system under test, the results
are categorized into four pass/fail criteria. Following is a list of
the pass/fail criteria giving examples how each might relate to
an RS-485 transceiver:
The test levels as defined in the IEC 61000-4-5 are shown in
Table 3. Level X can be above, below, or in between the other
levels. This is usually specified in the product standard. The
test levels should be selected according to the installation
conditions. There are six classes of installations defined in the
specification.
A. Normal performance; no bit errors would occur during or
after the transient is applied.
B. Temporary loss of function or temporary degradation of
performance not requiring an operator; bit errors might
occur during and for a limited time after the transient is
applied.
C. Temporary loss of function or temporary degradation of
performance requiring an operator; a latch-up event may
occur that could be removed after a power on reset with no
permanent or degradation to the device.
D. Loss of function with permanent damage to equipment.
The device fails the test.
•
•
•
Class 0 (well protected electrical environment)
Class 1 (partially protected electrical environment)
Class 2 (electrical environment where cables are well
separated, even at short runs)
•
•
Class 3 (electrical environment where power and signal
cables run in parallel)
Class 4 (electrical environment where the intercom-
nections are running as outdoor cables along with power
cables, and cables are used for both electronic and
electrical circuits)
Criteria A is the most desirable and Criteria D is unacceptable.
Permanent damage results in system downtime and the expense of
repair and replacement. For mission critical systems, Criteria B
and Criteria C are also unacceptable because the system must
operate without errors during transient events.
•
•
Class 5 (electrical environment for electronic equipment
connected to telecommunication cables and overhead
power lines in a nondensely populated environment)
Class X (special conditions specified in the product
specifications)
Class 0 has no surge transient treat associated with it. Class 5
has the most severe transient stress level.
Rev. 0 | Page 8 of 16
Application Note
AN-1161
THEORY OF PROTECTION
There are three main ways to prevent EMC problems.
market, each one with its own advantages and disadvantages.
Developing protection for a system usually requires the use of
both overvoltage and overcurrent protection devices.
•
•
•
Suppress the transient at source.
Make the coupling path as inefficient as possible.
Make the device less susceptible to the transient.
Figure 6 shows a typical design for a protection scheme.
The design can be characterized by having primary and
secondary protection. Primary protection diverts most of the
transient energy away from the system and is typically located
at the interface between the system and the environment. It is
designed to remove the majority of the energy by diverting the
transient to ground.
Often it is not possible to remove the source of the transient,
for example, it is not possible to control where lightning strikes
occur. Reducing the possibility of coupling is often beyond the
manufacturers control when the final product is installed. In
order to ensure the product is EMC compatible, it is often
necessary for the manufacturer to add protection to the data
ports to make the product less susceptible to these transients.
The function of the secondary protection is to protect various
parts of the system from any transient voltages and currents let
through by the primary protection. The secondary protection is
usually designed to be more specific to the part of the system it
is protecting. It is optimized to ensure that it protects against
these residual transients while allowing normal operation of
these sensitive parts of the system. It is essential that both the
primary and secondary designs are specified to work together
in conjunction with the system input/output to minimize the
stress on the protected circuit.
When designing protection circuitry to protect against
transients, consider the following:
•
•
•
It must prevent or limit damage caused by the transient
and allow the system to return to normal operation with
minimal impact on performance.
The protection scheme should be robust enough to deal
with the type of transients and voltage levels the system
would be subjected to in the field.
The length of time associated with the transient is an
important factor. For long transients, heating effects can
cause certain protection schemes to fail.
These designs typically include a coordinating element, such
as a resistance or a nonlinear overcurrent protection device,
between the primary and secondary protection devices to
ensure that coordination occurs.
•
•
Under normal operation conditions, the protection
OVERCURRENT
PROTECTOR
circuitry should not interfere with the system operation.
If the protection circuitry fails during overstress, it should
fail in a way that protects the system.
HARSH
ELECTROMAGNETIC
ENVIRONMENT
SYSTEM
There are two main types of protection schemes used to protect
against transients. Overcurrent protection is used to limit
peak current and overvoltage protection is used to limit peak
voltages. There is a broad range of overcurrent and overvoltage
protection technologies and components available in the
OVERVOLTAGE
PROTECTOR
OVERVOLTAGE
PROTECTOR
Figure 6. Protection Scheme—Block Diagram
Rev. 0 | Page 9 of 16
AN-1161
Application Note
RS-485 TRANSIENT SUPPRESSION NETWORKS
EMC transient events vary in time, so the dynamic performance
and the matching of the dynamic characteristics of the protection
components with the input/output stage of the protected device
leads to successful EMC design. Component data sheets generally
only contain dc data, which is of limited value given that the
dynamic breakdowns and I/V characteristics can be quite different
from the dc values. Careful design, characterization, and an
understanding of the dynamic performance of the input/output
stage of the protected device and the protection components is
required to ensure that the circuit meets EMC standards.
V
CC
ADM3485E
B
A
RO
RE
DI
DE
TVS
This application note presents three different fully characterized
EMC-compliant solutions. Each solution was certified by an
independent external EMC compliance test house, and each
provides different cost/protection levels for the Analog Devices
ADM3485E 3.3 V RS-485 transceiver with enhanced ESD
protection using a selection of Bourns external circuit
protection components. The Bourns external circuit protection
components used consist of transient voltage suppressors
(CDSOT23-SM712), transient blocking units (TBU-CA065-
200-WH), thyristor surge protectors (TISP4240M3BJR-S), and
gas discharge tubes (2038-15-SM-RPLF).
Figure 7. Protection Scheme 1—TVS
Table 5. Scheme 1 Protection Levels
ESD (-4-2)
EFT (-4-4)
Surge (-4-5)
Voltage
Level (Contact/Air)
Level Voltage Level Voltage
2 kV 1 kV
4
8 kV/15 kV
4
2
A TVS is a silicon-based device. Under normal operating
conditions, the TVS has high impedance to ground; ideally, it is
an open circuit. The protection is accomplished by clamping the
overvoltage from a transient to a voltage limit. This is done by
the low impedance avalanche breakdown of a PN junction.
When a transient voltage larger than the breakdown voltage
of the TVS is generated, the TVS clamps the transient to a
predetermined level that is less than the breakdown voltage of
the devices that it is protecting. The transients are clamped
instantaneously (< 1 ns) and the transient current is diverted
away from the protected device to ground.
Each solution was characterized to ensure the dynamic I/V
performance of the protection components protect the dynamic
I/V characteristics of the ADM3485E RS-485 bus pins. It is the
interaction between the input/output stage of the ADM3485E
and the external protection components that function together
to protect against the transient events.
PROTECTION SCHEME 1
It is important to ensure that the breakdown voltage of the TVS
is outside the normal operating range of the pins protected.
As demonstrated in Figure 8, the unique feature of the
CDSOT23-SM712 is that it has asymmetrical breakdown
voltages of +13.3 V and –7.5 V to match the transceiver
common-mode range of +12 V to –7 V, therefore providing
optimum protection while minimizing overvoltage stresses on
the ADM3485E RS-485 transceiver.
The EFT and ESD transient have similar energy levels, while
the surge waveform has energy levels three to four magnitudes
greater. Protecting against ESD and EFT is accomplished in
a similar manner, but protecting against high levels of surge
requires solutions that are more complex. The first solution
described protects up to Level 4 ESD and EFT and Level 2
surge. The 1.2 μs/50 μs waveform is used in all surge testing
described in this application note.
I
This solution uses the Bourns CDSOT23-SM712 transient
voltage suppressor (TVS) array which consists of two
bidirectional TVS diodes as illustrated in Figure 7. Table 5
shows the voltage levels protected against for ESD, EFT, and
surge transients.
V
= 7.5V
BR
V
V
= 13.3V
BR
Figure 8. CDSOT23-SM712 I/V Characteristic
Rev. 0 | Page 10 of 16
Application Note
AN-1161
overcurrent protection component with a preset current limit
and a high voltage withstand capability. When an overcurrent
occurs and the TVS breaks down due to the transient event, the
current in the TBU will rise to the current limiting level set by
the device. At this point, the TBU disconnects the protected
circuitry from the surge in less than 1 µs.
PROTECTION SCHEME 2
The previous solution protects up to Level 4 ESD and EFT, but
only to Level 2 surge. To improve the surge protection level,
the protection circuitry gets more complex. The protection
solution presented in this section protects up to Level 4 surge.
The CDSOT23-SM712 is specifically designed for RS-485 data
ports. The next two solutions build on the CDSOT23-SM712
to provide higher levels of circuit protection. In this solution,
the CDSOT23-SM712 provides secondary protection while the
TISP4240M3BJR-S provides the primary protection.
During the remainder of the transient, the TBU remains in
the protected blocking state, with very low current (<1 mA)
passing through the protected circuit. Under normal operating
conditions, the TBU exhibits low impedance, so it has minimal
impact on normal circuit operation. In blocking mode, it has
very high impedance to block transient energy. After the
transient event, the TBU automatically resets to its low imped-
ance state and reinstates the system allowing resumption of
normal operation.
Coordination between the primary and secondary protection
devices, and overcurrent protection is accomplished using
the TBU-CA065-200-WH. Table 6 shows the voltage levels
protected against for ESD, EFT, and surge transients with this
solution. Figure 9 shows a representation of the complete
solution.
Like all overcurrent protection technologies, the TBU has a
maximum breakdown voltage, so a primary protection device
must clamp the voltage and redirect the transient energy to
ground. This is commonly done using technologies, such as gas
discharge tubes or solid-state thyristors, such as the totally
integrated surge protector (TISP). The TISP acts as a primary
protection device. When its predefined protection voltage is
exceeded, it provides a crowbar low impedance path to ground,
thus diverting the majority of the transient energy away from
the system and other protection devices.
V
CC
ADM3485E
B
A
RO
RE
TBU
DI
TISP
DE
The nonlinear voltage-current characteristic of the TISP limits
overvoltage by diverting the resultant current. As a thyristor, a
TISP has a discontinuous voltage-current characteristic caused
by the switching action between high and low voltage regions.
Figure 10 shows the voltage-current characteristic of the device.
Before the TISP device switches into a low voltage state, with
low impedance to ground to shunt the transient energy, a
clamping action is caused by the avalanche breakdown region.
TVS
Figure 9. Protection Scheme 2—TVS/TBU/TISP
Table 6. Scheme 2 Protection Levels
ESD (-4-2)
EFT (-4-4)
Surge (-4-5)
Voltage
Level (Contact/Air)
Level Voltage Level Voltage
2 kV 4 kV
4
8 kV/15 kV
4
4
In limiting an overvoltage, the protected circuitry will be
exposed to a high voltage for the brief time period that the TISP
device is in the breakdown region, before it switches into a low
voltage protected on-state. The TBU will protect the down-
stream circuitry from high currents resulting from this high
voltage. When the diverted current falls below a critical value,
the TISP device automatically resets allowing normal system
operation to resume.
When a transient is applied to the protection circuit, the TVS
breaks down providing a low impedance path to ground to
protect the device. With large voltages and currents, there is a
need to protect the TVS and limit the current through it. This is
done using a transient blocking unit (TBU), which is an active
high speed overcurrent protection element. The TBU in this
design is the Bourns TBU-CA065-200-WH.
A TBU blocks current rather than shunting it to ground. As a
series component, it reacts to current through the device rather
than the voltage across the interface. A TBU is a high speed
As described, all three components work together in
conjunction with the system input/output to protect the system
from high voltage and current transients.
Rev. 0 | Page 11 of 16
AN-1161
Application Note
I
OVERVOLTAGE
BREAKDOWN
REGION
VOLTAGE
PROTECTION
LEVEL
SYSTEM
RATED
VOLTAGE
CURRENT AT
RATED VOLTAGE
TISP
V
SYSTEM
RATED
VOLTAGE
Figure 10. TISP Switching Characteristic and Voltage Limiting Waveshape
PROTECTION SCHEME 3
Protection levels above Level 4 surge are often required. The
protection scheme shown in Figure 11 protects RS-485 ports
up to and including 6 kV surge transients. It operates in a
similar fashion to Protection Scheme 2; however, in this circuit,
a gas discharge tube (GDT) is used in place of the TISP to
protect the TBU, which is, in turn, protecting the TVS, the
secondary protection device. The GDT will provide protection
to higher overvoltage and overcurrent stress than the TISP
described in the Protection Scheme 2 section. The GDT for
this protection scheme is the Bourns 2038-15-SM-RPLF. The
TISP is rated at 220 A vs. the GDT rating of 5 kA per conductor.
Table 7 summarizes the protection levels provided by this design.
impedance off-state to arc mode. In arc mode, the GDT
becomes a virtual short, providing a crowbar current path to
ground and diverting the transient current away from the
protected device.
Figure 12 shows the typical characteristics of a GDT. When the
voltage across a GDT increases, the gas in the tube starts to
ionize due to the charge developed across it. This is known as
the glow region. In this region, the increased current flow
creates an avalanche effect that transitions the GDT into a
virtual short circuit, allowing current to pass through the
device. During the short-circuit event, the voltage developed
across the device is known as the arc voltage. The transition
time between the glow and arc region is highly dependent on
the physical characteristics of the device.
V
CC
ADM3485E
IMPULSE SPARKOVER
VOLTAGE (TYPICAL 500V)
B
A
RO
RE
GDT
TBU
DI
ARC
REGION
DE
GLOW
REGION
TVS
Figure 11. Protection Scheme 3—TVS/TBU/GDT
Table 7. Scheme 3 Protection Levels
ESD (-4-2)
EFT (-4-4)
Surge (-4-5)
Voltage
Level (Contact/Air)
ARC VOLTAGE
(TYPICAL 10V TO 20V)
Level Voltage Level Voltage
2 kV 6 kV
4
8 kV/15 kV
4
X
TIME
Predominately used as a primary protection device, a GDT
provides a low impedance path to ground to protect against
overvoltage transients. When a transient voltage reaches the
GDT spark-over voltage, the GDT switches from a high
Figure 12. GDT Characteristic Waveform
Rev. 0 | Page 12 of 16
Application Note
AN-1161
CONCLUSION
This application note describes the three IEC standards of
interest that deal with transient immunity. In real industrial
applications, RS-485 communication ports subjected to these
transients can be damaged. EMC problems discovered late in a
product design cycle may require expensive redesign and can
often lead to schedule overruns. EMC problems should
therefore be considered at the start of the design cycle and not
at a later stage where it may be too late to achieve the desired
EMC performance.
This application note demonstrated three different EMC
compliant solutions for RS-485 communication ports, giving
the designer options depending on the level of protection
required. The EVAL-CN0313-SDPZ is industry’s first EMC-
compliant RS-485 customer design tool, providing up to Level 4
protection levels for ESD, EFT, and surge. The protection levels
offered by the different protection schemes are summarized in
Table 7.
While these design tools do not replace the due diligence or
qualification required at the system level, they allow the
designer to reduce the risk of project slippage due to EMC
problems at the start of the design cycle, thus reducing design
time and time to market. For more information, visit:
www.analog.com/RS485emc.
The key challenge in designing EMC-compliant solutions for
RS-485 networks is matching the dynamic performance of the
external protection components with the dynamic performance
of the input/output structure of the RS-485 device.
Table 4. Three ADM3485E EMC-Compliant Schemes
ESD (-4-2)
EFT(-4-4)
Voltage
Surge (-4-5)
Voltage
Protection Scheme
1. TVS
2. TVS/TBU/TISP
3. TVS/TBU/GDT
Level
(Contact/Air)
8 kV/15 kV
8 kV/15 kV
8 kV/15 kV
Level
Level
Voltage
1 kV
4 kV
4
4
4
4
4
4
2 kV
2 kV
2 kV
2
4
X
6 kV
Rev. 0 | Page 13 of 16
AN-1161
Application Note
REFERENCES
More information regarding interface and isolation products is
listed in this section (also see the Analog Devices website).
Electromagnetic Compatibility (EMC) Part 4-4: Testing and
Measurement Techniques—Electrical Fast Transient/Burst
Immunity Test (IEC 61000-4-4:2012 (Ed3.0)).
See the Bourns website for information on parts mentioned in
this document as well as for the First Principles document and
the Bournes Telecom Protection Guide.
Electromagnetic Compatibility (EMC) Part 4-5: Testing and
Measurement Techniques—Surge Immunity Test (IEC
61000-4-5:2005 (Ed2.0)).
ADM3485E Data Sheet. Analog Devices, Inc.
EVAL-CN0313-SDPZ. www.analog.com/RS485emc.
Electromagnetic Compatibility (EMC) Part 4-2: Testing and
Measurement Techniques—Electrostatic Discharge
Immunity Test (IEC 61000-4-2:2008 (Ed.2.0)).
Marais, Hein. “RS-485/RS-422 Circuit Implementation Guide.”
Application Note AN-960. Analog Devices, Inc.
Rev. 0 | Page 14 of 16
Application Note
NOTES
AN-1161
Rev. 0 | Page 15 of 16
AN-1161
NOTES
Application Note
©2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN10904-0-2/13(0)
Rev. 0 | Page 16 of 16
相关型号：
AN-118
Detecting Line Polarity Reversal Associated with BT Calling Line Identification Protocol
IXYS
©2020 ICPDF网 联系我们和版权申明