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Texas Instruments Protecting ethernet ports from surge events Application notes
Application Report
SLVAE50 – February 2019
Protecting ethernet ports from surge events
ABSTRACT
Ethernet networks are subject to many harsh conditions as a function of being in industrial applications.
One particularly harsh condition is a transient surge highlighted in IEC 61000-4-5. This surge pulse is
applied on the transmit and receive lines of the ethernet physical layer and can potentially damage the
ethernet controller, or phy. One statement that inevitably comes up during this discussion is that the
transformer in the system should isolate the sensitive phy from the pulse on the connector. However, as
shown in this application report, transient pulses can couple through the transformer and potentially
damage the ethernet phy. This means that adding protection between the transformer and the phy is
necessary in harsh industrial environments that will be subject to transient surges.
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Contents
Introduction ...................................................................................................................
Ethernet Protocol ............................................................................................................
Surge Pulse On Transformer ...............................................................................................
Conclusion ....................................................................................................................
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2
3
6
List of Figures
1
Ethernet Physical Layer..................................................................................................... 2
2
Transformer Schematic with Parasitics ................................................................................... 2
3
Differential Surge ............................................................................................................ 3
4
Common Mode Surge ....................................................................................................... 3
5
Differential Surge Waveform on Transformer ............................................................................ 3
6
Surge Pulse Setup with Transformer and Ethernet Phy ................................................................ 4
7
Diff Surge Pulse Just Before Phy Breaks
4
8
Diff Surge Pulse After Phy Breaks
4
9
10
11
................................................................................
........................................................................................
Surge Setup with Ethernet Physical Layer and ESDS314 .............................................................
Differential Surge Waveform with ESDS314 and Phy .................................................................
ESDS314 Protecting Phy From Differential Surge Pulse at 160 A ....................................................
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List of Tables
Trademarks
All trademarks are the property of their respective owners.
1
Introduction
Ethernet has been widely adopted in industrial markets as a robust way to transmit signals. In these
industrial markets the physical layer of the ethernet system is subject to harsh environment that can stress
the system as a whole. Surge transients are particularly dangerous for these system as they represent a
large amount of power that is coupled onto the transmit and receive circuitry of the ethernet physical layer.
This document goes through the dangers of surge pulses in ethernet applications showing why and what
protection is needed to make sure the ethernet controller can survive these transients.
(1)
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1
Ethernet Protocol
2
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Ethernet Protocol
Transformer
TX/RX +
TX/RX IO1
75
DP83867
Ethernet Phy
IO2
ESDS314
1 nF
GND
Figure 1. Ethernet Physical Layer
2.1
Physical Layer
The ethernet physical layer involves the ethernet phy (or controller), a transformer and a RJ-45 connector
as can be seen in Figure 1. Additionally there are shunt capacitors placed on the phy side while there is
an RC filter placed on the center tap of the transformer towards the connector side. When placing an
surge protection diode in the schematic, there is the option to place it before or after the transformer. On
the connector side of the transformer, the common mode voltage is uncontrolled. In the typical application
the voltages will be low but in power over ethernet (PoE) or a high voltage line coupling onto wires
extending from the connector can mean that the line is subject to very high common mode voltages.
Therefore the selection of surge protection diodes is extremely limited to ones that have a high enough
breakdown voltage to not turn on during the high common mode voltages but absorb the transients.
Additionally, having surge protection devices mean that they have to take the entirety of the surge pulse.
This is extremely problematic as surge events can go up to 6 kV with currents reaching 100 A. Extremely
few surge diodes are capable of handling that much energy and would have so much capacitance that it
would be impossible to have a high speed signal sent through.
2.2
Isolating Transformer
Having the transformer in the system means that there is isolation between ethernet wires such as CAT-5
or CAT-6 and the ethernet controller or phy. Transformers however do have parasitic elements associated
with them. Figure 2 shows the different parasitic elements present in a transformer specifically highlighting
one that is the culprit for why there is not full isolation in a transformer. On both the primary and
secondary side of the transformer there is a parasitic capacitance, resistance and inductance that is
present in all real world small signal analysis. These are due to the nature of the material and their
proximity to each other. However with having both the primary and secondary wound next to each other,
there is a winding capacitance introduced.
Winding
Capacitance
Primary
Side
Secondary
Side
Figure 2. Transformer Schematic with Parasitics
I = C * dV/dt
2
(2)
Protecting ethernet ports from surge events
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Surge Pulse On Transformer
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Equation 2 is the equation for a current through a capacitor. Since the surge pulse is providing a large
dV/dt on the primary side, the capacitor will have current flow from the primary side to the secondary side
but not fully isolating the two sides. This pulse seen on the secondary side is a percentage of the primary
side which at high levels can easily damage IC connected. Therefore this pulse must be measured and
protected against.
3
Surge Pulse On Transformer
Therefore the question that comes up is how necessary it is to have surge protection on the side of the
transformer that is not exposed to the harsh environment? There are two types of surge pulses that can
happen to the lines: differential and common mode surge. The common mode surge happens when a high
energy pulse capacitively couples onto all of the lines equally raising the common mode voltage. This can
happen during lightning strikes or other events but interestingly enough, the transformer can easily
dissipate this energy to the earth ground. This also means that the RC filter put on the center tap of the
transformer plays a role in protection. Looking at the isolated side of the transformer, it can be seen that
not much of the pulse gets through which means that the phy is protected until the transformer itself fails.
2
18 PF
Transformer
42
Transformer
0.5 PF
CWG
CWG
75
75
1 nF
1 nF
Figure 3. Differential Surge
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Figure 4. Common Mode Surge
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Surge Pulse On Transformer
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Figure 5. Differential Surge Waveform on Transformer
However, differential surge is quite different. Differential surge is when the surge pulse is coupled directly
on the signal line and returns on the other signal line. This type of surge pulse is very dangerous for
ethernet controllers because a portion of the surge pulse actually goes through the parasitic capacitance
of the transformer and causes a current spike. This pulse can be seen in Figure 5. It is important to note
that the reason there is no voltage on the secondary side of the transformer is because it is shorted
together.
3.1
Surge Protection Diode on Secondary Side of Transformer
Therefore since it can be seen that although the pulse that is transferred through the transformer is
reduced in peak and width, it could still potentially damage the phy. Using the same surge set up and
adding in the ethernet phy, the picture below shows the point just before and after it breaks the device.
Notice that the voltage that is measured on the secondary side is relatively high. This is due to the high
clamping voltage of the internal ESD protection of the phy. The waveforms below were taken measuring
the current on the primary side in blue, the current on the secondary side in green and the voltage on the
secondary side in yellow.
4
Protecting ethernet ports from surge events
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Surge Pulse On Transformer
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2
18 PF
Transformer
ESDS314
IO1
IO2
CWG
75
GND
1 nF
Figure 6. Surge Pulse Setup with Transformer and
Ethernet Phy
Figure 7. Diff Surge Pulse Just Before Phy Breaks
Figure 8. Diff Surge Pulse After Phy Breaks
Rerunning the same test but adding in the ESDS314 onto the signal lines that the surge was applied to, it
can be seen that the device is protected from this event.
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Conclusion
2
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18 PF
Transformer
TX/RX +
TX/RX -
CWG
DP83867
Ethernet Phy
75
1 nF
Figure 9. Surge Setup with Ethernet Physical Layer and
ESDS314
Figure 10. Differential Surge Waveform with ESDS314 and
Phy
Figure 11. ESDS314 Protecting Phy From Differential Surge Pulse at 160 A
4
Conclusion
Ethernet is widely used in a bevy of industrial end equipments. The ethernet protocol consists of a
transformer that is suppose to isolate the outside world and the ethernet controller. However due to the
parasitic nature of real transformers, there is not true isolation between the primary side and secondary
side. This causes issues when the ethernet physical layer is exposed to a surge event as the pulse
couples through the parasitic capacitance and can damage the controller on the secondary side.
Therefore it is necessary to provide protection on the secondary side of the transformer that can handle
the large currents produced. TI's ESDS3XX devices have high surge current rating that allow them to
dissipate this energy and protect the ethernet controller.
6
Protecting ethernet ports from surge events
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