How to Protect Communication Ports from Unwanted Interruptions: Part 1 - Power Over Ethernet (PoE / PoE++)
September 29, 2021
Designers often neglect to consider that electrostatic discharge (ESD) and power-line induced transients can impact the electronic circuitry not directly connected to the AC power line. If they are designing devices with communication links, they need to ensure the data port circuitry is operating reliably at all times.
At all times, a transmitter must send uncorrupted information, and the receiver needs to decode the transmitted data and not add additional distortion. Designers need to address these challenges by developing circuits that are robust to environmental effects that can damage communication data ports. Disturbances such as lightning, ESD, overcurrent surges, and overvoltage transients can propagate through a circuit and damage the data port circuitry.
This article discusses how to protect Power over Ethernet (PoE) networks. It is the first in a three-part series on protecting data communication ports. The two remaining articles will offer recommendations for protecting high-speed and low-speed communication ports.
PoE is a communication technology that includes power along with data on Ethernet cabling. A single cable provides power and data to devices such as data center network routers, security cameras using internet protocol, voice over internet protocol (VOIP) telephones, wireless access points, and industrial control systems. Standard IEEE 802.3 defines the PoE communication protocol and has evolved to permit the use of higher power. Table 1 shows the original and subsequent revisions of the IEEE standard to accommodate higher power transmissions. The 2018 evolution of the standard, 802.3bt, referred to as PoE++, allows a maximum power of 90 W and as much as 960 mA of current on the data lines. Low-power devices may not need a line-powered supply as the PoE++ channel will supply the needed power. In addition, this standard allows Ethernet communication rates up to 10 Gbps, 10GBASE-T. However, the introduction of power with low voltage digital signals requires that PoE circuits be protected from conditions, such as current overloads, voltage transients including lightning and ESD, and other fast transients that can occur on an AC power line.
Table 1. IEEE 802.3 Standard: Evolution to enable transmission at higher power
Protecting a PoE++ Port
An example PoE++ design is shown in Figure 1 including recommended protection components for current overload protection and transient voltage protection. The circuitry between the RJ45 connector and the protection network is designed to protect both the powered device (PD) controller and the Ethernet Physical Layer (PHY) circuitry.
Figure 1. PoE++ protocol circuit: Recommended protection components
Consider using a fuse to protect each of the eight data lines from current overloads. A slow blow fuse will avoid nuisance shutdowns from current surges due to powering a switching power supply. Another condition that the fuse can help avoid is damage due to an incorrectly wired or shorted power line. Ensure that the fuse is compliant with standards such as IEC 62368-1, Telcordia GR-1089, and FCC 47 Part 8 Surge Specifications. Fuses that meet these requirements have working current ratings of about 2A or less. Consider a fuse with an interrupting rating of as much as 100 A so that the fuse can open and not vaporize even under the worst-case overload condition. Fuses that meet the referenced standards can open in approximately a second to a 250% overload. To enable low-cost assembly of the PC board (PCB), select a surface mount version that is suitable for reflow soldering.
On the center tap of the isolating signal transformers, we recommend using a protection thyristor connected to earth ground to absorb and prevent voltage transients, including lightning strikes, from passing through the signal transformers. Protection thyristors, like Littelfuse SIDACtors®, are crowbar-type devices with low on-state voltage and have the capacity to absorb high currents from transients. Versions of protection thyristors can:
- Crowbar transient voltages to as low as 6 V
- Absorb surge currents as high as 200 A
- Minimize voltage overshoot
- Provide a low capacitance of around 100 pF
- Absorb either polarity transient
- Avoid degradation from multiple surge events.
Note that a fuse combined with a protection thyristor satisfies the global regulatory standards for protecting telecommunications equipment, GR 1080 and IEC 62368-1.
Protecting the Ethernet Physical Layer Chipset
ESD strikes, cable discharge events, and electrical fast transients on the data lines are the major transients that can cause damage to an Ethernet PHY chipset. Transient voltage suppressor (TVS) diode arrays can provide the necessary protection. To protect all eight data lines, use two 4-channel TVS diode arrays (Figure 2). The benefits of using TVS diode arrays include:
- ESD protection against strikes as high as ±30 kV
- Absorption of transients with as much as 1000 W pulse power or as much as 45 A peak current
- Minimized signal distortion with only 2.5 pF per pin to ground
- Low power drain with 0.5 µA
- Space-saving µDFN-10 surface mount package.
Figure 2. Four-channel TVS diode array for protecting an Ethernet physical layer (PHY) circuit incorporating bi-directional diode pairs and a Zener diode for more clamping protection
Protecting the Powered Device (PD) Controller
The PD controller is a DC/DC converter that provides the DC power for energizing the PoE++ port. Separate circuit blocks show the AC/DC rectifier circuits. The rectifier circuits interface with incoming signals from the RJ45 connector. To protect the rectifier circuits from voltage transients, place bi-directional TVS diodes across the input lines. Versions of these series diode pairs are capable of absorbing a surge current of 200 A or as much as 1500 W of pulse power. TVS diodes have a very quick response time to transients, typically in less than 1 ps. Also, their leakage current is under 1 µA to minimize circuit power consumption.
To complete the PD controller’s protection, use a uni-directional TVS diode at the output of the rectifier and the input to the PD controller DC/DC supply. The TVS diode provides a fast response to transients. Select a TVS diode with an appropriate clamping voltage based on the semiconductor components in the circuit design.
Protecting a PoE Network in a Building
An intra-building PoE network is a less harsh environment; and, the PoE network only carries a maximum of 15.4 V or 350 mA. For this circuitry, consider using a 2-channel TVS diode to protect the PHY against harmful ESD events. The block diagram on the left side of Figure 3 details an example PoE network and shows the TVS diode array on the input/output lines to the Ethernet PHY chipset. A 2-channel TVS diode array schematic is shown in Figure 4. Consider using a protection diode array with the capability to absorb an ESD strike up to ±30 kV and a current surge in the range of 40 A. To minimize signal distortion of the Tx and Rx signals, look for TVS diode arrays with no more than 2 pF capacitance to ground. Also, look for a TVS diode array with low leakage current such as leakage current under 1 µA.
Figure 3. PoE indoor and outdoor circuits: Recommended protection components
Figure 4. Two-channel TVS diode array IC for voltage transient protection of an Ethernet PHY circuit
Protecting a PoE Network in an Outdoor Environment
The outdoors is a much more severe environment for electronics than an indoor environment. There is a higher risk for power-cross to cause over-current faults and also a higher risk of lightning-induced surge events. Like the PoE++ protection circuit, a time-delay fuse is recommended on each I/O line for any outdoor and harsh environment PoE circuits to protect against power-cross events. (See the example shown on the right side of Figure 3.) Due to the challenging environment, in addition to fuses, gas discharge tubes (GDTs) should be placed across the I/O lines. A GDT provides crowbar protection from lightning or other hazardous transients. GDTs have the following properties:
- The capacity to absorb and survive a current surge up to 1000 A
- Low capacitance, < 1 pF, independent of the voltage applied across the component
- Versions that have surface-mount packaging.
Note: The fuse and GDT combination should meet all the regulatory requirements as described for the PoE++ standard.
As with the other circuits, a TVS diode array can protect the Ethernet PHY chipset. In the case of the outdoor environment PoE circuit, consider a higher power TVS diode array. One such higher power, TVS diode array is the 2-channel component (Figure 5). These diode arrays are capable of absorbing 3000 W of transient power or 150 A of surge current. The internal Zener diode provides transient protection across the component, and the internal suppressor diodes deliver differential transient protection across individual channels.
Figure 5a. Two-channel TVS diode array package pin-out Figure 5. Two-channel TVS diode array transient protector
5b. Functional diagram
A Robust Design’s Value
Incorporating overload protection in communication system designs prevents damage to electronic circuits due to environmentally-induced overloads and transients. There is a cost for adding the protection components; however, the benefits of reduced service costs and an enhanced reputation for product quality far exceed the low cost of the additional components. Include protection requirements in the original design definition, and development time will be insignificantly impacted. Development time might even be reduced if re-work is not required due to the failure of a certification or compliance test. Also, to save development time, take advantage of a manufacturer’s expertise for assistance with the design and selection of protection components. The manufacturer can help with solutions that are cost-effective and offer the lowest cost of ownership. Some manufacturers can offer guidance on standards compliance; and, some manufacturers can perform pre-compliance testing to facilitate getting a product approved by a standards body without multiple rounds of testing. A product with a reputation for high quality can sustain a higher level of profitability and increase revenue.
To learn more, download the Circuit Protection Products Selection Guide, courtesy of Littelfuse, Inc.
Todd Phillips is the Global Strategic Market Manager for the Electronics Business Unit. He joined Littelfuse as a sales engineer in 2006 for the industrial POWR-GARD business unit. Todd joined the electronics business unit in 2011 as a regional sales manager. His current responsibilities include development of marketing collateral material, management of marketing activities for new product launches and performing market studies and feasibility analyses for new product ideas. He received his BSEE from Milwaukee School of Engineering. Todd can be reached at [email protected].