Protecting Power Over Ethernet (PoE / PoE++) Communications

This article is the first in the "Protect Your Ports! Top Design Tips to Keep Your Communications Connected" series from Littelfuse.

Transmitting and receiving information between data ports on an information channel is the primary objective for the design of data communication systems. In addition, the communication systems need to be reliable so that they can be functional at all times ensuring critical data is not lost or interrupted. To develop robust and reliable communication systems, designers need to consider the impact of the external environment on their data ports. Disturbances such as lightning, electrostatic discharge (ESD), overcurrent surges, and overvoltage transients can damage the communication integrated circuits (ICs).

Designers have the challenges of incorporating protection against these disturbances into their designs without compromising the performance of their circuitry, maintaining the size requirements of the design, and controlling its cost. This article, the first in a series of three, will assist electronics design engineers in overcoming these challenges by offering protection schemes for their data port designs.

Part 1of this series will present protection recommendations for Power over Ethernet (PoE). The second and third parts will cover protecting high speed and low-speed interfaces. 

PoE is a transmission technology that passes power along with data on Ethernet cabling. A single cable provides power and data to devices such as voice over internet protocol (VOIP) telephones, security cameras using internet protocol, wireless access points, data center network routers, and industrial control systems. The IEEE standard for PoE is 802.3 and has been evolving since 2003 to enable the use of higher power.

Table 1 shows the original and subsequent revisions of IEEE standard 802.3 to accommodate higher power transmissions.

The 2018 evolution of the standard, 802.3bt, commonly referred to as PoE++, allows a maximum power of 90 W and as much as 960 mA of current on the data lines. Furthermore, this standard allows Ethernet transmission rates that can reach 10 Gbps, 10GBASE-T. However, the inclusion of power with low voltage digital signals requires that PoE circuits be protected from current overloads and voltage transients such as lightning, ESD, and other fast transients that propagate on an AC power line. 

Protecting a PoE++ Port

Figure 1 illustrates an example PoE++ design and includes 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 Ethernet Physical Layer (PHY) circuitry and the powered device (PD) controller.

A fuse is recommended to protect each of the eight data lines from current overloads. Consider a slow blow fuse to avoid nuisance shutdowns from current surges due to a switching power supply or lightning surges. Another condition that the fuse can help avoid is damage due to an incorrectly wired or shorted power line. Ensure that the fuse you select 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. 

Look for 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 the efficient assembly of the PCB, select a surface mount version that is suitable for reflow soldering. 

On the center tap of the isolating signal transformers, use 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 the capacity to absorb high currents from transients.

Versions of protection thyristors can:

• Crowbar a transient voltage to as low as 6 V
• Absorb a surge current as high as 200 A
• Minimize voltage overshoot
• Have a low capacitance of around 100 pF
• Absorb either polarity transient
• Avoid degradation from multiple surge events.

A fuse combined with a protection thyristor complies with global regulatory standards, GR 1080 and IEC 62368-1, for protecting telecommunications equipment. 

Protecting the Ethernet Physical Layer Chipset

For the Ethernet PHY chipset, the major transients that can cause damage are ESD strikes, cable discharge events, and electrical fast transients on the data lines. A transient voltage suppressor (TVS) diode array can provide the necessary protection. To protect all eight data lines, use two 4-channel TVS diode arrays as shown in Figure 2.

Benefits of using a TVS diode array include:

• ESD protection to 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.   

Protecting the Powered Device (PD) Controller

The PD controller is a DC/DC converter that provides the DC power to power the equipment. The AC/DC rectifier circuits are shown in separate circuit blocks. The rectifier circuits interface directly with incoming signals from the RJ45 connector. To protect the rectifier circuits from voltage transients, bi-directional TVS diodes across the input lines are recommended. Versions of these series diode pairs are capable of absorbing as much as 1500 W of pulse power or a surge current of 200 A. TVS diodes respond very quickly to transients with a response time that is less than 1 ps. Also, their leakage current is under 1 µA to minimize circuit power consumption. 

Complete the protection of the PD controller with a uni-directional TVS diode at the output of the rectifier and the input to the PD controller DC/DC supply. You will need to select an appropriate clamping voltage based on your circuit design. The component will provide a fast response to a transient.  

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 W or 350 mA. Here, protecting the PHY against harmful ESD events with a 2-channel TVS diode array is recommended.

Figure 3 (block diagram on the left) details an example PoE network and shows the TVS diode array on the input/output lines to the Ethernet PHY chipset.

Figure 4 shows the schematic of the 2-channel TVS diode array. 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 packages with no more than 2 pF capacitance to ground. Also, look for a TVS diode array with a low leakage current, such as less than 1 µA.

Protecting a PoE Network in an Outdoor Environment

The outdoors is a much more severe environment for electronics than indoors. There is a higher risk for power-cross to cause over-current faults and also a higher risk of lightning-induced surges 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. An example is shown on the right side of Figure 3. For this challenging environment, in addition to fuses, gas discharge tubes should also be placed across the I/O lines. A gas discharge tube provides crowbar protection from lightning or other hazardous transients. A gas discharge tube has 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 that the fuse and gas discharge tube 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 shown in Figure 5. 

omponents like this 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.

The Value of Robust Designs

Including overload protection in your communication system designs will prevent failures from damaging environmental disturbances. The benefits of reduced service costs and an enhanced reputation for product quality far outweigh the small cost for the additional components added to the product designs.

If you include protection as part of the initial design definition rather than as an afterthought, your development time will be minimally impacted. Furthermore, you can save yourself precious development time when designing and selecting protection components by taking advantage of a manufacturer’s expertise. The manufacturer can help with solutions that are cost-effective and offer the lowest cost of ownership. Your product’s high-quality reputation will enhance your company’s revenue and lead to higher profitability. Your value will be enhanced as well.

by Todd Phillips