Building a PoE Power Subsystem

Design Decisions

With Power-over-Ethernet (PoE), you can provide both data interconnection and power to devices over a single cable. In this article, Maxim Integrated’s Thong Huynh and Suhei Dhanani explore the key issues involved in implementing rugged PoE subsystems. Topics covered include standards compliance, interface controller selection, DC-DC converter choices and more.

Power over Ethernet (PoE) is a technology that allows us to send both data and power over existing CAT5/5e/6 Ethernet cables. PoE systems consist of some form of power sourcing equipment (PSE)—such as a network switch or a midspan injector switch—along with a powered device (PD) as shown in Figure 1. The primary benefit of PoE is, of course, the reduction in the number of cables running to the end equipment because just a single Ethernet cable now carries both data and power. This simplifies installation and also provides for centralized power management—the PDs can be remotely powered off and on and can provide continuous operation in spite of an AC power outage if the PSE has uninterruptible power source (UPS) power.

Of course, there are limitations to the amount of power that can be efficiently provided to an end-point and the distance of that end-point from the network switch that provides the power. The maximum distance specified is 100 m (333’). That’s the distance from a PoE-capable switch to the PoE PD. A PoE Ethernet extender, however, can lengthen that span.

Power is delivered using the twisted data wires in an Ethernet cable. Each Ethernet cable has four pairs of data wires. In Gigabit Ethernet—by far the most common today—all four pairs carry data. Power can be delivered using either two pairs of the wires as shown in Figure 2. One is called the Alternative A and the other is called Alternative B. To be IEEE-standards compliant, a PD must support both Alternative A and Alternative B, whereas a PSE may support either Alternative A or Alternative B, or both.

As far as the amount of power that can be delivered to the end-point, that is governed by the IEEE standard. The first standard, called the IEEE802.3af, was ratified in 2003. This was rated to put out 15.4 W/port using two pairs of wires in the Ethernet cable. At a 100 m distance, this means that a PD would get 12.95 W of power. In 2009, the IEEE ratified the PoE+ standard—802.3at. With this new standard, PDs can get 25.5 W of power delivered at 100 m. This standard is backward compatible so that the older PDs would work with the new PSE.

In September 2018, IEEE approved a new standard that can deliver 71 W to a PD over 100 m: IEEE 802.3bt. This means that the PSE can now put out 100 W over a single Ethernet cable. This will help expand the PoE market to LED lighting, large-screen displays and so forth. The voltage range sourced by the PSE and received by the PD is shown in Table 1.

	IEEE 802.3af	IEEE 802.3at	IEEE 802.3bt
Output by PSE	44 V – 57 V	50 V – 57 V	50 V – 57 V
Received by PD	37 V – 57 V	42.5 V – 57 V	42.5 V – 57 V

TABLE 1 – Voltage range sourced by PSE and received by PD

THE POE MARKET
As Internet becomes ubiquitous, it makes more and more sense to power end-point systems with PoE. Many types of end products can be powered via PoE. The most popular are surveillance cameras, home/building controls, digital signs, VoIP phones within enterprises and Wi-Fi access points. All market research reports point to a significant growth in the deployment of PSE and PD systems globally.

Some examples of this are:
“The global Power over Ethernet (PoE) chipsets market size is likely to reach $1.22 billion (USD) by 2025, according to a new report by Grand View Research, progressing at a CAGR of 12.6% during the forecast period. Increasing applications of Power over Ethernet (PoE) chipsets in the residential sector, for instance, in IP telephones, webcams & closed-circuit televisions (CCTVs) and complete wireless local area network (WLAN) coverage are poised to stoke the growth of the market over the forecast period.” [1]

“The power over Ethernet (PoE) solutions market was valued at $451.1 million (USD) in 2015 and is estimated to reach $1,048.3 million (USD) by 2022, at a CAGR of 12.56% during the forecast period.” [2]

“Based on estimates from Dell’Oro group we should see significant growth of PoE enabled ports. The growth should come from wireless access points as well as from other systems like security cameras.” (Figure 3) [3]

With the adoption of the new IEEE 802.3bt PoE standard, it is clear that more types of end-point systems will now be able to be powered by PoE. This should result in an even higher adoption rate.

PD POWER SUBSYSTEM
The power subsystem of a generic PD may look like the image shown in Figure 4. The power subsystem of a PD comprises a PD interface controller that accepts power from the Ethernet cable and a DC-DC converter that then regulates the power down to the supply rails required for circuit functionality.

The maximum power amounts available for the PD to draw from the PoE connector are listed in Table 2 below. The PD must classify itself, via the PD interface controller’s classification, to receive the right amount of power according to its class. You can consult the IEEE802.3bt standard for more details on PD classification.

PD Class	Maximum Power PD Can Draw PClass_PD (W)
1	3.84
2	6.49
3	13
4	25.5
5	40
6	51
7	62
8	71

TABLE 2 – Power draw from PoE connector

Before choosing a suitable power solution, think through the following questions to determine your PD requirement:

How much power does your PD need? Knowing the maximum power your PD requires, choose the appropriate class to match this need. It’s a good practice not to overclassify your PD power. One reason is that higher power adds cost to your power solution. Another reason is that it would reduce the remaining available power the PSE can allocate to other PDs connected to the same PoE network.

Is isolation needed? PDs and PSEs provide isolation between all accessible external conductors, including frame ground (if any) and all media-dependent interface (MDI) leads including those not used by the PD or PSE. There are two electrical power distribution environments to be considered that require different electrical isolation properties:

Environment A: When a LAN or LAN segment—with all its associated interconnected equipment—is entirely contained within a single low-voltage power distribution system and within a single building. A multiport network interface device (NID) complying with Environment A requirements does not require electrical power isolation between link segments.

Environment B: When a LAN crosses the boundary between separate power distribution systems or the boundaries of a single building. In this environment, equipment with multiple instances of PSE, PD or both must meet or exceed the isolation requirement of the MAU/PHY with which each is associated.

To state it in a simple way, if your PD is a single device without any external connector and is fully enclosed in a plastic casing (for example a security camera, a PoE-LED bulb, a low-cost IP phone or similar device), then isolation is not required. In this case, choose a non-isolated power solution for simplicity and lower cost.

Does your PD need to also be powered from a wall adaptor? An IP phone, for example, is most likely to have an AC power adaptor input for usage in a building where PoE is not yet available. If your PD is to be used where PoE is not yet available, then choose a PD interface with wall adapter interface feature.

Does your PD need low-power standby mode to comply with some agency requirement? More and more agencies are requiring “green power” features where equipment such as an IP phone during idle time (not used during the day) and sleep time (outside of work hours) consumes as little power as possible. Pick a PD interface controller whose features maintain power signature (MPS) and low-power sleep mode to guarantee compliance and also help contribute to a greener world.

Is high efficiency important? High efficiency results in lower power loss, easing the heat dissipation requirement for your PD. Lower heat dissipation means lower operating temperature, which translates to higher reliability. In cases where your PD needs a lot of power to operate, 60 W for example, an 80% efficiency PD will require an input power of 60 W / 80% = 75 W, which exceeds the maximum PoE power of 71 W, rendering it non-PoE compatible.

However, a 90% efficiency PD requires input power of 60 W / 90% = 67 W, which falls nicely under class 8 (71 W). In this situation, high efficiency is a must. Also, it is always desirable to classify your PD at the lowest power class, so that the remaining PoE power in the system can be allocated to more PD devices. High efficiency could bring your borderline PD to the next lower power class.

PD INTERFACE CONTROLLER
When selecting a PD interface controller, consider the following important features:

• • IEEE 802.3af/at/bt compliance
• • Type 1~4 PSE classification indicator or an external wall adapter indicator output
• • Simplified wall adapter interface
• • Multi-event classification 0–8
• • Intelligent MPS
• • Sleep mode and ultra-low-power sleep mode

These are features that would satisfy most of the PD requirements previously mentioned. The rest of the requirements will be addressed by the DC-DC controller, which will be discussed in later sections. The selector guide presented in Table 3 presents some recommended PD interface controllers and their key features.

	802.3af/at Compliant	CoC Compliant	70 W
MAX5969	X
MAX5981	X	X
MAX5982	X		X

TABLE 3 – Recommended PD interface controllers

THE DC-DC CONVERTER
Once you determine that your PD doesn’t require isolation, a high-voltage buck converter would be an appropriate choice for your DC-DC converter need. Efficiency, total solution size and cost are important considerations. Devices with features such as synchronous rectification, a wide input voltage range and a high level of integration support these considerations. Figure 5 shows an example of a non-isolated DC-DC converter solution that addresses efficiency and size requirements. This is a class 3 PD. The output is 5V/2.5A with 92% peak efficiency. Figure 6 provides an example of another non-isolated DC-DC converter, a class 1 solution using a tiny power module at 5V/300mA.

If your PD requires isolation, then a flyback converter would fit the bill, providing power up to 40 W or so (Class 5 and below). A device that minimizes the number of components required can save board space and cost. For example, an isolated flyback controller that doesn’t require an opto-coupler to provide feedback for output voltage regulation can save multiple external components and the associated board space and cost. Furthermore, an opto-coupler degrades over time, so no opto-coupler also means higher reliability.

Figure 7 shows an example of an isolated DC-DC converter solution. It is a class 2 PD, 5V/1A output, and can operate with 12 V to 57 V wall adaptor input voltage. There is an option to further improve the efficiency of the flyback DC-DC converter by replacing the output rectification diode with synchronous rectification. The Figure 8 schematic shows an example of this.

For output power beyond 40 W, the flyback converter can still work fine, but an active clamp forward converter is recommended for higher efficiency. At higher output power, efficiency is very important to reduce the amount of heat dissipation in the PD. An active clamp forward converter also has a lower EMI signature due to its soft switching edges. Figure 9 presents an example of an active clamp forward DC-DC converter. The converter delivers an isolated output voltage of 57 V at 700 mA, totaling 40 W at peak efficiency of 91.5%.

CONCLUSION
Globally, many more devices are getting networked. In fact, being connected to an IP network is increasingly the only option to control and manage the device. As devices get networked, it becomes very convenient to run them on just one Ethernet cable that provides both the connectivity and the power. Besides, having the power delivered via a centrally managed switch allows for other enhancements such as remote ON/OFF and uninterrupted operation even in the case of a local power outage.

With the advent of the newest IEEE 802.3bt PoE standard, even more types of devices can now be powered via PoE. As the amount of power delivered increases, so does the need for higher efficiency and wider input voltage ranges to accommodate the complex power system designs. This requires careful considerations regarding the selection of both the PD controller as well as the DC-DC converter to generate the required regulated voltage to be used by the system. 

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Maxim Integrated | www.maximintegrated.com

PUBLISHED IN CIRCUIT CELLAR MAGAZINE• JUNE 2019 #347 – Get a PDF of the issue

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