Power over Ethernet fundamentals - Embedded.com

Power over Ethernet fundamentals

Power over Ethernet (PoE) transports short haul (up to 100 meters) DC power across Ethernet cables between Power Sourcing Equipment (PSE) and Powered Devices (PDs).

[Editor’s Note: The convenience of delivering power and data over the same cable is compelling, and just as USB has become a ubiquitous source of power for many consumer devices, Power over Ethernet (PoE) brings multiple benefits to commercial and industrial applications. The previous article in this two-part series described PoE’s role in some of those applications. ]

According to the National Fire Protection Association (NFPA), the third leading cause of commercial fires in the United States is Electrical and Lighting Equipment. The typical root cause is old or defective wiring, overloaded circuits, loose connections, faulty fuses, imbalanced electrical loads, and many other electrical or lighting problems. These can lead to overheating, resulting in sparks that can ultimately ignite a fire.

Mains power transports long and short haul AC power across three insulated copper wires: live, neutral and earth. The live wire carries the alternating potential difference (120 VAC, or 230 VAC). The neutral wire completes the circuit and is kept at or close to earth potential, or 0V. The earth wire is a safety wire that grounds the circuit in the event of a fault. In short, along with fuses and circuit breakers, mains power dedicates 33% of its total copper, the earth wire, to safety.

Figure 1: Cross section of 2.5 mm2 solid copper mains wire (left), next to solid copper 23 AWG CAT6 cable (right) at the same scale (Source: Ethernet Alliance)

Power over Ethernet (PoE) transports short haul (up to 100 meters) DC power across Ethernet cables between Power Sourcing Equipment (PSE) and Powered Devices (PDs). Depending on the PoE standard, up to eight copper wires are used to carry DC power, including the return path. In short, PoE does not dedicate any copper to safety. Philosophically and architecturally, the PoE standard moves safety control from the copper (mains) to the silicon. There are two benefits here; silicon is much less expensive than copper, and you can code silicon. You can’t code copper.

2-Pair Power vs. 4-Pair Power

Ethernet uses the RJ45 connector, which features eight contacts. These are divided into four differential (diff) pairs (Figure 2). In 10BASE-T (10 Mbps) and 100BASE-TX (100 Mbps) networks, only two out of the four diff pairs available are used to transfer data, which leaves two pairs unused. In Gigabit Ethernet (1 Gbps) networks, all four diff pairs are used for data transfer.

Leveraging existing 10/100/1000 Ethernet infrastructure, IEEE 802.3af (now known as PoE), which provides 350 mA/pair, 57 V max, and IEEE 802.3at, which provides 600 mA/pair, 57 V max (known as PoE 1) deliver power using these unused pairs, implementing two alternative modes; Alternative A or B:

A. Alternative A (PSE), or Mode A (PD) transports power on diff. pairs 2 and 3

B. Alternative B (PSE), or Mode B (PD) transports power on diff. pairs 1 and 4

Meanwhile, PoE 2, or IEEE 802.3bt, operates on 4-pair power by using all four diff. pairs at 960 mA/pair to a maximum of 57. This achieves 90 Watts at the PSE.

Figure 2: 2-pair power vs. 4-pair power

IEEE 802.3bt (90 W) Classification

The Ethernet Alliance further slices the four types into eight distinct classes, depicted in Figure 3. For the Power Sourcing Equipment (PSE), each PoE 2 class (5-8) is a 15 W slice while each PoE 2 class is an 11 W slice for the Powered Device (PD). Finer slicing of classes vs. types optimizes a multi-port PSE’s efficiency to provide a variety of power to connected PDs, especially as the number of connected PSE ports grows.

Figure 3: IEEE 802.3bt Classification

IEEE 802.3af/at/bt Power Provisioning Phases

PoE power provisioning between the PSE and the PD follows the five distinct phases, illustrated below and in Figure 4.

  • Phase 1: Detection
  • Phase 2: Classification
  • Phase 3: Startup
  • Phase 4: Operation
  • Phase 5: Disconnect

The PSE contains an Rsense resistor in series with the return current path for measuring any current sinking performed by the PD. There is also a 25k pulldown signature resistor on the PD, which is used to notify the PSE of a detection.

Figure 4: PoE Power Provisioning Phases (Source: Ethernet Alliance)

Phase 1. Detection

When a PSE and PD are connected by an Ethernet cable, the PD presents a 25 kΩ pull down resister (Figure 4 right) to the PSE. The PSE then performs two current measurements within a 500-millisecond window:

1) force V 2.8 V, and measure I

2) force V 10 V, and measure I

By calculating a ∆V / ∆I, if the PSE measures from 19 KΩ to 26.5 ΩK, the PSE can accept the detection as valid. Otherwise, the PSE must reject the detection. The benefit of performing a differential measurement is that any surrounding noise (aggressor) will be common to each measurement and will therefore be rejected (common mode rejection).

Phase 2. Classification

During Classification Phase, a PD announces its requested class signature, or power requirements, to the PSE. The Classification Phase is divided into five class events or time slots, as depicted in Figure 5.

1) Class Signature 0:           1 mA to 4 mA

2) Class Signature 1:           9 mA to 12 mA

3) Class Signature 2:           17 mA to 20 mA

4) Class Signature 3:           26 mA to 30 mA

5) Class Signature 4:           36 mA to 44 mA

Figure 5. Class Signatures Produced by the PD

This figure captures which Class Signature (row) is required during each Class Event (column), in order to identify the PD class (1 – 8). For example, a Class 7 PD will provide 40 mA during Class Event 1, 40 mA during Class Event 2, and 18 mA during Class Events 3 through 5.  The PSE measures the PD’s current sinking during each time event to learn the PD’s Class.

The PSE is responsible for forcing the voltages depicted in Figure 6 below, while the PD is responsible for sinking up to five different current levels called class signatures.

Figure 6: Class Signatures and Current Levels


As shown in Figure 5, Class event 1 is longer than the other class events. This is unique to 802.3bt, and not the case with 802.3at or 802.3af. If the PD is also 802.3bt compliant, the PD can change to class signature 0 (1 to 4 mA) 81 milliseconds into Class Event 1, which informs the 802.3bt PSE that the PD is also 802.3bt and supports Autoclass.

After the PD turns on, the PD provides its maximum power for ~1.2 seconds. The PSE measures the PD power, adds some margin, and this becomes the new optimized power level provided by the PSE.

Autoclass optimizes PSE power allocation. For example, if a PD requires a maximum of 65W during operation that PD would identify itself as a class 8 to the PSE, in order to guarantee 65W at the PD. Without Autoclass, the PSE would allocate 90W, to ensure the PD gets 65W. With Autoclass, the PSE may read only 66.5 W (short cable length), + 1.75 W margin = 68.25 W allocation. The power savings is 21.75 W, or ~25%. Though this may not seem significant, if the PSE switch has eight 802.3bt ports, Autoclass can optimize each port (with a variety of cable lengths) for a total potential efficiency saving of hundreds of Watts.

Phase 3: Startup

During Startup Phase, the PSE is responsible for limiting inrush current to 450 mA for Classes 1 to 4, and 900 mA for Classes 5 to 8.

During the startup phase the PD is responsible for limiting the load current to 400 mA for Classes 1 – 6, and 800 mA for Classes 7 – 8.

Phases 4-5: Operation, Disconnect and MPS

Maintain Power Signature (MPS) is a keep alive function, where the PD sinks periodic current pulses from the PSE in order to inform the PSE that the PD has not disconnected. If a PSE does not receive an MPS from the PD after 400 milliseconds, then the PSE must disconnect power to the PD.

IEE 802.3bt PD Application Block Diagram

Figure 7 depicts a typical 802.3bt application diagram for a Powered Device (PD). Moving left to right, transformers AC couple the Ethernet 10/100/1000 data to a nearby processor. Full wave rectification is accomplished by GreenBridge™ 2, consuming less power than the traditional silicon diode bridge. The NCP1095 from ON Semiconductor ® (pin 7), presents the 25kΩ detection pull down resistor, while pins 2 and 3 determine the PD’s power requirements by Class (resistor values), communicated to the PSE during the classification events after attachment. Pins 6, 8, 9 and 10 collectively control inrush and over current protection (OCP) with an external Rsense and pass gate. Three-bit communication to a companion processor is accomplished on pins 13, 15 and 16. Pin 14 PGO pin informs a downstream DCDC device when the power output is good. Pin 4 allows the NCP1095 to power up from a local auxiliary supply, while pin 6 controls Autoclass, a new feature of 802.3bt.

Figure 7: 802.3bt Application Diagram

ON Semiconductor also offers the NCP1096 controller, which integrates both the external FET and Rsense.

You can code silicon

Fuses, circuit breakers and earth wires are relatively blunt instruments for preventing electrical fires, particularly when compared against the features of IEEE 802.3bt. The power provisioning features it offers, such as Classification, Autoclass, inrush and MPS, are far superior. For example, with mains power, rodents hidden in the walls or ceiling can easily cause an electrical fire without any warning. In contrast, if the PD does not provide an MPS to the PSE every 400 msec, the PSE automatically disconnects power to the PD.

One can easily imagine coding a PSE to capture unplanned disconnects, which triggers an early warning flag to the IT department, potentially preventing catastrophic events like building fires. Meanwhile, Classification and Autoclass intelligently allocates the exact power a load will require. This is a very safe and efficient way to distribute power. Like mentioned earlier, silicon is a lot cheaper than copper and you can code silicon, but you can’t code copper.

>> This article was originally published on our sister site, Power Electronics News.

Bob Card is Americas Marketing Manager, Advanced Solutions Group at ON Semiconductor.

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