How to Power Smart Buildings with PoE

by  Bob Card  - 08-19-2020 

How to Power Smart Buildings with PoE

Power Over Ethernet (PoE) offers many benefits for powering smart buildings. Most office buildings have an existing Ethernet infrastructure with centralized network closets equipped with battery-powered UPS (Uninterruptable Power Systems) in order to keep valuable network equipment operational during power outages. PoE connectivity allows the IT department to selectively power critical electronic devices like security cameras up to 100 meters away from the network closet. For further cost reductions, PoE installations do not require electricians, while maintaining international safety compliance. The union of power and data makes applications like lighting, security, signage, much smarter, while Wi-Fi performance is augmented when strategically located away from power outlets.

In October 2018, a new IEEE specification for PoE (IEEE802.3bt) was ratified, providing support for up to 90 Watts of power and additional features including Autoclass. By supporting up to 90 Watts, a significant increase compared to previous standards, IEEE 802.3bt opens the door for higher-power applications including Connected Lighting and signage within smart buildings. This blog will provide an overview of PoE systems and highlight new features offered by IEEE 802.3bt.



Figure 1. IEEE Specifications for Smart Building Applications

PoE Topology

A PoE system is very simple. The PoE network switch is the Power Sourcing Equipment (PSE) that provides power to a PoE Device (PD), across Ethernet cable (typically CAT5).

Figure 2. Basic PoE Topology

2-Pair Power vs. 4-Pair Power

A major difference between IEEE 802.3bt and previous PoE specifications is that the new standard uses all four pairs of the RJ45 connector, allowing significantly more power to be transferred. The 802.3bt specification categorizes the pair types into eight distinct classes. From the perspective of the Power Sourcing Equipment (PSE), each PoE 2 class (5-8) is a 15 W slice, while from the PD perspective each PoE 2 class is 11 W slice. Finer slicing of classes vs. types optimizes a multi-port PSE’s efficiency by providing a variety of power to connected PDs, especially as the number of connected PSE ports grows.

Figure 3. 2-Pair vs. 4-Pair Power

Autoclass and Power Efficiency

Once a PSE is connected to a PD, it recognizes its detection signature by measuring a 25 kΩ pull-down resistor presented by the PD. The PD then produces its requested class signature (power requirements) from the PSE during up to five Class Events/Mark Events.

The IEEE 802.3bt specification introduced a new feature called Autoclass. During a 1.2 second window, after the PSE has learned the classification of the PD, and the PD is powered up and turned on, the PD will sink its maximum current. The PSE then measures that current, adds some margin based on the PD classification, and that becomes the final allotted power for that particular PD.

Autoclass helps optimize the PSE power allocation. For example, a PSE network switch might service eight 802.3at ports plus four 802.3bt ports for a total of 600 W (30W x 8 =240 W, plus 90W x 4 = 360W). If two of the four 802.3bt PDs require 65 W each, without Autoclass, the switch would have to allocate 180 W for those two ports (90 W x 2 ports). With Autoclass, assuming short CAT5 cable length to both bt PD’s, and adding 1.75 W margin for each port, the PSE network switch will allocate ~136.5 W for the two 802.3bt ports (~66.5 W per port with a short cable length, plus 1.75 W margin per port = 68.25W/port). Now the PSE network switch can use the saved 43.5 W (180 W – 136.5 W) towards the other ten ports.

IEEE 802.3bt PD Application Block Diagram

The diagram below outlines the components within an energy-efficient IEEE 802.3bt Powered Device. Moving left to right, AC transformers couple the Ethernet 10/100/1000 data to a nearby processor. A high-efficiency GreenBridge™ 2 MOSFET solution accomplishes full-wave rectification while consuming less power than traditional silicon diode bridges. Pin 7 on the IEEE 802.3bt compliant NCP1095 controller presents the 25 kΩ detection pull-down resistor. Pins 2 and 3 determine the PD’s power requirements by Class (resistor values), and communicates this 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 tells a downstream DCDC device that the power out is good. Pin 4 allows the NCP1095 to power up from a local, auxiliary supply, and pin 6 controls the Autoclass feature for enhanced efficiency.

Figure 4. IEEE 802.3bt Powered Device (PD) Application Diagram

Future of PoE

By providing up to 90 Watts of power alongside new features like classification, Autoclass and MPS, IEEE 802.3bt will help open the door for many new smart building applications for PoE including Connected Lighting.

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