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    Trip the light fantastic: DC powered, energy efficient, Ethernet connected LED illumination

    Guest Blog

    April 24, 2017

    By Steve Pietkiewicz and Dave Dwelley

    Having largely displaced incandescent illumination, LEDs are now poised to replace standard and compact fluorescent lamps (CFLs). It makes sense: LEDs save energy, virtually eliminate replacement cycles, reduce heat load, and are cost-competitive with respect to bill of materials and installation.

    The widespread adoption of LEDs in lighting fixtures has given rise to another trend: The use of Power over Ethernet (PoE) as the DC LED supply, over the use of the traditional AC line, which requires conversion circuitry, internal to the bulb or fixture, to provide LED-compatible current and voltage.

    It’s the evolution of the PoE standard that’s responsible. The original PoE standard (IEEE 802.3af) defined supplying lower-power, remote devices — such as IP-enabled desktop phones in an office — up to nearly 13 W. A few years later, IEEE 802.3at was released (often called PoE+) which supported loads up to about 25 W; it was soon widely adopted for security cameras, badge readers, touch screens and other widely dispersed applications, since it could provide both DC power and data connectivity on the same hard-wired link, (see Figure 1). This eliminated the need for a nearby AC line to power each peripheral device.

    Figure 1: A basic PoE system uses a Power Sourcing Equipment (PSE) unit – a specialized power supply – to support a wide variety of Powered Devices (PDs) such as cameras, displays, touch panels, phones, microcells, and badge readers.

    The next upgrade to PoE, IEEE 802.3bt (PoE++), is nearing formal adoption. This step-up increases the power significantly via a 90-W source, delivering up to about 70 W to the load. That’s enough to power lots of LEDs.

    Given this level of delivered power, it makes sense to use the available copper of the Ethernet cabling to power more than small peripherals.

    There are several obvious benefits to using Ethernet cabling and PoE to deliver the LED power:

    • The wiring may already be in place and available;
    • It is low voltage, and so does not need to be installed by a licensed electrician;
    • For similar reasons, it does not need to meet electrical codes related to conventional power-wiring safety;
    • It saves on the component and labor cost of installing separate wiring for power and data to many corners of a building

    But the benefits of PoE for LED-based lighting go beyond wiring issues. What it also does is enable new possibilities for implementing smart buildings. A PoE-based lighting system not only provides control of lighting zones from a central site, it also allows the status of each fixture to be monitored. Further, it can be the nerve system of a building automation system, including HVAC, to minimize energy consumption.

    Why stop there? Building-wide PoE also becomes the foundation of a widespread IoT (Internet of Things) installation offering power, monitoring, and control – through sensors and transducers — for other local parameters (temperature, airflow, occupancy, and even security).

    Looking ahead: It may be possible to use the LED emitters as data transmitters, as promoted in LiFi network equipment, thus creating local network connectivity. As light does not go through walls, such networks would be more difficult to hack.

    PoE lighting takes planning, design effort

    Implementing PoE-based LED illumination isn’t  a “no-brainer,” for several reasons:

    • Even with suitable Ethernet wiring there are thermal issues; excessive temperature rise along the cable is potentially dangerous, especially in densely packed the cable trays and raceways with near-zero airflow and surrounding Ethernet and AC cables. Underwriters Laboratories has run tests and developed recommendations for such runs, and building codes are adopting these (see Reference 1).
    • Additionally, putting DC power on the Ethernet cables requires more than just simply coupling a DC supply into the Ethernet data cable at a source end, and then somehow “extracting” that power at the far end. A PoE installation needs the supply and magnetics for coupling it to the Ethernet cable, plus ICs which manage the DC power, implement the complex PoE initiation, signaling, sequencing, and protocol and provide resilience against various faults (see Figure 2).

    Figure 2: The circuitry of a PoE system shows the specialized ICs, coupling magnetics, and associated passive components on both the PSE and PD controllers; not shown is the signaling protocol and handshaking between PSE and PD which is implemented by the ICs.

    Despite these challenges, the many immediate benefits and longer-term potential of using PoE for LED lighting are driving adoption in real-world installations. Although PoE for LEDs may seem to be a solution to a non-problem when there are so many AC-powered LEDs available, it’s worth fair and objective consideration.

    Authors: Steve Pietkiewicz, Vice President & General Manager, Power Products; Dave Dwelley, Office of the Chief Technology Officer, Linear Technology Corp./Now Part of Analog Devices, Inc.

    References

    1. Cabling Installation and Management, “Practical considerations for complying with the 2017 NEC requirement for power over communications cable,” January 2017
    2. Cabling Installation and Management, “Intelligent building cabling solutions support PoE lighting for a better education environment,” January 2017
    3. Linear Technology Corp., “Power over Ethernet Solutions”