Shaking Hands with PoE

Fully integrated, powered device modules help simplify the implementation of Power over Ethernet.

Power over Ethernet (PoE) brings wide-ranging benefits to the design, implementation, and long-term usability of wired Ethernet localarea networks (LANs). Cost, flexibility, and even safety are all enhanced.

PoE overcomes the major limitation often encountered by system designers in which they must locate powered network devices in close proximity to ac power outlets. Either that, or have the extensive cost and disturbance of installing them.

With PoE, both data and power at a safe nominal 48Vdc are carried over the same Ethernet cable. If network devices can be configured to run from powered Ethernet, it eliminates the need for external or internal ac-dc power supplies. Thus, the freedom to position devices where they’re needed rather than where power cords dictate, plus the long-term flexibility to relocate devices to suit the changing operational needs of the workspace and the business, are both highly desirable benefits for any organisation.

Another key benefit of using PoE technology is single-point backup. PoE systems can use a single uninterruptible power supply (UPS) to provide power over the network. Consequently, if the power goes down, all PoE-enabled products remain operational. A UPS also provides valuable protection against damage to equipment caused by mains spikes and surges.

The guarantee of 100% up time is useful in any business scenario. But it’s especially beneficial in industries where telecom equipment is critical to operations, such as emergency services and brokerage houses.

Typical powered-Ethernet devices, such as VoIP phones, cameras, and Wi-Fi access points, may need to be specially designed or adapted to run from Ethernet 48Vdc. In network devices, dc-dc converters will normally be required to drop and isolate the 48V supply down to a lower voltage (say, 5V), suitable for the device circuitry. To take full advantage of PoE, the network devices will also be required to handle the handshaking process. In this case, the network hub is able to recognise a device as being PoE-enabled and understand and manage its specific power needs.

Responding to this requirement, and tapping into PoE’s potential, C&D Technologies developed a family of fully integrated powered-device (PD) modules. The company believes the modules will help reduce component count, design time, and installation cost of PoE applications. The NMPD series of compact power-extraction modules feature on-board dc-dc conversion, and provide a full IEEE802.3af functionally compliant data and power interface to an Ethernet line (including short-circuit protection).

The NMPD modules target PoE designs in applications ranging from security equipment and public-address systems, to remote environmental monitoring, telemetry, and industrial control. Operating with a maximum power rating of 10W, the modules extract power from a CAT5 twistedpair Ethernet cable when sourced by IEEE802.3 af-compliant power-sourcing equipment (PSE).

Each module integrates dataisolation transformers, PSE handshaking and signaturecontrol functionality, and a ULapproved dc-dc converter that delivers an isolated dc output voltage. This eliminates the need for external power sources and isolation transformers. Typical signature resistance and signature capacitance is 25k and 100nF respectively, while classification current is 4mA.

The modules, which work with a nominal 48V input voltage, feature isolated output voltage options of 3.3V, 5V, 12V, and 15V. Isolation is rated at 1200V (for one minute). In accordance with the IEEE802.3af standard, NMPD PDs allow for voltage input on the Ethernet data lines or spare pair wires.

All modules in the NMPD series incorporate an LED to indicate output voltage presence, while on-board thermal shutdown and overcurrent protection further reduces the need for external components. A module enters overcurrent mode when maximum output power is exceeded, and automatically recovers when the overload condition is removed.

In an ideal situation, PoEenabled versions of equipment should be housed in the same basic enclosure used by the externally powered versions on which they’re based. This can help reduce design time and avoid costs associated with tooling for new enclosures. Similarly, the minimum amount of impact on the circuitry and the unit’s internal design will help simplify and speed the transformation of an externally powered product to a PoEenabled derivative. NMPD modules support this philosophy being supplied in a vertically mounted SIP form factor with dimensions of 91.8mm by 25.1mm. Suitability for use in a diverse range of applications is reinforced by the fact that the parts will operate at full load in ambient temperatures ranging from 60°C to 110°C.

PoE will undoubtedly change the way many pieces of electrical equipment are powered. The benefits and flexibility derived from powered-Ethernet devices are huge, and businesses will ultimately be unable to ignore them. For companies building new premises or those refitting their existing locations, it’s well worth considering becoming early adopters of Power over Ethernet.

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