Power Electronics

Vendor Supports 2-Pair Solution for high-Power PoE

Phihong USA, a provider of power-over-Ethernet (PoE) solutions, presented its proposals on achievable limits of 2-pair and 4-pair solutions at the January 12th meeting of the IEEE 802.3at task force, along with several other member companies. Based on these presentations, the task force concluded that the standard’s objective of providing enough power for multiband access points and controllable security cameras can be met with a 2-pair solution, which is lower in cost and simpler to implement than 4-pair solutions.

Designated PoEPlus, the IEEE 802.3at standard now in development aims to more than double the wattage available to powered devices. If implemented as a standard, PoEPlus will allow at least 30 W of power and potentially up to 45 W.

“The specification is far from finalized but most members believe a source voltage from the PSE [power sourcing equipment] of 51 V to 57 V with up to 840 mA is a reasonable expectation,” said Keith Hopwood, vice president of marketing for Phihong USA. “The drop per pair total in a worst case scenario of 100 m cable and connectors is 12.5 Ω, which would provide over 30 W at the powered device. The limiting factor is the temperature rise in bundled cabling,” said Hopwood.

The cable working group is testing 196 cables bundled in a conduit and measuring the temperature rise in all the conductors when current runs through them. The testing does not reflect a safety concern or fire hazard, but the cables will permanently deform above certain temperatures, causing data attenuation.

“At Phihong, we offer both 2-pair and 4-pair solutions, so we know the 2-pair option is feasible,” said Hopwood.

In addition, at the meeting, a group was formed to define a detailed classification proposal. The new classification will not only give a much higher resolution of current required by the powered device but will also signal the powered device of the source power capability of the power sourcing equipment.

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