Power Electronics

Vendor Supports Motion On High-Power PoE Standard

Phihong USA (www.phihong.com), a provider of power-over-Ethernet (PoE) solutions, supported the motion that was passed at the May 23, 2006 IEEE802.3at committee meeting to set the output voltage for power sourcing equipment (PSE) to 50 V to 57V. This voltage range is an increase from the 44 V to 57 V specified in the IEEE802.3af standard. The powered device (PD) voltage will remain the same as the IEEE802.3af standard at 36 V to 57 V.

“The PD voltage is not being increased because the higher current in long cables will result in more drop,” said Keith Hopwood, vice president of marketing for Phihong USA. “This will reduce the total current and heating in cables, along with maintaining a decent high voltage level at the PD after all of the cable drops.”

In addition to the higher output voltage, the IEEE802.3at committee agreed on a motion to increase the number of classification levels for the PD to up to 10 classes for 2-pair systems, including existing IEEE802.3af classes.
The lowest classification level will be 2 W, with a maximum number of 10, level 1 classifications on a 2-pair device, including the existing IEEE802.3af standard. Level 1 is the hardware interface, while level 2 is the management software that controls and manages the overall network. In level 2, there is a method to classify loads in 0.1-W increments.

Even though products will be available that will comply with the new high power specification, many powered device manufacturers still plan to allow their high power PD’s to provide limited functionality when plugged into an IEEE802.3af-enabled PSE. This will result in multiband radios operating at lower ranges or not supporting all bands even though the access point will still work, a VoIP phone with a large color display that works at low brightness only, or security cameras that still transmit pictures but cannot carry out the pan, tilt, and zoom functions.

The IEEE802.3af classification was determined by measuring a dc current at a low voltage before the device was turned on. Because of the need for more levels and more information, the IEEEat devices will do additional classification via a series of pulses for a multi-step result. Although complexity is increased to get the benefit of higher power, the IEEE802.3at standard will be backward-compatible with “af.”

“The major barrier to the completion of the new IEEE802.3at standard is that there are still no current and wire temperature definitions from the cabling standards committee,” said Hopwood. “Even though several representatives from the cable standards group were present at the Austin interim meeting, no progress has occurred. Several members of the IEEE802.3at working group will attend the next standards meeting to petition for a result.”

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