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Power over Ethernet Moves Forward Focus: The new IEEE 802.3at standard for power over Ethernet (PoE) (which was ratified in September of 2009) increases the power available to powered devices (PDs) up to 25.5 W, which is nearly double the 13 W specified in the original IEEE 802.3af PoE standard. But with the higher 48-V power delivery, there are also increased distribution losses in the Ethernet cable. These losses are in addition to the losses associated with converting the ac mains voltage to 48 Vdc and the losses incurred in stepping down 48 V to the voltage or voltages required by the PD. Consequently, the power conversion efficiency of a PoE system powering a PD can be much lower than that of an individual ac adapter when powering the PD at full load. This article presents data on an example application (a motorized security camera) to illustrate this comparison and discusses how use of power management techniques in the PoE system can offset the higher full-load losses of the PoE system, and make these systems more efficient overall than adapter-powered systems. Relevant power management features within IEEE 802.3at are discussed.
What you’ll learn: - How to improve the overall power conversion efficiency of IEEE 802.3at-compliant high-power PoE systems
- How to compare the efficiency of a PoE-based power delivery with ac adapter power
Notes: Article appears on pages 42-43 of March issue in article archive. You must register to access articles in this magazine's archive.
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Author & Publication: Koen Geirnaert, Product Marketing Manager, ON Semiconductor, Bodo's Power Systems, Mar 01 2009
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