Power Electronics

PoE Offers Many Outlets for Creative Power Design

Since the ratification of the IEEE 802.3af standard nearly four years ago, Power over Ethernet (PoE) technology has seen widespread deployment. Through the use of midspans and endspan equipment, a multitude of Ethernet ports now provide dc power along with data to network peripherals such as Voice over Internet Protocol (VoIP) phones, WLAN access points and numerous other applications. According to Marianne D'Aquila, principal analyst at Information Resources, the number of installed power-sourcing equipment (PSE) ports has risen from 20 million in 2004 to 47 million ports in 2006, and is projected to reach 92 million ports this year.

When IEEE 802.3af went into effect, it defined specifications for managing power distribution over Ethernet cabling. Semiconductor vendors then began providing control chips to help implement the standard in PSE and powered devices (PDs). Since those early controller ICs were introduced, vendors have moved beyond simply implementing the key PoE functions to looking at all aspects of PSE and PD designs with the goal of optimizing them for efficiency, size, cost and ease of implementation.

Some chip developers have sought to further integrate the power-management functions, in some cases together with power-conversion circuitry. Others have developed power-conversion chips or modules specifically for PSE and PDs. Meanwhile, passive components suppliers have also gotten involved, crafting transformers and connectors to ease or optimize PoE implementations.

Some of the more recent component development has been aimed at satisfying the requirements of the emerging IEEE 802.3at standard for high-power PoE. That standard promises to extend the use of PoE into a host of new applications. D'Aquila predicts that if the IEEE is successful in getting the PoE Plus standard ratified by 2008, we can expect to see a similar curve of early product adoption as with the current IEEE 802.3af standard. “The number of installed high-power PSE ports could rise from less than 1 million prior to the standard to about 5 to 10 million ports by 2009,” says D'Aquila.

This month Power Electronics Technology is featuring an online section addressing different aspects of PoE design. The authors of these four articles aim to aid designers in optimizing or simplifying their PSE and PD implementations.

Daniel Feldman, product line manager at Microsemi and chairman of the Ethernet Alliance's PoE/PoE+ Technical Committee, outlines the requirements of the emerging IEEE 802.3at standard, which will increase the power available to the PD from 12.95 W to 59 W. Feldman discusses the impact of these requirements on issues such as cable selection, power classification, power allocation and compatibility with IEEE 802.3af.

John Gallagher of Pulse Engineering explains how to design the flyback transformer when implementing a flyback power converter in the PD. As Gallagher explains, the continuous-mode flyback topology is particularly well suited to PDs, because it represents the simplest and lowest-cost isolated topology at the PD's 13-W power level.

As in all power applications, reliability is a concern in PoE. In the third article, Jean Picard, systems engineer in the Power Management Group of Texas Instruments, discusses how the IEC 61000-4 transient immunity standards apply in PoE. Picard examines the test methods associated with these standards, and provides guidelines for implementing circuit protection.

Naturally, some unique PoE designs are made possible by recent component developments. Dilian Reyes, applications engineer at Linear Technology, describes how a new single-port PSE controller enables novel applications such as a PSE wall adapter or power-forwarding unit.

Whether designers are taking their first stab at a PoE design, or are looking to improve on a previously developed product, the design techniques and solutions in this special section may help engineers to avoid potential pitfalls and get their products to market sooner. Perhaps these articles will also inspire some designers to discover new uses for PoE technology or other approaches to addressing the issues raised in these features. Given the rapidly expanding universe of powered Ethernet ports, the opportunities for innovation in power design seem limitless. Feel free to share your PoE-related ideas and inspirations by writing to me at [email protected].

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