Power Electronics

PoE Chips Reach For Higher Power and New Features

With millions of power-equipped Ethernet ports shipping each year in switches, routers and midspans, power over Ethernet (PoE) is now a mainstream technology that enables VoIP phones and numerous other types of network peripherals to use the RJ-45 connector as both data port and power outlet. Chip vendors continue to support the adoption of PoE technology by developing controllers that manage 48-Vdc power distribution from the power-sourcing equipment (PSE) to the powered devices (PDs).

Recently developed PSE controllers further simplify the task of managing power distribution by integrating more of the control and power functionality required by the IEEE-802.3af PoE standard. The new controllers also include the first power managers to address demands for higher power per port. While IEEE 802.3af provides nearly 13 W of power to each PD in the system, the emerging high-power version of the standard, known as IEEE 802.3at (or PoE Plus), makes provisions for 29.5 W or 59 W to be delivered to a PD.

Although IEEE 802.3at won't be completely finalized and ratified until at least 2008, semiconductor vendors can now offer PSE controllers with support for higher power levels and some degree of compliance with the developing 802.3at standard. These first chips will likely be stepping stones to fully 802.3at-compliant controllers once the standard is ratified.

A Milestone for High-Power PoE

Efforts to increase the power levels available to powered devices took a big step forward when the special interest group (SIG) for developing the IEEE 802.3at high-power PoE standard met in January 2007 in Monterey, Calif. At that meeting, the Telecommunications Industry Association (TIA) recommended to the SIG that 802.3at support 720 mA over two pairs, more than doubling the current and power levels available per port. IEEE 802.3af limits the current per port to 350 mA, which is delivered over one pair (either the data pair or the spare pair in the Ethernet cable). The agreement on current levels and other aspects of the high-power PoE standard paves the way for formal ratification of 802.3at.

The availability of either 29.5 W per port or 59 W per port under the 802.3at standard presents new opportunities for the adoption of PoE in numerous applications. For example, the higher power levels may be used to power laptops in commercial applications, particularly those designated as “ultraportables” or “thin and light,” says Daniel Feldman, product line manager for PoE ICs at Microsemi and an 802.3at SIG participant.

New PSE Controllers

Microsemi, which acquired PoE specialist PowerDsine last fall, has launched an IC-based PSE solution that claims compliance with the emerging IEEE 802.3at standard. To do so, Microsemi has taken its PD64012 and PD64004 controller chips and modified them to support the higher current levels associated with the proposed 802.3at standard. The new PD64012GH is a 12-port PSE controller that supports the higher currents and is RoHS compliant. Meanwhile, the new PD64004AH is a 24-port PSE controller that supports the higher currents while also supporting industrial temperature range operation.

Feldman says that these parts do not support all 802.3at functions. For example, they support layer 2 power classification of power but not two-event classification. As a result, the chips are fine for use in PoE-enabled switches but not in midspans. However, Feldman notes that before the standard is ratified, the company expects to offer a solution specifically optimized for the 802.3at standard.

The company also expects to offer PD controller chips in the future. Feldman observes that under the 802.3af standard today, it is still cheaper to implement the PD functionality with discrete devices. But the 802.3at standard will make PD designs more complicated, making a chip-based implementation cost effective, according to Feldman.

Prior to the recent developments in high-power PoE, some vendors introduced devices in anticipation of the 802.3at standard that would allow PoE system designers to deliver more than the 12.95 W provided by the 802.3af standard. Texas Instruments' TPS23841 quad-port power manager is such a device.

Introduced in December, the TPS23841 is a fully integrated PoE management device that allows multiport enterprise systems to deliver up to 25 W of power over a standard Ethernet cable while operating over the -40°C to +125°C temperature range.

The TPS23841 quad-port PoE IC provides power-management control and protection, while safely delivering up to 665 mA per port from input voltage levels of 21.5 V to 57 V. The device's wide input-voltage capability allows designers to support emerging Ethernet-powered 24-V medical and industrial applications, such as nurse call center systems and human machine-interaction equipment.

The TPS23841 simplifies PoE design by integrating several components onto a single chip — including integrated switches, sense resistors and four individual 15-bit ADCs — which allows the device to accurately measure signature resistance, voltage, current and die temperature. The device also can be designed together with TI's TPS2384 power-source equipment manager to enable high-power and standard PoE operation in a system running on the same comprehensive software.

The TPS23841 provides three different options — automatic mode, semi-auto mode or manual operation — that give designers added flexibility to meet specific PoE system requirements. While in manual mode, the TPS23841 can work with a microcontroller such as TI's MSP430 to detect legacy capacitance loads.

The TPS23841 high-power PoE device matches all the power requirements of TI's 8-pin, TPS2376-H controller for PDs. The high-power device controller contains all the safety features needed to develop a PD with programmable, 600-mA current limit and thermal shutdown, auto-retry and fault protection. The TPS2376-H also allows a designer to implement a nonstandard PD that draws up to 26 W from the PSE with a minimum of 52 V of input and more than 100 m of Cat-5 cable.

Available now, the TPS23841 is packaged in a 64-pin HTQFP and priced at $7.50 each in quantities of 1000. Reference designs featuring the TPS2376-H and TPS23841, application notes and technical documentation are available through www.power.ti.com/poe.

While the high-power PoE standard moves toward finalization, the ongoing growth of 802.3af-compliant applications also continues to inspire the development of controllers. For example, in November 2006, Linear Technology unveiled a single-port PSE controller that offers autonomous operation to the extent that no external MCU is needed. Although Linear Technology has described its previously introduced quad PSE controllers as autonomous, even those devices required some intervention from an external microcontroller.

By eliminating all coding and requiring few external components, the LTC4263 simplifies the task of implementing a single PSE port. This capability may be used to install a PSE within a wall jack for the purpose of providing a 48-V power repeater. This chip may also be used to inexpensively install a single PoE port in a router.

The IC, which integrates a power MOSFET, detects, classifies and provides isolated 48-V power to PDs connected to it. It offers standard-compliant ac or dc methods to sense the removal of a PD. Using an internal sense resistor, the LTC4263 provides precision inrush, current limit and short-circuit control to further ensure IEEE 802.3af compatibility.

Specified over the full commercial and industrial temperature ranges, the LTC4263 is offered in 14-pin, 4-mm × 3-mm DFN and SO-14 packages. Pricing begins at $2.95 each in 1000-piece quantities.

Online PoE Special Section

This month, the editors of Power Electronics Technology have created an online section of special Power over Ethernet articles that discuss magnetics design for PoE power converters, how to design for IEEE 802.3at, how to implement a single-port PSE and how to implement transient protection in PoE systems. To learn more, go to www.powerelectronics.com.

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