Power Over Ethernet (PoE)
Design Factors
The concept of providing power along with data is as old as plain old telephone service, but the formal standard for providing power along Ethernet lines is much more recent. Though the original Power-over-Ethernet (PoE) specification was approved in 2005, the IEEE ratified a superseding standard in summer 2009—the IEEE 802.3at. Both standards specify behavior for devices receiving power across Ethernet lines, known as "powered devices" (PDs), and methods for injecting power onto the line, used in equipment known as "power sourcing equipment" (PSE).
Powered Devices (PDs)
A powered device typically has a front-end IC to pull power off of the Ethernet line and safely pass it through to the rest of the system. It is important that these PD front ends be robust enough to withstand 100-V surges while ensuring that the current is ramped in a controlled fashion into the PD. Most PD front ends, like the TPS23753A, incorporate a DC/DC converter to down convert the PoE voltage to a suitable voltage. The 802.3at Type I PDs (adherents of the original 802.3af standard, with some updates) are guaranteed at 12.95 W, while 802.3at applications are allowed up to 25.5 W of power.
Power Sourcing Equipment (PSE)
PSE is typically found in either Ethernet switches ("endspans") or a device between the Ethernet switch and the PD ("midspan"). In both cases, power is injected onto the line for a PD to use. PSE is responsible for querying the PDs and legacy clients to ensure that they want power to be applied and that they do not pull more power than allowed. All PSE performs some level of hardware authentication for initial power application to PDs, while newer 802.3at PSEs can query with either an additional hardware layer or a software layer of communication to provide more than 12.95 W of power to the PD.
The TPS23754/6 incorporates a high-power PD front end along with a DC/DC converter capable of highly efficient, isolated down conversion topologies. With full 802.3at compliance and an additional gate driver for secondary-side active clamp rectification, the TPS23754 is an easy choice for high-power PoE applications.
For more information please visit www.ti.com/poe.
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