Reality is quite different. Adapting a data-communications medium—here, Ethernet wiring and interfaces—for power is very tricky, as it must be safe, protected, low loss and not interfere with the original Ethernet function. Note that the dilemma is not unique to PoE, as major challenges occur when using USB for power delivery, the AC power line as a data path, or a microphone audio cable for preamplifier power.
A glance at a typical PoE installation shows the complexity of ensuring that power and data can coexist, yet guarantee that performance and problems with one function do not interfere with the other function, Figure 1.
The PoE solution includes a set of hardware requirements with "magnetic" for isolation, power-supply units of various types (including mid-span "boosters") and a complex multilevel protocol where the power sourcing equipment (PSE) and the powered device (PD) end must interrogate each other via a handshaking set of queries and sensing of current flow to identify each other and confirm their situation and status, Figure 2.
This original PoE specification (2003), designated as 802.3af, allowed for 48 V DC power at up to 13 W, but 13 W was not adequate for some requirements. In 2009, the enhanced standard informally called PoE+ (formally, 802.3at) increased the voltage and current requirements to provide 25.5 W of power. Finally, the not-yet-formalized specification referred to as PoE++ boosts the power to around 90 W, which is substantial.
To meet the multifaceted needs of PoE standards, ICs such as the LTC4279 Single-Port PSE Controller from Linear Technology play a major role in supporting PoE, PoE+ and PoE++, Figure 3. Not only does it supply the DC power needed, it also implements the protocol sequence and protection. This IC is an autonomous single-port power sourcing equipment (PSE) controller that is fully compliant with IEEE 802.3 and the proposed PoE++ implementation, and does not require a local microcontroller, just a single-ended power supply and a few passive support components.
The vendor maintains that it delivers lowest-in-industry heat dissipation via a low RDS(ON) external MOSFET and a 0.1 Ω sense resistor, thus eliminating the need for heat sinks. Its "PD discovery" mode uses a proprietary dual-mode, four-point detection mechanism for enhanced immunity from false PD detection. The LTC4279, which is available in 20-Pin QFN and 16-Pin SO packages, includes a status LED pin, and various protection modes including robust short-circuit protection as well as a VOUT pin that is protected against cable surges to ±80 V.
Note that there are also major concerns about I2R loss and, thus, self-heating when delivering substantial power (and current) through the fairly thin, high-resistance Ethernet wires. This is a major concern for the National Electrical Code perspective on PoE++ as this heating and temperature rise can become a safety hazard for wires in walls or running in tightly packed cable trays and raceways.
For more information, including a tutorial on PoE, go to the LTC4279 data sheet at http://cds.linear.com/docs/en/datasheet/4279f.pdf .