The PoEP standard and how to get the most from it

Overview
Under the existing IEEE 802.3af standard, designers can supply up to 12.95 watts to the system's powered devices (PDs) over distances of up to 100 meters of CAT 5 Ethernet cable. The standard classifies PDs into four classes depending upon how much power they require for operation. Class 1 defines devices requiring small loads up to 3.84 watts such as print servers. Class 2 represents devices such as handheld computers or magnetic card readers that need between 3.85 and 6.49 watts. IP phones, security cameras and other devices that need anywhere from 6.5 watts to 12.95 watts fall into a Class 3 category. Finally, designers building low cost solutions can also use a general default class 0 designation which describes any PD requiring up to 12.95 watts.

PDs in an 802.3af-compliant network typically receive power from "endpoint" power sourcing equipment (PSE) such as a hub or switch, or via a midspan or midpoint PSE located between the switch or hub and patch panel. In both cases, power is delivered to the PD after a negotiation process determines that both a compliant PSE and PD are present. A PSE interrogates the PD by providing a voltage ramp (2.5 to 10 volts) to the receiving device. If the PSE detects the proper impedance signature (24.9 kilohms), indicating the presence of a PD, it proceeds to a classification stage where it applies a higher voltage ramp between 15 and 20 volts.

The PSE again measures current flow to determine the device's specific class. Until it detects a signature, the PSE delivers no energy. The power supply section of the PD is held inactive by an under-voltage lockout (UVLO) function, which isolates the switching stage until the signature and classification processes are complete. Once the process class is established, the PSE supplies full operating voltage to the PD which releases its UVLO to activate its DC/DC converter.

Enhancements
As PoE has taken off, demand has grown for support of applications, such as tilt/zoom/pan security cameras and POS terminals, which require more than 12.95 watts. To address this need, an IEEE Task Force has begun developing a higher power version of the standard. The IEEE802.3at, or PoEP, standard will support power levels up to 25 watts or more by using a new Class 4 category for PDs requiring more than 12.95 watts. While the standard is still in draft form and not scheduled for final publication until 2008 or 2009, we know the general guidelines upon which the new standard is being built.

Backward compatibility, for instance, is a requirement. IEEE802.3at-compliant PDs will recognize both IEE802.3af and IEEE802.3at-compliant PSEs. At the same time 802.3at-compliant PSEs will recognize PDs compliant with either standard. Also, IEEE802.3af-compliant PSEs will recognize new Class 4 IEEE802.3at-compliant PDs (Class 4 was included in the IEEE802.3af standard but reserved for future use). The primary challenge for those engineers writing the new standard was how to avoid initiation problems when a PD does not know if it is connected to a PSE compliant with either the older IEEE802.3af standard or the new IEEE802.3at standard. To avoid these issues, the task force added a mechanism in the new standard in which a PD requiring up to 25 watts and connected to an IEEE802.3af-compliant PSE can signal its system that it will not be able to access the power it needs.

This requirement necessitates a more sophisticated negotiation protocol. The recognition procedure (signature) in the new standard uses the same process as the IEEE802.3af standard. That is, it uses the same signature resistance and PSE voltage range for discovery. However, instead of sending a single voltage pulse to poll the PD, a PSE compliant with the new IEEE802.3at standard uses two classification-voltage pulses. After the initial pulse is sent, the PSE momentarily cuts off the classification voltage and allows the voltage level to drop (pulled down via the impedance of the PD). After a pre-determined delay, an IEEE802.3at-compliant PSE ramps the voltage back up to the classification point and effectively apply a second pulse.

If an IEEE802.3at compliant PD detects the voltage drop and the second class query, it recognizes that it is connected to an IEEE802.3at-compliant PSE. PDs compliant with the existing IEEE802.3af standard will not react to the second query signal because the duration of the classification cycle occurs within IEEE802.3af classification time limits. Instead they will respond to the second class query as it were an extension of the initial class query. The IEEE802.3at compliant PSE will be able to determine if it is connected to an IEEE802.3at PD because that device will respond to a classification query by presenting a Class 4 impedance. As the response to Class 4 is undefined in the IEEE802.3af standard an IEEE802.3af PSE may or may not power up an IEEE802.3at PD. To support higher power, the output range of an IEEE802.at-compliant PSE will be narrower than it is for an IEEE802.3af-compliant PSE.

Improved Power Distribution
The second key differentiator between standards is in PoEP's ability to control power distribution. Under the existing standard, once PSEs identify a PD's class, they simply provide the maximum power allowable under the class definition. One of the goals of the task force defining the new standard is to enable the transfer of additional information on the PD's power requirements as part of the classification process.

To reduce the complexity of the PD's hardware interface, the new standard will convey this information via layer 2 software instead of the layer 1 common mode power path. Accordingly, PDs requiring more than 12.95 watts under the 802.3at standard will support a layer 2 classification mechanism. By allowing each PD to determine peak and continuous power requirements, this new feature will allow designers to budget and match total system power capabilities more closely to actual PD load requirements.

In this way an IEEE802.3at PSE can allocate a minimal amount of power to a security camera when it is in a set position, but budget for more power when it is needed to operate a motor to pan or zoom (it is unlikely that multiple cameras will zoom at the same moment so the extra power could be part of a shared additional power budget). The task force has not yet completed the work on how this new software based description will be implemented in IEEE802.3at.

Tradeoffs—Flyback versus Forward
To take full advantage of the PoEP architecture, you must carefully consider your power supply design options to maximize system efficiency and drive down cost. Actually, the design rules are initially not too difficult since the voltage and current requirements of the PD will, in most cases, dictate the converter topology. Typically, designers building DC/DC converters for PDs in PoEP applications will choose between a flyback or forward topology.

If the load current remains under 6 amps, a flyback topology offers the least expensive option. Typically used in applications with output voltages greater than 2.5 volts, flyback converters offer excellent isolation between output and input, and eliminate the need for output inductor. However, as compared to other architectures this topology (Figure 1, below) offers limited efficiency and higher output ripple current.