Opportunities abound in PoE

New standard promises to expand applications

BY ANDREW SMITH
Power Integrations, San Jose, CA
http://www.powerint.com

Momentum is building behind Power-over-Ethernet (PoE), the IEEE industry standard for power transmission over Ethernet cable. By delivering power over the same CAT-5 cable that supplies Ethernet data, this emerging technology simplifies the installation and deployment of powered devices (PDs), wireless networks, security cameras, and VoIP phones. In the process, it reduces cost and increases utility.

The current IEEE802.3af PoE standard allows designers to supply up to 12.95 W to the PD over an Ethernet cable while meeting Safety Extra Low Voltage (SELV) requirements. Under IEEE802.3af, PDs are categorized into four different classes, depending on how much power they require for operation. Designers assign their devices to a particular class depending on power requirements. Class 1 defines devices requiring up to 3.84 W. Class 2 identifies devices requiring between 3.85 W and 6.49 W. Class 3 covers power ranges from 6.5 W up to the maximum 12.95 W. Low-cost solutions can use a general default class designation (Class 0), which describes the PD as requiring up to 12.95 W.

A PSE determines the class of a PD by presenting a voltage ramp to the PD, which rises from 2.5 to 10 V, see Fig. 1 . If it detects the proper impedance signature (current drawn) of a PD (24.9 kΩ), the PSE proceeds to increase the voltage. If no signature resistance is detected, the PSE will not energize the CAT-5 cable. In the classification stage, the PSE applies between 15 and 20 V to the PD and measures current flow to determine the PD’s specific class. During this phase, the power supply portion of the PD is held inactive by an undervoltage lockout (UVLO) circuit that isolates the switching stage until the signature and classification phases are completed. Once classification is achieved, the PSE supplies full operating voltage to the PD.

Fig. 1. A PSE determines the class of a PD by presenting a voltage ramp to the PD, which rises from 2.5 to 10 V.

Topology decisions

To develop efficient power supplies for PoE-compliant PDs, designers need low-voltage converter ICs, power MOSFETs, in-rush protection, UVLO, and signature and classification circuitry. Equipment designers are using two approaches to address this requirement.

Some use an integrated approach that combines in-rush protection, a dc/dc controller, and the switching MOSFET, along with current limit and other protection features. The designer then uses discrete circuits to add signature and classification functions, the UVLO, and a pass switch.

This approach offers excellent scalability across all PoE classes and gives the designer maximum opportunity to fit the solution to a particular application. Power Integrations, for example, offers 14 dc/dc conversion ICs with different MOSFET and package options to suit a wide range of PoE requirements.

Designers using this approach require only a small number of external components for the signature circuit (for Class 0, just one resistor), the classification circuit, and UVLO: in total, perhaps seven discrete components are required. A fully configurable multiclass IEEE802.3af-compliant PD interface using this approach is shown in Fig. 2.facnpower1aug2007Fig. 2. To develop efficient power supplies for PoE-compliant PDs, designers need low-voltage converter ICs, power MOSFETs, in-rush protection, UVLO, and signature and classification circuitry.

Other power semiconductor manufacturers take a different integrated approach and combine the in-rush protection circuit and dc/dc controller with the UVLO, switch, and signature and classification functions (see Fig. 3 ). Typically these circuits do not include the dc/dc switching MOSFET, and current limit circuitry.

The integration of the 802.3af front end simplifies the design of the (already simple) PoE interface. This approach typically requires the customer to add a discrete switching MOSFET and external current limit components. In taking responsibility for the switching components in this system, the designer must be familiar with the switching and control issues of an analog power switching stage.

The designer must also be aware of the stability issues associated with PCB layout and high-frequency switching. These issues are not trivial and require a good understanding of power circuits and require additional time for board design and testing.

facnpower2aug2007Fig. 3. Other power semiconductor manufacturers take a different integrated approach and combine the in-rush protection circuit and dc/dc controller with the UVLO, switch, and signature and classification functions. (a) Integrated PD and controller function. (b) Integrated dc converter function with discrete PD.

New standard

An IEEE task force is currently working on the next version of the standard. Called Power-over-Ethernet Plus (PoEP) IEEE802.3at, the new standard will extend power levels to 25 W or more by defining Class 4 (a class described in IEEE802.3af, but reserved for future use) for devices requiring more than 12.95 W.

By supporting higher power levels, industry experts are already predicting that this new standard will broaden PoE applications to new areas such as WLAN, point-of-sale devices, routers, switches and pan/tilt/zoom security cameras. The task force is developing the new standard with publication expected in late 2008 or 2009. Although the new standard is not complete, the following comments describe what looks to be some of the key features.

Since the new standard will be backward compatible with 802.3af, one of the primary problems the task force faces is how a PSE can determine whether it is connected to an 802.3af or the new 802.3at-compliant PD (and how could the PD make a similar determination about the PSE). To solve this problem, the 802.3at standard will use the same signature employed in 802.3af to indicate a PoE-compatible PD, but will add a second voltage pulse to indicate the PSE’s 802.3at-compliant status.

The 802.3at-complaint PDs will detect the second voltage pulse and recognize they are connected to an 802.3at-complaint PSE. They will respond by indicating they are a Class 4 device indicating to the PSE that they are an 802.3at-compliant PD. Since the duration of the classification cycle occurs within 802.3af time limits, PDs compliant with the existing 802.3af standard will view the second pulse as simply another initial query and respond accordingly.

One of the most exciting new changes in the PoEP standard will be the ability it offers to control power distribution in much finer detail than the existing standard. PSEs compliant with the 802.3af standard identify a PD’s class and simply provide the maximum power allowable under the class definition. Once an 802.3at system is powered up, standard-compliant PSEs will be able to use Level-2 software communications with each PD to determine peak and average power requirements.

This new capability will allow designers building PoE-compliant systems to budget power and match total system power capabilities to system load requirements much more closely. It will also be possible to communicate power about peak loads and duration of high-current demand (camera tracking motors, for example, have higher current requirements for a short period). This Level-2 communications has not yet been defined in the new standard. ■

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