Power over Ethernet (PoE) grows up: it’s now PoE+

In 2003, the IEEE 802.3af standard for Power over Ethernet (PoE) introduced a new facet to Ethernet networking, delivering DC power in tandem with 10/100/1000 Mbps data. The standard defined nominal power delivery at 12.95W, which was more than adequate for early adopter applications, including standard VoIP phones, security cameras and wireless access points (WAPs).

Since then, PoE infrastructures have become abundant in the industry and the demand for additional features and higher power has grown significantly. Fixed security cameras are gaining full motion, wireless access points are providing stronger signals at greater distances, and VoIP phones are providing video and peripheral support. To support the increased functionality, these powered devices (PDs) demand higher power from a PSE (power sourcing equipment) than what was enabled by the original PoE standard. As a result, IEEE 802.3at (also known as PoE+), was developed to on the IEEE 802.3af specification in order to accommodate the new wave of high-power applications.

One of the areas that required careful engineering design was the new classification mechanism that would be used to allow PSEs and PDs to mutually identify each other. With this mutual identification comes the ability for PSEs to properly power both (also known as Type 1 hardware) and 802.3at (Type 2 hardware) PDs, the ability for 802.3af PDs to be powered by 802.3at PSEs, and for PDs to know if they have the full power available that their higher loads demand. Every combination needs to have a well defined and consistent behavior so that interoperability within the 802.3 standard is maintained. This mutual identification has been implemented in PoE+ with the use of a more elaborate hardware-classification mechanism along with a new data-layer mechanism.

PoE+ adds a new hardware classification known as 2-event classification and involves the PSE essentially repeating the 802.3af voltage probe twice. Each voltage probe of the PD results in drawing a single pulse of current (Figure 1 ), which corresponds to a particular power level.

Figure 1: 2-Event classification and link layer discovery protocol
(Click on image to enlarge)

To begin, the PSE asserts a voltage pulse in the range of 15.5 V to 20.5 V on the data or spare pairs. The PD responds with a current of up to 40 mA which conveys one of four power classes to the PSE. The double pulse is a signal to the PD that the connected PSE is indeed a high-power PSE, able to source the higher power levels associated with 802.3at power.

The 802.3at PD responds with a class 4 current, thereby communicating to the PSE that it is a high-power PD requiring the full power available. The layer 1 classification method in 802.3af provided an optional method for the PSE to query the PD to determine the PD power needs. However, in the 802.3at specification, a Type 2 PSE is now mandated to perform, at a minimum, the single-event hardware classification.

In addition to the upgrade in hardware classification, the PoE+ task force defined a new data layer (layer 2) classification known as Link Layer Discovery Protocol (LLDP) for communication between the PSE and PD. Once a link is established, the PSE and PD can use LLDP to determine the power needs of the PD. The use of LLDP allows the PSE to query the PD repeatedly and determine the status of the PD and its power needs.

With this mechanism, it is now possible to implement dynamic power allocation, where the PSE can continuously allocate power to the PD in 0.1 W increments and the PD can request, and then subsequently relinquish, power. Communication over layer 2 enables advanced features to poll for more information such as peak power, average power and duty cycle. This new dynamic power allocation is certain to become an important feature as systems move toward a greener power environment. LLDP is an optional classification mechanism for the PSE, but is required to be implemented by the PD. If the PSE only performs a single-event classification, then the PD may negotiate higher power via the LLDP protocol. Figure 1 shows both classification methods used in PoE+.

There are two types of PSEs: midspan and endspan. Midspan controllers (or power injectors) literally inject power onto existing Ethernet cables and are situated between the LAN switch and the powered device. Data is routed through a midspan PSE without modification. These controllers are especially convenient for PoE installation into existing networks, since replacement of the switch is not necessary.

An endspan device , on the other hand, is a switch that has built-in PoE capabilities (therefore, no midspan is needed). An endspan PSE is used when building a new network from the ground up. Since midspans only have access to the power layer, they use the new 2-event classification in PoE+ to communicate high power capabilities. LLDP uses the data layer, so endspan controllers have the option to negotiate power with the PD using this additional classification method.

For PoE systems, there are two distinct locations where power is defined–at the PSE output connector and at the PD input connector. One of the more important developments in the PoE+ specification is the capping of the current to 600 mA. A PSE must now be able to continually source at least 600 mA with a minimum output voltage of 50 V. This translates to a PSE output power of 30 W. The cable resistance is modeled no larger than 12.5 O, resulting in 25.5 W available power at the PD connector. It is necessary to take into account the 48 V conversion efficiency, so that in the end, there is about 24.6 W available to the PD load.

The need for higher power is, of course, driven by the market and, today, there is already strong demand for PD power solutions beyond the existing 12.95 W limit. There are numerous power-hungry network devices that need more power now. So how does a circuit designer solve his high-power needs today? The solution is to design with new PoE+ compliant PSE and PD products (such as from Linear Technology) which address the power limits set under the new standard, and also accommodate even higher power for proprietary applications.

The challenge now is for PSE manufacturers to rapidly deploy high-power PoE+ ports in the field. Upgrading an existing PSE design for PoE+ requires:

  • Improved Ethernet magnetics that can take more bias current without increased bit error rates.
  • New PSE controller chips with higher cutoff current thresholds.
  • Depending on which controller chip is used, larger MOSFETs with larger Safe Operating Areas (SOA) may be needed.
  • Larger main power supply.
  • Miscellaneous components such as connectors, fuses, common-mode chokes, transient voltage suppressor diodes, current-sense resistors, and EMI filters may need to be upgraded for higher currents.

All of these components are already available and, often times, 802.3at magnetics and chips are simple drop-in replacements for their 802.3af counterparts. Although many design changes are needed when transitioning a PSE from 802.3af to 802.3at, we will focus on the key component facilitating the transition, the PoE+ PSE controller.

Linear Technology's LTC4266 (Figure 2 ) is the first fully compliant 802.3at quad PSE controller on the market, and is backward compatible with the popular 802.3af compliant LTC4259A. Not only does it provide PDs with power levels mandated by the new standard, but it is also backward compatible with the original PoE standard, allowing users to mix and match up to four PoE and PoE+ PDs. As mentioned earlier, to be 802.3at compliant, a PSE must be able to provide 30 W at the output of the PSE connector so that after cable losses are accounted for, 25.5 W is available to the PD. The LTC4266 provides the 30 W, but does so while drastically minimizing heat dissipation.

Figure 2: LTC4266 quad-channel PoE+ PSE Controller and LTC4269 PoE+ PD Controller with integrated switching regulator
(Click on image to enlarge)

When designing next generation PSEs, it is important to choose a PSE controller that is capable of sourcing the higher power levels, can perform the new classification mechanisms, and provides a reliable PoE system that can deliver power efficiently. The LTC4266 has extremely low heat dissipation that makes thermal design significantly easier than designing with PSE controllers that integrate less rugged, normally higher RDS(ON) , MOSFETs. The LTC4266 supports the use of external MOSFETs and, if a port fails due to a MOSFET failure, the failure will not “domino” and cause adjacent channels to collapse, which is a concern with internal MOSFETs.

The accuracy of the LTC4266 allows the use of low-value sense resistors and, more importantly, low RDS(ON) MOSFETs when managing line currents and voltages. Values can be as low as 0.25 O for the sense resistor and 0.09 O for the MOSFET, providing a maximum total channel resistance that is half that of other PSE controllers. As a result, heat dissipation is significantly reduced, allowing designers to easily and reliably use the LTC4266 without a heat sink if desired.

Power Sourcing Equipment using Linear Technology's IEEE802.3at compliant quad PSE controller has already been deployed. Off-the-shelf midspan and endspan devices that provide 30 W over each port are now available. For PSE designers who do not wish to deal with the headaches of designing from scratch, the new PoETec PSE ICM (Integrated Connector Modules) offered by a host of vendors including Molex, Tyco and Belfuse, offer an elegant 8-port and 12-port drop-in solution using the LTC4266 PSE controller.

PoE+ PDs
Transitioning from 802.3af to 802.3at on the PD side can be somewhat simpler, or at least the designer needs to consider changing fewer components, since the only things that may need an upgrade are the bridge rectification components, PD controller, DC/DC controller and transformer, so that the PD load's power requirements are ultimately met.

Heat dissipation may be less of an issue in a PD compared to a PSE, but power efficiency is still a high priority. Designers must also decide whether a PD will be able to support auxiliary power from a wall adapter or if isolation of the PD load is needed. Similar to the PSE when upgrading to PoE+, the success of a PD's design highly depends on the PoE+ PD controller.

To maximize PD efficiency, certain key decisions need to be made. For isolated designs, it is best to avoid using optocouplers that are normally used in the converter feedback loop. But perhaps the most important decision is choosing a flexible PD controller that enables these high efficiency techniques. As a benchmark, Linear Technology's LTC4269 offers up to an impressive 94% efficiency for isolated designs.

The LTC4269 is a fully compliant IEEE 802.3at PD controller, and is the counterpart to the LTC4266 (Figure 2). The LTC4269 is a full-featured PD controller with an integrated switching regulator, and has auxiliary support down to 16 V. Although 802.3at caps the power to a PD at 25.5 W, the LTC4269 does not have a current limit and can comfortably source 30 W+, enabling proprietary power levels and unlocking PD features beyond that of the PoE+ standard. Reliability is reinforced by integrating a rugged 100 V Hot Swap??? MOSFET, which isolates the PD controller and DC/DC converter during detection and classification, while providing 100 mA of inrush current for smooth power-up transitions with any PSE.

Two versions of the LTC4269 are available in order to optimize PD designs. The difference between both versions lies in the specific switchers used. The LTC4269-1 integrates a synchronous flyback controller, while the LTC4269-2 integrates a synchronous forward controller. The flyback converter provides a low part-count design, with the advantage that additional outputs can be obtained by simply adding more windings, while the forward controller provides slightly better efficiency over the flyback at higher load currents.

In both cases, synchronous rectification provides the benefit of higher output power, increased conversion efficiency and improved cross-regulation in applications with multiple outputs. Furthermore, in low-noise system designs, the controllers can be synchronized to an external oscillator.

It is also worth noting that the LTC4269-1 incorporates Linear Technology's patented No-Opto feedback topology to provide full IEEE 802.3 isolation without the need for of optoisolator circuitry (Figure 3 ).

Figure 3: LTC4269-1 PoE+ PD Controller with integrated synchronous no-opto flyback switching regulator
(Click on image to enlarge)

This avoids the disadvantages of implementing optocoupler feedback, including variable loop gain due to optocoupler tolerance, high-temperature sensitivity and higher cost. The traditional optocoupler and shunt regulator used in the feedback loop are replaced by an additional winding on the existing transformer for improved regulation, efficiency and a simpler circuit.



Linear Technology's LTC4266 and LTC4269 are spearheading new PoE+ designs, enabling PSE and PD manufacturers to produce next-generation devices. Linear actively participates in the IEEE802.3at task force and has more IEEE 802.3at compatible products in development. Table 1 provides a list of Linear's released PoE+ devices.

(Click on image to enlarge)

These new products will provide pin-compatible upgrade paths from Linear's existing PD products for a smooth transition to the new PoE+ standard. Linear Technology's products continue to offer rugged design, field-proven reliability, and are supported with a wealth of technical experience based on years of designing PoE products into a wide variety of applications.

About the author
Christopher Gobok is Product Marketing Engineer for Mixed Signal Products at Linear Technology Corporation, Milpitas, CA

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