Power Electronics

Emerging Standard Drives High-Power PoE Design

Key details of the IEEE802.3at standard, which is to be ratified next year, are available now for designers looking to take advantage of the nearly 60 W allowed to power PDs.

Power over Ethernet (PoE) is about to go through a transformation. The increase in the power available to powered devices (PDs) from 12.95 W to 59 W will transform the RJ45 connector into the first truly universal power plug. With the IEEE 802.3at standard expected to be ratified in the second quarter of 2008, and prestandard equipment already available, the full deployment of high-power PoE is rapidly approaching.

To create systems that meet this emerging IEEE standard, designers must have guidelines for cable selection; voltage levels, power levels and classification for power-sourcing equipment (PSE); and power allocation values, as well as the enabling of backward compatibility with the existing IEEE 802.3af PoE specification.

In November 2004, the IEEE 802.3 Working Group approved the creation of the PoE Plus Study Group, with the intent of exploring ways to enhance the successful IEEE 802.3af-2003 PoE standard. The PoE Plus Study Group work culminated in September 2005, with the formation of the IEEE 802.3at Task Force, which is currently drafting the IEEE 802.3at-2008 standard.

Trying to increase the expanding market for PoE systems, the IEEE 802.3at Task Force has taken upon itself several objectives. First, 802.3at should operate on Cat 5e and higher infrastructures, unlike 802.3af, which had to account for the limitations of Cat 3. Also, 802.3at should follow the power safety rules and limitations pertinent to 802.3af. A High Power 802.3at PSE must be backwards compatible with 802.3af, being able to power both 802.3af and high-power 802.3at PDs.

Furthermore, the 802.3at standard should provide the maximum power to PDs as allowed within practical limits — at least 30 W — and 802.3at PDs, when connected to a legacy 802.3af PSE, will provide the user an indication that an 802.3at PSE is required.

The group will research the operation of midspans for 1000BASE-T and the operation of midspans and endspans for 10GBASE-T. It will also create a PD management information base.

Electrical Specifications

In IEEE 802.3af, Cat 3 cabling, as a minimum, is assumed and only four wires are available. These are organized into two pairs, which serve to conduct the current from the PSE to the PD. A key factor is the overall resistance of this loop, which is 100 m long. With Cat 3 cabling, it is 20 V. When using Cat 5e, there are eight wires in the cable, which means that two such loops can coexist, in a 232-pair configuration. In addition, due to the higher gauge of Cat 5e, each 100-m-long loop has a resistance of only 12.5 V.

At this point, the guidelines for the maximum current determination used by the IEEE 802.3at assume that bundles include 100 cables maximum at a temperature of 45oC or lower. Also, the maximum current is the same for any type of cable better than Cat 5e, such as Cat 6, Cat 6A and Cat 7. Fig. 1 shows the different configurations of PoE, including IEEE 802.3af’s two-pair medium-power PDs and four-pair high-power PDs.

The power-sourcing equipment (PSE) output-voltage range is one of the most important factors in the amount of power available for the PDs. The IEEE 802.3af standard establishes this range as 44 V to 57 V. The number that determines the maximum power the PD can take is the lower end of the range. Therefore, the IEEE 802.3at Task Force intends to raise the minimum PSE output voltage from 44 V to 50 V, increasing the power available to the PD by 16%. The implications this will have for design depend on the type of PSE.

Modular PSEs such as chassis switches, which are already deployed in the field, would need the IEEE 802.3at modules/line cards to have voltage-boosting circuitry. This would ensure that the minimum voltage is 50 V or above. It is important to note that with the higher maximum current of IEEE 802.3at, the voltage drop in the system tends to increase. In practice, power-supply designs must account for 0.7-V voltage drops across diodes to guarantee an output voltage of at least 53 V, especially when ac disconnects are used.

The latest recommendations from the Telecommunications Industry Association (TIA) state that the maximum guaranteed current to be adopted by the IEEE 802.3at Task Force is 720 mA, with the cabling limitations previously discussed. This means that there would be a new ICUT limit (which determines the level after which the PD may be disconnected, if enough time passes) that must be set by vendors between around 720 mA and 820 mA.

The original IEEE 802.3af standard stipulated ICUT to be between 350 mA and 400 mA. In the same manner, ILIM moves from the 400-mA to 450-mA range and into the 820-mA to 920-mA range. ILIM is the maximum instant current a PD can draw; if it tries to get more, the current is limited to the maximum level. The nominal value of 720 mA for the maximum current means a two-pair PD can get 29.5 W and a 232-pair PD could get as much as 59 W. Table 1 shows the voltages and currents for different configurations.

Component Design

PSEs must have thicker internal pc-board traces to support the higher currents. In addition, increased current through components such as ac disconnect diodes, sense resistors and power MOSFETs will lead to increased power dissipation in these devices. This means that in many cases the components need to have more space between them on bigger boards to improve heat resistance. Other alternatives include heatsinks or improved ventilation.

Another issue is the magnetic cores of the pulse transformers used for Ethernet communications. As a rule of thumb, to keep a reasonable current imbalance, the cores have to be bigger than those of conventional pulse transformers. Since there is a limit to the width and height of the cores — as they need to fit inside 238, 236 and 234 RJ45 connectors — these can only grow in depth, becoming more like oval toroids.

Many vendors have demonstrated to the IEEE 802.3at Task Force that it is feasible to implement RJ45 connectors with integrated pulse transformers supporting 720 mA, but these would be deeper than current RJ45 connectors and, therefore, would not fit the same mechanical footprints.

System Considerations

PSE power supplies, which must have higher minimum voltages, must also be bigger, as each port can take either 36 W or 72 W. As in IEEE 802.3af switch design, this does not mean that full power needs to be allocated to every port. Rather, it depends on the application being targeted.

As an example, IEEE 802.3af switches supporting Voice-over-Internet-Protocol (VoIP) phones typically have power capacity of about 7 W per port on average. Therefore, a 24-port switch would require a 170-W power supply. If a switch is designed for deployment in an enterprise environment to power IP phones and laptops, then laptops could take 50 W and IP phones 7 W, so the average would be 28.5 W instead of 72 W per port.

Another way to cope with the varying power requirements is to have hardware capable of providing full power on each port by using a relatively small internal power supply and a dc input; this allows end customers to pay for only the power they use. This is common practice in modular and high-end fixed-configuration (stackable) PoE switches today, and is expected to become even more popular with the introduction of IEEE 802.3at.

The power-supply sizing and the usage of a combination of power supplies to power IEEE 802.3at PDs bring about the need for excellent power management. The power management scheme must be able to dynamically change the allocation of power to PDs to enable the maximum utilization of the power budget and to account for failure of the power supply (which would have the ripple effect of an instant change in power budget) without disrupting transmission of power to high-priority devices. The power allocated to each PD at any given time can be calculated by the PSE using three methods:

  • · Preconfiguration is normally not used, but possible.
  • · Classification is a reliable possibility because the PD can never take more power than its rated power.
  • · Measurement ensures the best power-supply utilization but must be done carefully to avoid the disconnection of ports in the event of a momentary power fluctuation.

Classification

Classification, which in IEEE 802.3af was an optional feature at both PD and PSE, becomes mandatory in IEEE 802.3at. This needs to happen so there can be mutual identification between IEEE 802.3at PDs and PSEs, avoiding the situation in which a high-power PD attempts to draw power beyond what a low-power PSE it is connected to can supply. The IEEE 802.3at Task Force has devised two classification methods:

  • The two-event classification is an extension of the existing classification method, where there are two classification pulses, with a voltage drop in the middle (Fig. 2). The PD uses these two events to differentiate between a low-power PSE and a PSE capable of delivering high currents. Table 2 lists the latest values agreed by the IEEE 802.3at Task Force to be used in two-event classification.
  • Layer-2 (or data layer) classification is performed after the PD is powered, enabling the PD to negotiate and renegotiate its power requirements according to the application needs. It has the advantage of never needing to turn off a PD because of an unexpected power increase, but it still allocates to PDs a maximum power consumption, which may be required.

Layer-2 classification has not been fully defined but should be based on link layer detection protocol (LLDP; IEEE 802.1ab) and LLDP-MED (multimedia related), defined by the TIA. To ensure interoperability, PDs must support both of these classification methods, while PSEs must choose to implement at least one method.

Another important option, especially since gigabit midspans are within the scope of IEEE 802.3at, is to be able to power two pairs of a PD with an endspan and the other two pairs with a midspan that could be added at a later time (Fig. 3). This, again, allows the customer to deliver higher power through the use of four pairs, but only on designated ports (such as those power laptops, for example), without the need to support this capability on all ports.

The IEEE 802.3at Task Force has overcome the most important hurdles in the process of defining the future of PoE, and PSE and PD vendors can now begin developing devices that will be compliant with IEEE 802.3at, once the standard has been ratified.

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