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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. PoE brought with it a unique set of problems and new ways of thinking that were unfamiliar to many engineers with experience designing Ethernet equipment. PoE is now commonly used for VoIP phones, wireless access points and security cameras. As PoE evolves, there is a need for enhancements to the standard to enable emerging applications.
By way of review, the PoE link allows a Powered Device (PD) to draw up to 12.95W from the Power Sourcing Equipment (PSE). The PoE link or port is controlled by the PSE, which identifies PDs via detection and classification before powering and monitoring the port (ICUT, ILIM and disconnect). Much of the burden of PoE rests on the PSE, which must perform detection and disconnect flawlessly to avoid damaging legacy devices. If the PSE does not adequately perform classification, power delivery and monitoring, intermittent failures and instabilities may result. The PSE cannot control everything; when it applies power, it trusts the PD to follow the standard, turn on without oscillating and avoid drawing more power than requested. Because both types of devices must cooperate, PD and PSE designers should consider issues from the perspective of both devices.
Higher power for emerging applications
13 watts is adequate for basic IP phones, but motorized cameras, multi-radio access points, and devices with large color screens are seriously constrained. The IEEE is currently working a higher power standard, dubbed PoE+ (officially IEEE 802.3at) that will coexist with 802.3af devices available today. The ultimate power levels defined by the new standard have not been established, but as of today, it is likely we will see 30-watt 2-pair systems and 60-watt 4-pair systems. The IEEE 802.3at committee has taken on a daunting task to define a world standard that will define a safe, higher-power standard that is backward compatible and interoperable with currently deployed 802.3af equipment. Because of the complexities of writing this specification, we are not likely to see a final specification for 1 ½ to 2 years from now.
Although typical CAT 5 cabling includes four twisted pairs, the 802.3af standard only allows two pairs to carry current at any given time. One option is to allow additional current down the third and fourth pairs, doubling the available power. A second option is to raise the current limit, allowing more power down the same two pairs. Each of these techniques has appeared in proprietary PoE systems. However, each has drawbacks, complicating the choice between them.
Implementing pre-standard high power PSEs
In the interim, some applications require high power and cannot wait for the new standard. There are several ways to do this. The following circuits build on the basic 802.3af-complaint PSE circuit using the LTC4258, shown in Figure 1a, and the basic PD circuit using the LTC4257 shown in Figure 1b. The LTC4259 can be substituted in the PSE circuit if the application requires AC disconnect; the LTC4267 can be substituted in the PD circuit if the application requires an integrated switching regulator.
Figure 1a. Basic 802.3af-compliant PSE circuit using LTC4258
Figure 1b. Basic 802.3af-compliant PD circuit using LTC4257
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