Seizing the lead in IEEE 802.3bt PoE++, ADI launches PoE++ PD and PSE controllers

The IEEE 802.3 Ethernet specification requires that the network segment (including the PoE circuit) be electrically isolated from the chassis ground and PHY. By placing the LTC4291-1 on the non-isolated side and the LTC4292 on the isolated side, up to 6 expensive optocouplers and 1 isolated power supply can be replaced with a cheaper and more reliable 10/100 Ethernet transformer. This topology not only saves costs, but also enables a more stable, reliable and easy-to-manufacture PSE design.

802.3bt introduces two different PD signature configurations: single-signature and dual-signature PD. Single-signature PD (Figure 3) is a PoE++ PD that shares the same detection signature and classification signature between two pairs. Dual-signature PD is a PoE++ PD with independent signatures on each pair; allowing each pair to have completely independent classification and power allocation. Dual-signature PD solutions are very complex and cost twice as much as single-signature PD.

Figure 3. Topological structures of single-signature and dual-signature PDs.

Additionally, it is important to note that despite sharing a common architecture, 802.3bt dual-signature PDs are not equivalent to previous standard UPoE devices. The LTC4291-1/LTC4292 supports an updated PoE++ PD detection process that includes a new connection check sub-process to determine which PD signature configuration the PSE is connected to.

After performing the connection check, the LTC4291-1/LTC4292 will begin to verify that the connected PD complies with the IEEE standard. Although the IEEE requires the PSE to use either a 2-point voltage or 2-point current detection scheme to detect valid PD characteristics (25 kΩ), the LTC4291-1/LTC4292 uses both types of detection schemes to achieve a more robust and reliable scheme. This multi-point (multi-voltage and multi-current) detection mechanism can be used to eliminate false alarms and avoid damage to network equipment that is not designed for PoE DC voltage tolerance.

PoE++ delivers power over 4 pairs of conductors (8 conductors) compared to the traditional 2 pairs (4 wires) used by previous PoE standards. Not only does this enable new higher power levels, but using more conductors can improve the efficiency of older systems with lower power levels because the power loss in the cable is cut in half. For example, to ensure that a PoE+ PD can receive 25.5 W, 30 W of power needs to be supplied to the PoE+ PSE, as the loss over 100m of CAT5E cable is 4.5 W. Powering the same 25.5 W PD using the PoE++ standard typically reduces the loss to less than 2.25 W, increasing the overall power delivery efficiency from 85% to 92.5%. Considering the number of PoE PDs worldwide, this means a significant reduction in power consumption and, in many cases, a 7.5% reduction in carbon emissions.

PoE++ introduces four new high-power PD classes, bringing the total number of single-signature classes to nine (see Table 1). Classes 5 through 8 are new to PoE++ and correspond to PD power levels from 40W to 71.3W. PSEs still have the choice of using the physical layer (i.e., 5-event classification for 71.3W) or the data link layer (e.g., Link Layer Discovery Protocol (LLDP)) to classify PDs, and PDs must still be able to support both classification schemes to comply with the standard. Keep in mind that because each pair operates independently in a dual-signature PD, each pair can be a different class. For example, Class 1 (3.84W) on the first pair and Class 2 (6.49W) on the second pair would result in a dual-signature Class 1, Class 2 (10.3 W) PD.

Table 1. PoE++ PD Classification and Power Levels

PoE++PD can also implement an optional extension of physical layer classification (called Autoclass), in which a PoE++PSE (such as the LTC4291-1/LTC4292) measures the actual maximum power drawn by the connected PD. In this way, using this power management feature, the LTC4291-1/LTC4292 can allocate the remaining power to other bulbs (if a certain bulb is measured and consumes less than its classified power due to a lower brightness setting or a short cable).

It goes without saying that PoE++ is backward compatible with the older 25.5W PoE+ and 13W PoE standards. A lower power PoE+ or PoE PD can be connected to a higher power PoE++ PSE (such as the LTC4291-1/LTC4292) without any problems. Furthermore, when the situation is reversed, that is, a higher power PoE++ PD is connected to a lower power PoE+ or PoE PSE, the PD can operate in a negotiated lower power state, which is called demotion. If the PD ignores the demotion and operates in its highest power state, the high power PD will cause the PSE to repeatedly turn on, reach its current limit, and then turn off, which actually causes the PSE to generate low frequency parasitic oscillations. Therefore, both PoE+ and PoE++ require demotion, but unfortunately demotion is ignored in many implementations.