实现更强大的以太网供电功能(英)

Power over Ethernet (PoE) technology provides power via standard Cat-5 cables, eliminating the need for wall adapters or other external power sources to drive networked equipment. Two major components are required in such a system: the power sourcing equipment (PSE) which provides power; and the powered device (PD) that receives and uses this power.

The Jack PC from Jade Integration (Newcastle upon Tyne. U.K.) uses PoE for single-wire operation.

Numerous practical PoE applications have been developed to date. However, new applications have emerged that need more power than the power limit defined by today’s 802.3af standard, which is about 13 W at the powered-device (PD) end. A new standard, PoE+, is required to address these increased power needs, but it has not yet been defined. Until it is, it must be possible to feed PD loads requiring more power than what is currently available.

High-power PoE system requirements
Any high-power solution based on Cat-5 cable must meet the following basic criteria:

• Must not overheat the Ethernet cables, power sourcing equipment (PSE), or PD

• Must not cause a current imbalance to avoid the risk of transformer saturation

• Must maintain an operating voltage within the 802.3af standard voltage range

If possible, any solution should provide the following features:

• Compatibility with 802.3af standards: discovery, classification, current consumption, undervoltage lockout, in-rush currents, current limiting, etc.

• Power management capability

• Broken-wire detection in cable installations

Operating at higher voltages
The 802.3af standard stipulates a PSE output voltage from 44 to 57 V, with a PSE I_CUT (represents a level beyond which power consumption is regarded as an overload) of 350 mA min and an I_LIM (represents the highest consumption level possible) of 400 mA min. Using 100 m of cable with a worst-case feed resistance of 20 W, the result is a limit of 12.95 W at the PD input, when operating at the minimum voltage allowed.

One simple way to provide more power involves operating at a higher bus voltage closer to the 57-V limit. The benefits of such a solution include better system efficiency and decreased losses in the cable and at the PSE and PD input stage.

Existing Cat-5 cable installations can still be used because cable dissipation does not increase, and the risk of wire imbalance and magnetic saturation is no greater than in standard PoE setups as long as the design rules are followed to help guarantee that no situations become problematic. Also, such a solution is in compliance with 802.3af standards and uses magnetic components the same as those for standard 802.3af devices.

For this solution to work, the switching power supply that provides the input voltage must regulate at a tighter tolerance. For example, if the variation is from 53 to 57 V, it requires a ±3.5% voltage tolerance in addition to the power supply’s overvoltage protection.

Today, Cat-5 copper communications wiring is recognized as the minimum for broadband services. Its specific standard designation is EIA/TIA-568. Cat-5 cable is designed with 24-AWG conductors, resulting in a worst-case feed resistance of 12.5 W.

Providing power over four pair
For higher power requirements at higher voltages, using all four pairs and two PSE operating ports in tandem is a possible solution. In this case, all previous benefits discussed are still applicable, and the designer at the PD end can choose dc/dc power supplies connected in either series or parallel.

To have a working solution, the design must ensure an acceptable current sharing between each power feed (current sharing must not be confused with balancing within a pair). One must be careful, as each power feed has a separate diode and diode bridge (negative temperature coefficient), fusing, and resistive elements and each PD has a switching power supply with negative input resistance. Also, two different voltage sources could be used at the PSE end to drive each set of pairs, which means two different voltage levels. Without proper sharing, the current limit will be reached at a much lower total current than expected.

This quad-port PSE manager eliminates two external components per port.

Providing more power over two pair
For higher power with only two pairs, a possible solution involves a higher current per pair at even higher voltages. In this application, the current limit on both PD and PSE is set higher than the limit defined by today’s 802.3af standards.

PD and PSE power components must have higher current capabilities, particularly important with the data transformer assemblies. It is recommended to anticipate a higher dc current imbalance between two wires of the same pair than 802.3af standard specifications, and critical to keep the signal’s integrity in spite of the dc current.

Existing Cat-5 cable installations still can be used, but be careful to ensure that the maximum-rated cable temperature is not exceeded. That’s because the ambient and the cable temperatures rise from dissipation in the wires.

It is highly recommended to limit higher currents to only a few of the cables within a cable bundle to prevent high temperature elevations for the innermost wires within the bundle. Additionally, connectors must be chosen according to their mating/unmating resilience (capacitive and inductive loads), as well as their current-carrying capability.

For PD loads requiring higher power than the 802.3af standard, one can use two- and four-pair solutions. However, these solutions do not provide a means for the PSE to estimate how much more power (than the 802.3af standard limit) the PD requires and what its configuration should be梕ither single port or two ports in tandem. It is not also possible for the PD to "ask" the PSE how much more power it is able to provide over the standard level.