Efficiency Considerations in Power-over-Ethernet Systems

Power over Ethernet offers benefits to both commercial and industrial networks as it allows end devices such as IP security cameras, VOIP telephones, wireless access points and other devices to get both data connection and power supply from the same physical connection (up to 25W per port). This can clearly reduce the amount of cables required for any network installation, as individual device power cables are no longer needed, saving money. PoE end nodes are also easy to install as no mains power connections are required; the technician doesn’t need to be a qualified electrician, promoting network scalability. Networked devices can also be placed wherever they are required, and can easily be moved, too.

Other benefits of PoE include safety features to protect devices from receiving too much or too little power. Reliability is increased as power for the whole network comes from a central source, which can be backed up by a UPS if necessary.

As with any power system, efficiency is of course extremely important in PoE systems, especially those used in tightly controlled environments such as data centres. The efficiency of all the parts of the PoE system may be considered separately and then multiplied together to get a final figure for the efficiency of the system.


Parts of the System

At the very beginning of the network will be a power supply, or supplies, plugged into the mains, which converts mains voltage to a voltage that can be used by the power sourcing equipment (PSE) it is powering. PSE options might be a dedicated PoE Ethernet switch, which has PoE built in, or a PoE injector, which is added to an existing Ethernet switch to upgrade it to PoE. An example of a PoE injector is this Phihong 8-port 125W PoE injector, which has an efficiency of 75%.

Using a separate injector is a good idea where there is already a perfectly good Ethernet switch installed; this separates the data and power parts of the infrastructure which makes the network more flexible and scalable for the future. This spreads the heat dissipation between two devices which could be useful in sensitive applications such as datacentres. Injector manufacturers also argue that their products can offer better power efficiency than PoE switches can, since they are better able to provide the exact amount of power required by the network at any time (and no more).

Power then travels around the network using components such as cables and connectors. Since standard Ethernet components are used, that means CAT5 cable and RJ45 connectors.

Power lost in the cable may be calculated as I2R, where I is the current carried, and R is the resistance of the cable. The resistance of the cable of course depends on its length; length is therefore a limiting factor. Note also that CAT5 cables contain four twisted pairs of wires and that PoE solutions that use all four twisted pairs minimise power loss; it is halved compared to using only two pairs. Phihong injectors are available for either the two-pair or the four-pair setup.

Any loss in the connectors may be similarly calculated using P= I2R; the R in this case being the contact resistance of the connector. It is possible to add up the losses for connectors in the whole system by calculating the connector loss then multiplying it by the amount of connectors in the system. As an example, this typical Sentinel 1.5A RJ45 connector has a contact resistance of 10mΩ.

At the powered device (PD) end of the network, if the PD is PoE compatible, a built-in PoE extraction module takes power straight from the network cables and converts it to useable form inside the PD. Extraction modules such as this PoE extractor module from Murata Power Solutions are available; this one offers efficiency of 77%.

If existing PDs without PoE capability are used, a PoE splitter will be needed. Splitters, like this 21W version from Phihong which is 80% efficient, separate the power from the Ethernet signal before it reaches the device. The power and data are sent through separate channels to the PD, in the same way it would expect from a non-PoE network.

The latest PoE injectors, extractors and splitters offer high efficiency ways to implement PoE practically anywhere an Ethernet network exists. RS offers PoE components, including cables and connectors, for any size of network.