Posted on 01 March 2019

Power over Ethernet Moves Forward

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Brains as well as brawn boost efficiency for 25.5-Watt Ethernet Powered Devices

To deliver environmentally sound solutions using the latest 25-Watt and higher Power over Ethernet adapters, designers need to take advantage of the intelligent features included in the PoE standard.

By Koen Geirnaert, Product Marketing Manager, ON Semiconductor


It’s Good, but is it Green?

Power over Ethernet (PoE) has won acceptance as a convenient and flexible way to connect powered devices such as VoIP phones, wireless access points, or security cameras to a LAN. Benefits include reduced costs as well as valuable flexibility for network managers and users.

Among the main attractions of PoE, consolidating data and power on a single cable save significant cabling and installation costs. Connection points to the LAN can be added easily and at low cost, and the locations of Ethernet Powered Devices (PDs) can also be altered quickly and easily. A further benefit is that, while conventional mains wall socket formats vary around the world, PoE utilises the globally recognised and approved RJ45 so that any PD can be used in any country.

There is now a growing demand to increase the specified maximum rating for Ethernet PDs, which is currently 13W. The next-generation standard, soon to be ratified as IEEE802.3at, will allow PDs up to 25.5W to be powered from the LAN cable. This will open-up the PoE market to a far wider range of equipment and applications. Some independent end-to-end PoE solutions are targeting even higher power ratings, using components such as ON Semiconductor’s integrated PoE-PD controller and DC-DC converter IC for Ethernet-connected devices up to 40 Watts.

But as PoE moves forward to address increasingly power-hungry applications, there are valid doubts about overall system efficiency. Despite its established advantages, PoE must also now prove its green credentials.

Natural Barriers

On first inspection, the indicators are not good. At the higher power ratings, distribution losses in the Ethernet cable will be greater than in an ordinary AC line. Although PoE technology seeks to take advantage of distributing power over an existing cable, the cable itself actually restricts PoE’s potential. The Ethernet cable can be a twisted pair CAT5, CAT5e or CAT6 cable. These types of cables differ in loop resistance - CAT5 has a loop resistance of 20Ù and CAT5e or 6 one of 12.5Ù. The fact that the data cable need not be run in conduit and is considered unprotected, and that the technology requires a DC voltage, also limits the maximum voltage level on the cable to 57V for safety reasons.

However, the lower the supply voltage, the more current is needed for the same power level and the more conductance loss in the cable. Therefore PoE has a disadvantage against the mains network since voltage is lower and the cable has a higher resistance. Since the voltage level in the mains network is much higher the loss incurred in the mains cable is minimal.

Consider a motorized, high-power security camera rated at 20W with the power supplied either over the Ethernet cable according to the IEEE802.3at standard, or via 120V or 220V mains through a wall adapter. The latest version of the Energy Star specification (2.0) requires efficiencies up to 82% for “wall wart”-style mains adapters. The figure is valid in the 20W range for full power operation. At this required efficiency level, 24W is taken from the power grid to supply a camera of 20W.

Using PoE and a (PSE) platform, the voltage is converted from the mains to an output voltage in accordance with the IEEE802.3at PoE specification. Figure 1 shows a PoE camera with the required voltage levels on the cable according to the IEEE standards. 20W can only be supported by the new higher power IEEE802.3at standard. On the cable some power is lost due to the 12.5Ù resistance of the cable. On the PD side there is again a DC-DC converter converting the DC voltage to the appropriate level for that device.

PoE configuration

The power-balance chart of figure 2 compares the losses in the PoE system with those in the line-powered arrangement. Due to the three steps in a PoE configuration, its power balance appears to be inferior since the input power is 10.3W higher to deliver 20W.

Power balance PoE versus wall socket

However, recall that PoE is designed to distribute data and power over the same cable. Hence a convenient data connection is available, which can be used as a communications channel to help manage more intelligent power distribution. Implementing power saving algorithms in the Power Sourcing Equipment (PSE) can compensate for power losses in the cable. In this way overall system efficiency can match or exceed that of an alternative line-powered arrangement.

The 10.3W difference between a PoE approach and a wall socket implementation is true only at full-load operating conditions. In practice, for the majority of the time the converter is not operating at full load but in standby or somewhere in between. With this in mind, and anticipating improved power management using intelligent communications, PoE has a greener character than many have recognized.

Thinking Green

By adding intelligence to the PSE within the switch, individual ports can be switched off completely during periods when the equipment connected to that port is not in use. This is difficult with a conventional mains adapter. WLAN access points and VoIP phones in an enterprise environment are two good examples to illustrate the potential for valuable savings. Typically the equipment is only used during office hours, but each unit is usually powered all the time. Manually turning them on and off at night time and at weekends could result in an ‘off time’ of around 65%.

The upcoming IEEE802.3at standard includes several features to implement power-down of the PD side from the PSE side. The layer 2 protocol contains Power status communication frames, for example, which allow the PD board to send a message to the PSE to stop powering the port. If at the same time the detection resistor is also removed the PSE will not re-power the port until the detection resistor is again available. The detection resistor can be disconnected via a switch on the application. This switch can be mechanically operated (such as by taking a phone off the hook) or electronically controlled via a small power source. This could be a battery or solar cell on the application.

Visible Improvement

Considering again the example of a security camera, the power demand is greatest when the motors are moving the camera – such as to follow a moving object. During these periods the power demand is close to the maximum level of 20W. But for most of its operating life the camera is motionless and power is required only to enable monitoring. In this state, the actual power requirement is far less than 20W; in fact typical power consumption is only around 25% of full load. In these operating conditions a single power supply is working with efficiency of approximately 50% - far less than 82% for maximum load conditions.

On the PSE side several ports must be served. There may be 24 or 48 ports, and by satisfying demands on a per-port basis through communication between the load and the port, the overall power can be budgeted. By taking the average of the overall power budget, the supply of the switch can work most of the time around its optimum efficiency point. Based on this principle the power supply on the PSE side can be much more efficient in total than individual wall wart converters.

Furthermore, since not all ports demand full power at all times it is also feasible to install a quality-of-service algorithm on the PSE switch. This delivers an additional benefit by allowing the use of a much smaller power supply to achieve extra cost savings in the switch hardware.


The initial success of Power over Ethernet has led, inevitably, to demands to support higher power ratings. But in today’s environmentally aware world, the 25.5W standard arriving in 2009 must demonstrate acceptable power efficiency if it is to find a place. This can be achieved through improved management of Ethernet PDs, taking advantage of the cable’s data channel to support intelligent power-control techniques.



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