Power over Ethernet (PoE) is a technology whereby standard Ethernet wiring does double duty, both as a data transport facility and a means to supply power to remote devices. This provides many advantages in installation simplicity and interconnect efficiency. PoE has found wide use in applications like security cameras, IP telephones, wireless access points, and light fixtures for smart buildings. However, there are shortcomings to standard PoE, and this blog suggests some techniques to help improve it.
Traditional Unidirectional Power over Ethernet Systems
The original PoE standard is IEEE 802.3af, which specified 15.4W of 48V launched into standard CAT-5twisted pair Ethernet cables. Later variants include 802.3at. which can launch 30W, and 802.3bt, which can launch 60W or 100W. There are also a number of non-standard implementations that can achieve higher powers or longer reach.
PoE systems work based upon the flow of energy supplied from a power sourcing equipment (PSE) endpoint, through the CAT-5 cable, and into a powered device (PD). This energy flow doesn’t hamper the delivery of bidirectional Ethernet data over exactly the same conductors. Because Category 5 twisted pair Ethernet cables are optimized for high-speed digital signals and not power delivery, there can be significant I2R losses in the cable. The standards allow for approximately 15 percent of the power launched at the PSE to be lost in the cable before it reaches the PD. DC-DC power converters at the PSE and PD convert between the approximately 48VDC on the PoE line and the internal voltages used in the devices at each end.
Advantages of Bidirectional Power over Ethernet Systems
Notice the energy flow in standard PoE is unidirectional, always from PSE to PD. This limits the applicability of PoE, and also may require redundant PoE links in each direction if the PSE and PD ever need to exchange roles. There are several reasons why we may want bidirectional flows on those wires. One scenario involves a distributed energy system. Remote endpoints may have internal batteries, or could be associated with their own energy sources. A prime example could be a streetlight with an integrated photovoltaic array and battery. During extended dark stretches, the network supplies power over the PoE to keep the lights on and the fixture battery charged. However, when the sun is shining, and the internal battery is fully charged, the watts from the photovoltaic( PV )array on each streetlight could back-drive the PoE and help power the routers, stoplights and other loads deeper in the smart highway network, or sell surplus energy back to the grid. Another reason for bidirectional PoE is fault tolerance. In mission-critical IoT networks, routers and edge computers are often run in duplex pairs. A predominant failure mode of those devices is their local energy source or line-operated power supply. If a bidirectional PoE port interconnects two redundant routers, if the AC grid connection or power-supply module running either router fails, the bidirectional PoE can borrow some energy from its mate, transmit it in the appropriate direction over the PoE, and operate the failed node until it can be repaired. A third possible use case for bidirectional PoE could daisy-chain a linear array of IoT nodes, where power is fed into both ends, each of the two Ethernet ports on each device on a chain could either source or sink power. Finally, there are potential uses similar to USB-C ports on modern laptops, where sometimes the port drives a load and other times accepts a charger cable. Many potential IoT applications could use these bidirectional capabilities.
So, how does a single endpoint function both as a PSE and PD? The trick is to make the power converters at each end two quadrant. In traditional PoE, the power converter can only take in a specific voltage (say 380VDC right from the Power Factor Correction circuits on the AC input), and bulk convert it to the 48V (often set a bit higher, like 54V) used to drive the PoE power inserter transformers that in turn drive the cables. At the PD, again there is typically another DC-DC converter that takes the 48V from the PoE cable and creates the intermediate bus voltage (often 12V or 3.3V) used by the PD’s circuits. There is no way these one-quadrant supplies could ever reverse their energy flow. In bidirectional PoE, the power converters on both ends are converted to two quadrant operation, so when commanded, the internal switching regulator circuits can accept energy from either port, perform the appropriate voltage conversions, and deliver the energy to the opposite port. In today’s multi-output PoE devices like switches and routers, there is often one large bulk converter for all PoE ports. To convert it to bidirectional operation, there would have to be an individual dedicated two-quadrant power converter for each PoE line served because a subset of the active lines will be operating as PSEs, and the remainder as PDs.
Controlling the operation of these power converters could be tricky. The system would have to monitor the voltages and current flows on both ports of all power converters, and instruct the converters on each end of each line how to program its conversion direction, switching frequency, waveform, etc. In devices with capacity limits—like the main power supplies on central PoE routers, potentially with hundreds of ports—the control algorithms must be sure that the total energy in and out of all ports remains within those limits. The control system also has to exercise great care and precise synchronization as a PoE line switches direction, to avoid transients that, for example, could happen if both ends of a cable were programmed as high-power PSEs simultaneously.
US patent 9,531,551 includes details of how bidirectional PoE could work, the design of the two-quadrant power converters it requires, and the sophisticated control system it depends upon. Using this scheme, many advanced power distribution architectures are possible, which should enhance the versatility, efficiency and reliability of PoE-based networks.
Key Points:
- Designers can modify Power over Ethernet to transport energy in both directions.
- Many IoT applications could benefit from bidirectional PoE.
- Bidirectional PoE requires special power converter design and control algorithms.
