Since its first release in 2003, the IEEE PoE (Power over Ethernet) standard has evolved from 802.3af to 802.3bt in September 2018 and the power delivery capability has been increased from about 15.4W at 350mA to 90W-100W at 600mA-960mA. As the delivered power increases, PoE has found more applications heading into the future, such as IoT (Internet of Things), POS (Point of Sale) terminals, smart LED lighting, IEEE 802.11ax access point and PZT (pan-tilt-zoom) security cameras.
The new PoE standard drastically increases the maximum power level at the PD (Powered Device) and opens the door for new applications. IEEE 802.3bt uses all four pairs of the ethernet cable wires with the current flow spread out among the pairs. The power is delivered carrying along with the data up to 10GBASE-T. As the power increases and the data transmission rate maintains high, the requirements for the quality of cable and the electronic design of PD become more important for delivering the high power and reducing data errors. Cat5e structured cable is the minimum for reliable applications, while Cat6e cable dissipates much less power than Cat5e so more power can be delivered to the PD. With IEEE 802.3bt in effect, a compatible PSE (Power Sourcing Equipment) can differentiate devices that support PoE and are requesting power from devices that are either not PoE compatible or not requesting power by continuously sensing at the interface if a valid or a non-valid detection signature is present. The new IEEE 802.3bt standard also support dynamic power allocation using the multi-event classification process for mutual identification between the PSE and PDs that are connected by checking the pair sets for signature connections. When there are two signature PDs that are requesting power from the PSE, the system can have two independent rails using a single cable and allows to monitor them independently for more power saving by shutting down one rail while maintaining the other rail at full power. IEEE 802.3bt defines two more PD and PSE types, Type 3 identifying a PSE that can deliver 60W and Type 4 identifying a PSE that can deliver up to 90W using all pairs in an Ethernet cable.
In applications that requires many PDs, we may have a noticeable power loss in standby state, as the IEEE 802.3af/at standards requires minimum 10 mA and maximum 30% duty cycle to keep the connected port alive, which is called Maintain Power Signature (MPS). Considering a LED lighting system, we expect to have as much as 200mW of power loss per port and more than 2W for only 10 ports. With the new IEEE 802.3bt (that removes the AC MPS requirement and allow class 5-9 PDs to draw 16 mA), the duty cycle can be lower than 2% (minimum 7 ms on/310 ms off duty cycle), which cuts the loss per port by more than 15 times.
With the new PoE standard, it imposes challenges for new designs. Since the change of the standard includes higher current flow, we need increase the current limit not only on the PD side but also on the PSE side. We must increase the absolute ratings of the components, such as protection diodes, load switching transistors, and current sensing resistors. On the data transmission part, the isolation transformer must be sized to allow the worst-case current flow without affecting the data transmission.
Applications
TI TPS2373 is a PSE PD Interface IC for high-power PoE applications. The device provides all the features for designing a compatible IEEE 802.3bt Type 1-4 PD (Powered Device). With a low internal switching resistance, TPS2373 is capable of high-power applications up to 90W that translates into 71.3W at the PD over a 100m Cat5e cable. The following waveform shows the PoE startup sequence for TI TPS2373 high power PoE PD interface IC. The startup sequence includes detection, classification, and startup from a PSE with Type 3 Class 6 hardware classification. According to IEEE 802.3bt, a PSE must allocate Class 6 level of power to generate a minimum of two detection levels, four class and mark cycles, and startup from the fourth mark event.
The following schematic is a typical application of TPS2373 for Class 8 PoE PD design.
The design parameters for implementing the above application.
Read more: http://www.ti.com/product/TPS2373
