What is IEEE 802.3bt PoE?

The wireless network architecture is readily available, why are we still using wired connections? Well, wireless connections are convenient, but millions of miles of wired CAT5e cables are already in buildings and homes, so wired connections are still in use and wired It is more difficult for hackers to invade or intercept than wireless, and the cost of a long wired cable is also very low. Usually in many cases, you can get a good wireless signal (5G has better indoor coverage, but it will take time to fully deploy). If you want to access the Ethernet cable signal, you have to pass through the wall. Wireless networks are more susceptible to interference from other signals and radio waves, but wired cables are usually shielded and provide a "plug and play" experience, and have a better quality of service (QoS).

802.3bt system architecture

The 802.3bt system architecture uses Power Sourcing Equipment (PSE), a power controller that can power a Powered Device (PD) through an Ethernet cable. The IEEE 802.3bt standard states: "PD is a device that consumes power or requires power by participating in the PD detection algorithm; a device that can become a PD or has the function of drawing power from a backup power source and needs to be powered by a power interface (PI) It may draw power from a backup power source at the same time. "Typical PDs include IP phones, wireless access points, security cameras, and other devices that receive power from Ethernet cables. PI is the mechanical and electrical interface between PSE or PD and the transmission medium ;

The previous PoE standard used only four of the eight conductors in the Ethernet cable to transmit DC current. The IEEE working group chose to use all eight conductors for 802.3bt. Amendment 2 (Amendment 2) of IEEE Std 802.3bt-2018 states: "This amendment uses all 4 pairs of wires in structured wiring equipment to increase power transmission, thereby providing greater power to the terminal device. The amendment also reduces the standby power consumption of terminal devices and adds a mechanism to better manage the available power budget. "

The goal of the IEEE Standards Committee is to increase the power from PSE to PD. These rated power levels provided to the PD are as high as 71.3W (with 90W power from PSE), while greatly reducing the standby power consumption required by the PD during sleep.

Automatic classification function

Section 145.8.8.2 of the IEEE 802.3bt standard provides an optional extended function for physical layer classification, called Autoclass; when this function is enabled, the PSE determines the actual maximum consumption of the connected PD device power. Automatic classification only defines a single feature PD.

When the PSE performs the Autoglass function, PAutoclass is measured when POWER_ON and pd_autoclass are TRUE. Later in this article, an 802.2bt worst-case example will be mentioned, showing that the power sent to the PD does not reach the required full power, which can be corrected if the Autoglass function is enabled.

Single Feature / Double Feature

IEEE 802.3bt provides two new PD topologies, called single-signature / dual-signature. The single feature PD has the same classification, maintain power signature (MPS) and detection features between the two-wire pairs, and the dual feature PD has independent and different features between the two-wire pairs. The 802.3bt standard implements a differentiation function through a newly added Connection Check to identify differences between single-feature or dual-feature PD links.

A dual-feature PD will require two parallel PD interfaces, because in this topology two different pair sets are required; the power of each PSE is pooled after each PD interface. This is a higher-cost solution, and designers may choose a single-feature solution that is half the cost. For example, in a dual-feature security camera, one of the pairs is connected to the camera, and the other pair is connected to the heater or the pan/zoom motor.

For each data pair from the PSE, the PD usually also requires a transformer, an active bridge rectifier, an 802.3bt PD interface controller, and a DC-DC converter. Schottky diodes, resistors, and capacitors can also be understood as possible parts of PD additional components.

The maximum power that Type 4, Class 8 may consume is 71.3W. The minimum voltage of the PSE is 52V, the support channel resistance in the worst case is 6.25ohm, and the current of 1.73A will flow through the cable. The worst-case channel is 6.25 Ohms, and the load cross power is 71.3 W (Class 8). Each wire 1.73A or 0.433 is the highest-rated current that can flow in a compatible system.

Potential problems and feasible solutions on the PD side

The IEEE 802.3bt standard states: "The physical connection point of PD is specified, and the characteristics such as loss due to voltage correction circuit, power supply efficiency, the separation between internal circuit and external ground, or PI Other characteristics caused by the circuit after the connector are not specified. Unless otherwise specified, the restrictions defined for the PD are specified on the PI, not on any point within the PD. "

To create a truly durable design architecture, the following are the areas that design engineers should consider:

1. Note the current imbalance caused by other components (such as diodes, transformers, etc.) in the channel between PSE and PD (refer to Figure 3). As long as the design engineer is aware of this imbalance, he can use creative methods to alleviate this imbalance. This will depend on the design architecture. Some reliable rules are to use a good ground plane and a wide ground loop carrying large currents.

2. The current between the wires in the Ethernet cable is unbalanced. For this problem, cable suppliers rarely test or provide designers with line-to-line unbalance specifications. They usually only indicate the unbalance within the line pair.

3. Beware of cable heating. A large amount of cable heating data is usually available, but designers need to maintain temperature rise control. The IEEE working group determined that the temperature rise limit should be less than 10 degrees Celsius. They used 300mA to flow through all cable conductors, just like transmitting 51W of power for every 100 meters (m) of cable end without imbalance. Design engineers can try some solutions, such as using lower resistance cables to reduce I2R losses, using fewer cables in each harness, or only partially supplying power in the cable harness. The correct method to determine the dissipation (ie, heat generation) of any existing cable is to use a constant power heat sink as the load and a voltage source as the input power supply. Some cable heating studies will test the cable harness at 2.0A, so if you use 24AWG cable, the cable power density is 164 mW / m. Power density is the power dissipated per unit length of cable, so: 164 mW / m = ((2.0A) 2x 4.09 ohms) / 100m); the channel resistance (RCh) is based on the resistivity of 24 AWG solid copper at 20 ° C.

4. There is a nonlinear relationship between the power delivered to the PD (PD is a constant power load) and the PSE power output. The power requirements of the PD are different. If PD needs more current, it means that the voltage drop in the cable is higher and there is IR loss; the voltage obtained by PD is lower than the required voltage. It turns out that using a higher PD voltage at a lower current can stabilize this effect. For safety reasons, the PSE voltage should not exceed 57V.

PD test

If the manufacturer has a development board or reference design, be sure to use it to create your new application; these development boards are carefully crafted with proper wiring and grounding techniques to provide the best architectural performance. Usually, Gerber files can also be obtained from the manufacturer, please use them in the design, these methods will eliminate the need for extensive testing of the final design.

There are also some good solutions for the designed production test and the test in the actual system, such as Reeach Technology's PoE5 100W PoE tester, or RT-PoE5 IEEE 802.3bt Ethernet-powered PSE production tester. The University of New Hampshire InterOperability Laboratory (The University of New Hampshire InterOperability Laboratory) is an exclusive third-party testing organization that provides PoE certification. In addition, Sifos Technologies has a compact PowerSync analyzer that supports IEEE 802.3bt PoE, which facilitates 4-pair testing. The above solutions all help to ensure the ruggedness of the system.