How Is PoE Powered

- Sep 24, 2020-

PoE power supply process description:

1. Detection: The PSE device sends out a voltage pulse of 2~10V at the port to detect whether the PD connected to the cable terminal is a standard-supported powered device. Only when the PD is detected as a standard device, will the next step be continued.

2. PD classification (Classification): Since there are many types of PDs, the required power is also different, so after the power supply device correctly detects the power receiving device, it is necessary to detect the power level of the opposite PD device.

When the PD is detected, the PSE will classify the PD device. The PSE applies a 15.5V~20.5V detection voltage (current limit is below 100mA), the PD device will connect a grading resistor in series to the line to identify its own power, and the PSE can determine which PD device belongs to by testing the size of the returned characteristic current classification.

0 is the default category

PoE type




PD classification level










PSE Power(W)










PD  Power(W)










3. Power up: When the PSE detects that a standard PD is connected to the end of the cable and has classified the PD, it starts to supply power to the PD and output a DC voltage of 44~57V.

4. Power supply: PSE provides a stable and reliable DC voltage for the PD, and outputs the corresponding level of power according to the classification result of the PD.

5. Disconnection: If the connection cable connected to the PD is unplugged or the user turns off the PoE power supply function of the switch port from the software, the PSE will quickly (usually within 30-40ms) stop being PD power supply. At any time during the entire process of PSE powering PD, if the PD device is short-circuited, the power consumed during classification exceeds the power that the PSE corresponds to, or the power consumed exceeds the rated power, the entire power supply process will be interrupted and the first The step detection process starts.

PoE power supply line sequence description

Taking the PoE standard as an example, the standard defines two power supply line sequences, and PSE can adapt to PD using any power supply line sequence.

Idle line power supply: Use idle line pairs (4/5, 7/8) that are not used in the Ethernet cable to transmit DC power.


Figure: Idle line power supply

Signal line power supply: DC power (1/2, 3/6) is simultaneously transmitted on the core wire used for data transmission, and the power transmission adopts a different frequency from the Ethernet data signal to realize the multiplexing of transmission data and transmission current.


Signal line power supply

PoE+ uses the PoE line sequence, which is consistent with PoE.

How 802.3bt achieves higher power

As the market develops, the terminal's demand for high power is increasing. At the same time, as the power load increases, the cable temperature will also increase. When the temperature of the cable is too high, the cable may exceed its rated temperature, thereby reducing performance and reliability (may cause damage to the cable). Therefore, for devices with a power supply requirement of more than 30W, you cannot use only 4-core of the 8-core twisted pair for power supply like PoE and PoE+.

The IEEE 802.3bt standard defines the use of 8-core twisted pair for voltage output to meet the market's demand for stronger and more efficient Ethernet power supply capabilities. can be realised:

(1) The highest rate is compatible with 10GBASE-T;

(2) The minimum power of 40W and the maximum of 90W can be obtained at the PD end.


Implementation of IEEE 802.3bt on PSE&PD

When the PSE end realizes 802.3bt, voltage is applied to all 8 cores of RJ45. Load 54V positive electrode on 1/2 and 4/5 wire pairs, and load 54V negative electrode on 3/6 and 7/8 wire pairs. The PD end uses two diode bridges, and both diode bridges have current flowing. The chip at the PD end is required to control the two currents and output to the DC-DC conversion circuit.

PoE technology summary

Now that PoE technology has been widely used in smart cities, IoT and other industries, it is believed that PoE technology will get greater development and breakthroughs in the near future. With the official approval of 802.3bt this year, higher power terminals will soon be used in large numbers. Application will help the rapid development of the IoT industry.

HRUI products supporting PoE technology cover all kinds of park switches. The products are widely used in various industries such as new retail, Internet, education, medical care, transportation, energy, etc. Among them, the industrial-grade series also supports high-power 802.3bt output to achieve full scenarios Covering, the power output is more stable and safe.