Introduction and Application of PoE PD Function

What is PoE?

PoE (Power over Ethernet) power supply technology is a technology that uses a conventional Ethernet cable to transmit data and supply power without any modifications to the existing Ethernet cabling infrastructure. It ensures that the cable can provide DC power to Ethernet devices such as IP phones, wireless APs, and network cameras while transmitting data signals.

The PoE system consists of two parts, power supply equipment (PSE) and power receiving equipment (PD). The PoE power supply process requires successful communication and handshake between the PD and PSE before the PSE can supply 48V DC power to the equipment.

Before PSE supplies power to PD, it is necessary to first check for the presence of powered devices to avoid damaging non-standard PD devices with 48V power supply. PSE will use a detection voltage of 2.7V~10.1V to detect the characteristic impedance between the power output lines to determine whether the powered device is a standard PD device. The IEEE802.3 standard defines the characteristics of PD as follows:

a. The DC impedance is between 19K and 26.5Kohm;

b. The capacitance value shall not exceed 150nF.

2 Classification

Once it is detected as a standard PD device, PSE needs to determine the power consumption of the PD device in order for the system to manage the power supply. PSE provides a detection voltage of 14.5V to 20.5V to detect the power level of PD devices. PD indicates its maximum power requirement to PSE by pulling a constant current (graded characteristic signal) from the bus. PSE measures this current to determine which power level PD belongs to. The PSE current used during the grading period must be limited to 100mA to avoid damaging the PD, and its connection time cannot exceed 75ms to control the power consumption of the PD.

After successful detection and grading, PSE will officially supply power to PD.

What is the PoE power supply standard?

IEEE established the IEEE802.3af PoE power standard in 2003, which specifies that the maximum output power of PSE is 15.4W and the maximum power of PD is 12.95W. But with the diversification of PD device functions, a power supply of 13W can no longer meet the demand. In 2009, the IEEE committee formulated the IEEE802.3at standard, which expanded the output power supply capability of PSE and PD to about twice.

However, in the past two years, with the advent of the intelligent era and the popularity of AI technology, more and more devices that require PoE power supply have exceeded 25W. The demand for higher power Po power supply from IEEE802.3bt has been proposed, and we are also waiting for the official release of the new IEEE802.3bt standard.