Weak current people must understand standard PoE power supply! Otherwise, if you don't understand the IPC power supply power and line sequence, how can you do a good project?

What is Standard PoE?

PoE, or Power over Ethernet, is a technology that transmits data and power simultaneously on Ethernet cables. After applying this technology, network terminal devices only need to connect to the network cable to meet power supply needs. Standard PoE is divided into three categories according to power supply standards: IEEE802.3af, IEEE802.3at and IEEE802.3bt.

Why do we need PoE?

• Low cost: PoE power supply eliminates the need for DC power supply boxes, PVC pipes, and power cords, saving costs and being simpler and safer.
• Reliability: One PoE device can provide power to multiple terminal devices at the same time, realizing centralized power supply and power backup.
• Simple connection: The terminal device does not require an external power supply, only an Internet cable.
• Unified standards: Comply with international standards and use the globally unified RJ45 power interface to ensure docking with equipment from different manufacturers.

What are the standard PoE device types?

As shown in the figure below, the PoE power supply system includes the following two device roles:

• Power-sourcing equipment (PSE) is a PoE device that provides power to powered devices through Ethernet and provides functions such as detection, analysis, and intelligent power management, such as PoE switches.
• Powered Device (PD): such as wireless AP, IP phone, card reader, camera and other powered devices. According to whether they comply with IEEE standards, PD is divided into standard PD and non-standard PD.

Standard PoE power supply mode

According to the definition of IEEE standards, PSE devices are divided into two types: MidSpan (PoE function module is outside the device) and Endpoint (PoE function module is integrated into the device).

Endpoint PSE devices are divided into two power supply modes: Alternative A (1/2 and 3/6 line pairs) and Alternative B (4/5 and 7/8 line pairs) according to the power supply line pairs used.

• Alternative A power supply mode uses data pairs for power supply. PSE supplies power to PD through 1/2 and 3/6 wire pairs, with 1/2 link forming the negative pole and 3/6 link forming the positive pole. 10BASE-T and 100BASE-TX interfaces use 1/2 and 3/6 wire pairs to transmit data, and 1000BASE-T interfaces use all 4 wire pairs to transmit data. Since DC power and data frequencies do not interfere with each other, current and data can be transmitted simultaneously on the same wire pair.

• Alternative B power supply mode uses the idle pair to supply power. The PSE supplies power to the PD through the 4/5 and 7/8 wire pairs, with the 4/5 link forming the positive pole and the 7/8 link forming the negative pole.

Standard PoE negotiation process

After the PSE is powered on and the PD is connected to the PSE through the network, the PSE and PD begin power supply negotiation:

• Detect PD: PSE periodically outputs a small voltage with limited current at the port to detect the presence of PD devices. If a resistor with a specific resistance value is detected, it means that the cable terminal is connected to a powered device that supports the IEEE 802.3af standard or the IEEE 802.3at standard (a specific resistor with a resistance value of 19kΩ to 26.5kΩ, a normal small voltage of 2.7V to 10.1V, and a detection period of 2 seconds).
• Power supply capability negotiation is the PD device classification process: PSE classifies PDs and negotiates the power supply. Power supply capability negotiation can be achieved not only by parsing the resistance sent by PSE and PD, but also by discovering and announcing the power supply capability through the Link Layer Discovery Protocol (LLDP).

• Start power supply: During the startup period (generally less than 15μs), the PSE device starts to supply power to the PD device from a low voltage until a DC voltage of 48V is provided.
• Normal power supply: After the voltage reaches 48 V, the PSE provides a stable and reliable 48 V DC power to the PD device, and the power consumption of the PD device does not exceed the maximum output power of the PSE.
• Power off: During the power supply process, the PSE will continuously monitor the PD current input. When the PD current consumption drops below the minimum value, or the current surges, such as when the device is unplugged or the PD power consumption is overloaded, short-circuited, or exceeds the PSE power supply load, the PSE will disconnect the power supply and repeat the detection process.

Standard PoE power supply parameter standard

In order of time, IEEE has released PoE standards such as 802.3af (PoE), 802.3at (PoE+) and 802.3bt (PoE++). The later released standards are compatible with the earlier released standards, but the earlier released standards are not compatible with the later released standards.

• The maximum power supported by PoE based on the 802.3af standard is 15.4W
• The maximum power supported by PoE+ based on the 802.3at standard is 30W
• The maximum power supported by PoE++ based on the 802.3bt standard is 90W

Standard PoE power classification

According to the required power, PoE devices are divided into different classes (Class 0 to Class 8, a total of 9 classes):

• The 802.3af standard covers Class 0 to Class 3, with a voltage range of 36 V to 57 V and an output power of 15.4 W. This type of equipment is recommended for sensors and simple cameras.
• The 802.3at standard (also known as PoE+) introduced Class 4, which increased the output power to 30W within the same voltage range, but is only compatible with PoE+ PSE. This type of equipment can be used in complex cameras, LCD displays, and tablets.
• The 802.3bt standard (also known as PoE++) introduces Class 5 to Class 8, with output powers ranging from 45W to 90W. Such devices can support laptops, TVs, and electrical systems in buildings. The following figure summarizes these power classes and their associated input voltage, input power, and output voltage.