PoE Power Supply Standards

PoE (Power over Ethernet) refers to "Ethernet power supply," a technology that enables the delivery of electrical power to network devices via Ethernet cables. It allows for the simultaneous transmission of both electrical power and data signals, eliminating the need for separate power cables for devices. PoE effectively provides centralized power supply to terminals such as IP phones, wireless APs, portable device chargers, card readers, cameras, and data acquisition systems. It offers the advantages of high reliability, simplified connectivity, and standardized compatibility.

Based on whether PoE devices comply with the three PoE standards issued by the IEEE Standards Committee, PoE devices can be classified into standard PoE and non-standard PoE.

Standard PoE: Refers to PoE devices that comply with the three PoE standards issued by the IEEE Standards Committee (IEEE 802.3af standard, IEEE 802.3at standard, and IEEE 802.3bt standard).

Non-standard PoE: Refers to PoE devices that do not comply with the three PoE standards issued by the IEEE Standards Committee. These are considered non-standard PoE devices.

PoE Power Supply Standards and Technical Parameters

Power Supply Technology	PoE	PoE+	PoE++	PoE+++
Power Supply Standard	IEEE 802.3af	IEEE 802.3at	IEEE 802.3bt	IEEE 802.3bt
Power Supply Distance	100m	100m	100m	100m
Classification	0~3	0~4	0~6	0~8
Maximum Current	350mA	600mA	1200mA	1730mA
PSE Output Voltage	44V DC~57V DC	50V DC~57V DC	50V DC~57V DC	52V DC~57V DC
PSE Maximum Output Power	≤15.4W	≤30W	≤60W	≤90W
PD Input Voltage	36V DC~57V DC	42.5V DC~57V DC	42.5V DC~57V DC	42.5V DC~57V DC
PD Maximum Power	12.95W	25.5W	51W	71.3W
Maximum Transmission Line Loss	2.45W	4.5W	9W	18.7W
Cable Requirements	No special requirements	CAT-5e or above network cable	CAT-5e or above network cable	CAT-5e or above network cable
Power Cable Pairs	2	2	4	4

Components of the PoE Power Supply System

PoE Power Supply: The PoE power supply provides power to the entire PoE system. The number of PDs connected to the PSE is limited by the power capacity of the PoE power supply. Based on whether it is pluggable, PoE power supplies are classified into two types: built-in and external.
Power Sourcing Equipment (PSE): This refers to PoE devices that supply power to PDs via Ethernet, providing functions such as detection, analysis, and intelligent power management. Depending on compliance with IEEE standards, PSEs are categorized into standard PSEs and non-standard PSEs (e.g., standard PoE switches and non-standard PoE switches).
Powered Device (PD): This refers to IP terminal equipment that receives power from PSEs (e.g., IP phones, wireless APs, network cameras, and other powered devices). Based on compliance with IEEE standards, PDs are classified into standard PDs and non-standard PDs.

PoE Power Supply Principle

According to the IEEE 802.3af, IEEE 802.3at, and IEEE 802.3bt standards, within a specific timeframe, the PSE must complete the detection and classification of the connected network terminal device, then decide whether to supply power to it and how much power to deliver. This regulation ensures that incompatible network devices are not damaged by the 48V power supply. Therefore, the primary functions of the PSE include detecting whether a compatible device (PD) is connected to or disconnected from the system, classifying the power requirements of the PD, and accordingly supplying the appropriate power or cutting off the power supply.

PoE Power Supply Process

State	Function	Remarks
Detection	The PSE device detects the presence of a PD device. The PSE periodically outputs a low current-limited voltage on the port to detect the resistance and capacitance values of the PD device, thereby determining whether the PD is present. The typical voltage range for this detection is 2.7V to 10.1V, and the detection cycle is 2 seconds.	The PSE will only proceed to the next step if a valid PD is detected. The criteria for detecting a PD's presence are: a resistance value between 19kΩ and 26.5kΩ, and a capacitance value not exceeding 150nF.
Classification	PSE Determines PD Power Consumption. After the PSE detects a valid PD, it will supply a voltage between 15V and 20V to the PD and determine the PD's power class by measuring the current draw. Different power classes correspond to different power levels.	Power level classification and power grading table are shown below.
Power up	After the PSE determines the power class of the PD, it begins to supply power to the device according to the corresponding power level.	The PSE equipment starts supplying power to the PD equipment from a low voltage, increasing it up to 48V DC. This process occurs within a startup period (typically less than 15μs).
Real-time Transport Protocol (RTP) &Power Management (PM)	After the voltage reaches 48V, the PSE provides stable and reliable 48V DC power to the PD device, ensuring that the PD's power consumption does not exceed the PSE's maximum output power.	During this stage, the PSE continuously monitors for PD disconnection while supplying power. Output voltage: -48V.
Disconnection	During the power supply process, the PSE continuously monitors the input current to the PD. If the PD's current consumption drops below a minimum threshold or surges abruptly-such as when the device is unplugged, or in cases of PD power overload, short circuit, or exceeding the PSE's power supply capacity-the PSE will cut off the power and restart the detection process.	If the PD is disconnected, the PSE will quickly shut off the port's output voltage (typically within 300-400ms). The port status will then return to the Detection state.