In order to meet the increasing demand for network communication, PoE power supply as a higher speed, higher performance and more flexible power supply equipment is widely deployed on a large scale.
The high reliability of the standard PoE power supply is mainly reflected in the power supply principle and process.
After PSE and PD devices are connected by super five network cables, the specific method is to determine whether there is a legitimate PD device by detecting the resistance capacitance value between the power output line pairs. PSE equipment will output two voltages with a voltage difference >1V and both within the range of 2.8V~10V (interval time >2ms) through the power supply line, calculate the slope through the V-I curve, and obtain the common mode impedance of PD equipment.
In order to meet the PSE detection requirements, the 802.3af standard requires that the PD detection state input voltage range is between 2.7 and 10.1V, the design input impedance is between 23.75 and 26.25KΩ, and the input capacitance is between 0.02 and 0.12uF
Since there are many types of power receiving devices, the power required 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 peer PD device.
PSE applies a detection voltage of 15.5 to 20.5V (current is limited to less than 100mA), and the PD device will place a stage resistor in series into the line to identify its own power. PSE determines which grade the PD device belongs to by testing the size of the returned characteristic current. The classification process time is limited to between 10 and 75ms.
According to the 802.3af design specification, it is required that the input voltage range of PD start classification is 14.5~20.5V, the stability time is less than 5ms, and the classification characteristic current absorbed by PD has a 1mA margin.
After the detection and grading is complete, the PSE device begins to supply power from the low voltage to the PD device until 48V DC power is provided during a startup period with a configurable time (generally less than 15us). During power supply, PSE also monitors the power supply of each port to provide undervoltage and overcurrent protection.
PD sends a continuous working signal by maintaining power characteristics. The PD should be maintained at least 10mA and the AC impedance should be maintained at <26.25KΩ and the capacitor should be in parallel with more than 0.05uF to avoid PD drop.
When the PD device is disconnected from the network, the PSE will quickly (generally within 300 to 400ms) stop the power supply, and repeat the detection process to detect whether the terminal of the cable is connected to the PD device.
The IEEE802.3af/at standard specifies two methods for PSE to detect whether PD is disconnected, namely DC short circuit detection method and AC open circuit detection method.
Determine whether PD is online according to the size of the DC current flowing from PSE to PD. When the current remains below the threshold Imin(5~10mA) state duration >Tmpd(300~400ms), PSE considers that PD does not exist, thus cutting off the power supply. The disadvantage of this method is that when the PD is working in low power mode, the PD must periodically absorb a certain amount of current to avoid dropping the line.
