PoE Power Supply Process

If a PSE is installed in a network, the PoE Ethernet power supply process is as follows:

Figure 12-3 PSE Power Supply Process

• Detection: After a network device is connected to a PSE, the PSE first detects whether the device is a PD to ensure that the current is not supplied to non-PDs because supplying power to a device that is not a PD may damage the device. The PSE detects the resistance capacitance between the power output wire pairs to determine whether PDs exist. The PSE proceeds to the next step only after it detects PDs.
• Classification: After detecting PDs, the PSE classifies the PDs. It determines power grade of PDs by detecting power output current. During the power supply process, classification is optional.
• Power Up: Within a startup period which is configurable (usually less than 15 us), the PSE starts to provides low power voltage to PDs and gradually increases the power voltage to 48 V DC.
• Power Management: The PSE provides stable and reliable 48 V DC power for PDs. Once the PSE starts to supply power, it continuously detects PD current inputs. If the current consumption of a PD drops under the minimum value owing to various causes, such as the PD is disconnected, the PD encounters power consumption overload or short circuit, and the power load exceeds the PSE power supply load, the PSE regards the PD as not in position or abnormal. In this case, the PSE stops providing power to the PD.
• Disconnection: The PSE detects the current of PDs to determine whether PDs are disconnected. If a PD is disconnected, the PSE stop supplying power to the PD quickly (usually within 300 to 400 ms), and then the PSE returns to the Detection status.