The Power Sourcing Equipment(PSE) is a component of the PoE architecture that determines whether a PoE-compatible device, a Powered Device( PD), is connected and needs to be supplied with power. In doing so, the current flow is used to determine the internal resistance of the powered device.
Compatibility between a power sourcing equipment and a Powered Device is determined using the Resistive Power Discovery procedure. In this procedure, the PSE unit uses a measurement circuit to check the resistance and capacitance of the connected Powered Device. Only when the devices are compatible does the PSE supply power to the PD Device over the existing data line.
The output power of the PSE is divided into power classes. In IEEE 802.3af, the power sourcing equipment (PSE) output power is 4 W and 7 W. The 802.3atworking group has increased the PSE power to 15.4 W and 30 W. And 802.3bt brings it to supply powers of 45 W, 60 W, 75 W and 90 W. As for the supply voltages, they are 44 V for 802.3af, 50 V for 802.3at, and 50 V and 52 V for 802.3bt. The supply path is constantly monitored by the power sourcing equipment. A PSE unit is equipped with one or more RJ45 jacks that provide power and connectivity to ports in Integrated Connector Modules( ICM). PSE units can be integrated into a switch or intermediate power hubs can be installed. If the powered device is supplied directly by the PSE unit, this is the endspan concept; if it is supplied via intermediate power hubs, it is a midspan concept.
The supply voltages and the associated current flows are designed so that they do not result in too high a voltage drop at the plug contacts. The voltages are converted into the corresponding operating voltages in the Powered Devices (PD) via DC/DC converters. The end devices can be up to 100 m away from the PSE unit. PoE+ and PoE++ are available for higher powers of up to 90 W.