App note from ON Semiconductors regarding different bridge rectifier topologies and how to reduce power losses in these stages. Link here (PDF)
Power over Ethernet (PoE) is a technology which allows power sourcing equipment (PSE) to safely transfer power to a powered device (PD) through an Ethernet cable.
The majority of the power transferred to the PD normally goes to a DC−DC converter to supply loads such as a camera or an IP phone. However, not all the power delivered at the PSE output will be available to this load, due to a number of reasons. First of all, power loss occurs due to the resistance of the Ethernet cable. Additionally, the combined efficiency of the PD’s bridge rectifier and DC−DC converter is typically around 90%. It is important to keep in mind that the power is limited in a PoE system. When designing a powered device, reducing the power loss in the rectification and conversion stage means more power is available to the loads.