Proper PoE-PD Rectifier Bridge Circuits design.

By On-Semiconductor

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 maximum amount of power that can be transferred from the PSE to the PD is defined by the different IEEE 802.3 Power over Ethernet standards, of which there are currently three. PSE compliant with the 802.3af standard can deliver up to 15.4 W, PSE compliant with the 802.3at standard can deliver up to 30 W, and PSE compliant with the latest 802.3bt standard can support up to 90 W.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.

Different rectifier topologies have been studied, in this application note, to assess how to reduce the power losses in these stages: the diode bridge topology, the active topology (GreenBridge 2) and a diode + MOSFET topology. Measurements have been made to evaluate the power losses for each topology.

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