Electrical isolation requirements in power-over-Ethernet (PoE) power sourcing equipment (PSE)

Power-over-Ethernet (PoE) is becoming a popular way to deliver power to all types of end-point networking and peripheral equipment. Examples of equipment receiving power through Ethernet cable include wireless access points, surveillance cameras and voice-over-Internet (VOIP) telephones. There has been a lot written about the intricacies of the designing the 48 V power control circuits in the power sourcing equipment (PSE) and in the powered devices (PD). However, little has been said about how properly power both the 48 V sent from the PSE to the PD and the internal circuits of the PSE. The PoE standard, IEEE 802.af3, requires a high degree of electrical isolation between anything attached to the Ethernet cable and any circuitry sending and receiving transmissions over that cable. This paper starts by exploring the electrical isolation requirements of the PoE standard. It explains them in straightforward terms familiar to power supply and power system engineers. The paper then describes several power architectures for the equipment in the PSE that meet the PoE requirements. The conclusion is that there in no one right way to power PSE equipment and each equipment designer must understand the requirements and tradeoffs of their equipment in order to make the best choice.