Gigabit PoE Interface from an EMC Perspective
Usually, devices networked via Ethernet rely on a separate power supply. However, if these end devices can manage with low power consumption, "Power-over-Ethernet" (PoE) can be an alternative and data transmission and power supply can be achieved using one network cable. The RD022 reference design presents a GB Ethernet USB adapter with integrated PoE function from Würth Elektronik. This new AppNote, ANP122, highlights the EMC behavior of this reference design in detail and provides design tips.
The "GB-PoE+ Ethernet USB adapter" offers three interfaces:
The board is designed to familiarize users with PoE technology.
In this article, we'll discuss emission measurements of WE's reference design and compare them to common residential limits. If the device with PoE interface takes larger dimensions, or the secondary side is grounded, additional filtering is needed for the conducted emission. The interference emission is both conducted and radiated, dominated by the isolated converter and the emission of the digital circuit part is insignificantly low. To reduce the emissions from the converter, the islanding concept presented in the AppNote can be used to better control the interference from the isolated switching regulator.