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) is an alternative and data transmission and power supply can be realized via one network cable. The RD022 reference design presents a GB Ethernet USB adapter with integrated PoE function from Würth Elektronik. The AppNote ANP122 highlights the EMC behavior of this reference design in detail and provides design tips.

The realized "GB-PoE+ Ethernet USB adapter" offers three interfaces, a USB Type-C (USB 3.1), an RJ45 1 Gbit/s Ethernet interface with integrated PoE function, and a connector for a DC/DC converter with an adjustable output voltage of 6 to 18 V and a maximum output power of 25 W. The board is designed to familiarize users with PoE technology.

The ANP122 explains the emission measurements of WE's reference design and compares them to common residential limits. If the later used 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.

Contents:

• EMC behavior of the Gigabit Ethernet interface.
• Basic EMC considerations of Power-over-Ethernet: EMC test setup, operating parameters, influence of output voltage, output filter, influence of different load resistors, input filter, optimization of conducted emissions, noise immunity.
• EMC performance of reference design: conducted emission (CISPR 32), radiated emission (CISPR 32), radiated immunity (IEC 61000-4-3) / against burst (IEC 61000-4-4) / against surge (IEC 61000-4-5) / against continuous conducted disturbances (IEC 61000-4-6)
• EMC improvements: Island concept

Download: ANP122