Understanding a PoE interface from an EMC perspective

Würth Elektronik has published another Application Note on a special challenge in EMC. App Note ANP122 'Understanding a PoE Interface from an EMC Perspective' clarifies which EMI must be considered for an Ethernet interface also used as a power supply. The application example is the RD022 reference design, in which the company has developed an industrial GB Ethernet USB adapter with integrated PoE functionality.

Devices with low power consumption (below 100W) networked via Ethernet can be supplied via PoE. Data transmission and power supply are realised with a network cable. With the reference design, Würth Elektronik presents a GB Ethernet USB adapter with integrated PoE functionality up to 25W. App Note ANP122, now available, takes an in-depth look at its electromagnetic behaviour and provides design tips.

The Application Note describes the emission measurements from the reference design and compares them with the standard limit values. If the subsequently used device with PoE interface takes on larger dimensions or the secondary side is earthed, additional filtering is required for the conducted emission. It can be seen that both conducted and radiated emissions are dominated by the isolated converter, while the emission from the digital circuit is negligibly small.

The App Note offers an island concept to decrease emissions from the converter, whereby the interference from the isolated switching regulator is controlled better.

Besides describing the reference design, the paper also summarises the underlying EMC considerations of PoE: the EMC test setup, operating parameters, the influence of output voltage, output filter, the influence of different load resistors, input filters, optimisation of conducted emission and interference immunity.