How to suppress common-mode interference on the PoE port of an edge box?

For edge computing boxes using PoE power supply, the simultaneous transmission of data and power supply common-mode interference is particularly severe. Suppression solutions include:

1. Using a dedicated PoE common-mode transformer to isolate power and data and suppress common-mode current.

2. Connecting a common-mode inductor CMZ2012A-900T in series between the PHY and RJ45. Its name is CM common-mode, Z impedance is 2012, size is 2.0×1.2mm, 900T impedance is 90Ω, and it can effectively filter MHz-level common-mode noise at 100MHz.

3. Adding X and Y capacitors to the PoE power supply DC-DC section. A recommended common-mode inductor is CML4532A-101T connected in series in the 48V input path, and strengthening the Y capacitor to PE with 1nF 2kV.

4. Ensuring the center tap of the network transformer is connected to signal ground through a 1kV capacitor instead of being directly grounded.

Testing shows that this comprehensive solution reduces radiated emissions by 15dB and power ripple <50mV for PoE edge boxes using 100BASE-TX transmission, meeting the IEEE 802.3at standard.