How to optimize EMC for HMI Ethernet interfaces?

Optimizing the EMC of the HMI Ethernet interface requires attention to its differential signal pairs, power supply, and isolation transformer. First, selecting RJ45 connectors with integrated common-mode chokes and good shielding simplifies design and improves performance. The network transformer is crucial; its center tap needs to be connected to chassis ground via a capacitor, such as a 1000pF/2kV Y capacitor, to provide a discharge path for common-mode noise. A common-mode choke, such as the CMZ2012A-900T, can be connected in series on the differential lines between the transformer and the PHY chip to further suppress common-mode noise. The PHY chip's power supply must be very clean, using ferrite beads, such as the PBZ1608E102Z0T, and multi-stage decoupling with MLCCs of various capacitance values.

The analog and digital power supplies for the PHY chip should be separate and properly filtered. In terms of PCB layout, the Ethernet interface area should be relatively independent, differential pairs must be strictly controlled to 100Ω impedance, with equal-length and equidistant traces, and kept away from other signals. The ground plane of the interface circuit should be connected to the digital main ground at a single point via a ferrite bead. For applications requiring surge protection, a GDT or dedicated protection device can be added between the line and ground on the outside of the transformer. By using high-performance common-mode chokes, ferrite beads, and protection devices from Eintraelectronics, combined with rigorous PCB design, the HMI Ethernet interface can meet stringent EMC requirements and ensure long-term network communication stability.