How to eliminate common-mode interference in HMI (Human-Machine Interface) from the source?

Eliminating common-mode interference in HMIs requires addressing its source by controlling its generation, conduction, and radiation. First, identify the main common-mode noise sources, such as switching nodes in the power supply, high-frequency clock signals, and unbalanced high-speed data lines. For switching power supplies, adding a shielded winding between the primary and secondary sides of the transformer or using a shielded transformer can effectively reduce common-mode noise coupled through parasitic capacitance. A common-mode inductor, such as the CMZ3225A-101T, must be used at the power input; its high common-mode impedance effectively blocks the conduction path of common-mode noise. All cables entering and exiting the chassis are radiating antennas for common-mode noise; therefore, filter connectors or filter circuits should be installed at cable ports, such as using RJ45 sockets with integrated filters at network ports.

For internal circuitry, minimizing the high-frequency signal loop area is crucial, employing a complete ground plane and providing a low-impedance common-mode noise return path. Locally shielding sensitive or noise-generating circuits and ensuring proper grounding of the shield can prevent the spatial propagation of common-mode noise. Simultaneously, ensuring proper grounding of the system chassis provides a discharge path for common-mode current. By employing the CMZ series high-performance common-mode inductors from Eternity Electronics, suppression can be applied to specific frequency bands, significantly reducing the intensity of common-mode interference along the conduction path. Combined with good layout and shielding, this can achieve source elimination.