Common-mode (CM) interference in switching power supplies arises from unequal voltage potentials between the power supply’s ground and the system ground, caused by parasitic capacitances (e.g., between switching transistors and heat sinks, or input/output cables and chassis) and fast switching transients. To mitigate CM interference, a combination of hardware design, component selection, and layout optimization is required.

First, integrating common-mode chokes (CMCs) is the most effective solution. CMCs consist of two windings wound on a single magnetic core, carrying equal and opposite currents (from the power supply’s input or output). These currents generate opposing magnetic fields in the core, canceling out for differential-mode (DM) signals but creating a high impedance for CM signals, attenuating CM noise by 20–40 dB at frequencies between 10 kHz and 100 MHz. Selecting CMCs with high saturation current (matching the SPS’s maximum load current) and appropriate inductance (10–1000 μH) ensures effectiveness without compromising normal operation.

Second, using X and Y capacitors (compliant with safety standards like IEC 60384-14) suppresses CM noise. Y capacitors (connected between live/neutral lines and ground) shunt CM current to ground, while X capacitors (connected between live and neutral lines) reduce DM noise that can couple into CM paths. It is critical to select Y capacitors with low capacitance (typically 10–1000 pF) to limit leakage current (≤0.75 mA for Class II equipment) and avoid electric shock hazards.

Third, optimizing PCB layout and grounding minimizes parasitic capacitances. This includes separating analog and digital ground planes to prevent noise coupling, placing CM filters close to the SPS’s input/output ports to block noise at the source, and using short, wide traces for power paths to reduce parasitic inductance. Additionally, using a star grounding topology (connecting all ground points to a single reference) ensures uniform ground potential, reducing CM voltage differences.

Finally, shielding the SPS’s enclosure and cables reduces external CM noise coupling. Metal enclosures with proper grounding act as Faraday cages, blocking external EMI, while shielded cables (e.g., twisted-pair with foil shielding) prevent CM noise from coupling into input/output lines. Combining these solutions ensures compliance with EMC standards like CISPR 22 and FCC Part 15.

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