Radiation interference from switching power supplies (SPS) — electromagnetic energy radiated as radio frequency (RF) noise — can disrupt nearby sensitive electronics (e.g., medical devices, communication systems). Effective control measures combine design optimization, component selection, and shielding to minimize radiation and meet EMC standards like CISPR 22 Class B and FCC Part 15.

First, shielding is a foundational measure. The SPS enclosure should use conductive materials (e.g., aluminum alloy, galvanized steel) with a minimum thickness of 0.2 mm to block RF radiation. Seams and openings (e.g., for vents, connectors) must be properly sealed using conductive gaskets (e.g., nickel-coated foam, beryllium copper springs) to prevent leakage, as even small gaps (≥λ/20, where λ is the wavelength of the noise frequency) can significantly increase radiation. For internal components like transformers and inductors (major radiation sources), ferrite shields or copper tape can be applied to contain magnetic fields.

Second, optimizing the PCB layout reduces radiation at the source. This includes minimizing loop areas in power circuits (e.g., between switching transistors, diodes, and capacitors), as larger loops act as antennas for radiating noise. Placing high-speed switching components (e.g., MOSFETs) close to the input/output filters shortens current paths, reducing loop size. Additionally, separating analog and digital circuits on the PCB and using a solid ground plane (with no breaks) provides a low-impedance return path for high-frequency currents, preventing radiation from scattered currents.

Third, selecting low-radiation components is key. Using shielded inductors and transformers (with ferrite cores and copper shielding) reduces magnetic field radiation. For capacitors, low-ESR (equivalent series resistance) and low-ESL (equivalent series inductance) types (e.g., ceramic capacitors with X7R dielectric) minimize voltage spikes and current ripples that contribute to radiation. Additionally, implementing soft-switching topologies (e.g., LLC resonant converters) reduces the rate of voltage/current change (di/dt, dv/dt) during switching, a primary cause of high-frequency radiation.

Finally, adding external filters and damping circuits complements internal measures. A chassis-mounted EMI filter at the SPS input reduces radiation coupled to power lines, while RC snubber circuits across switching components dampen voltage spikes. Regular pre-compliance testing with a near-field probe and spectrum analyzer helps identify radiation hotspots, allowing targeted adjustments to meet standards.

Read recommendations:

5V2A Wall Plug UK Standard

24V low frequency transformer

6V3A 180W C14 Switching Power Supply

12V UK Standard Switching Power Supply

Dual-Port Car Charger