Switch Mode Power Supplies: Their Frequencies, EMC Compliance Considerations, and an EMC Mitigation Checklist

 Switch Mode Power Supplies (SMPS) operate by rapidly switching electronic components on and off to regulate voltage and current efficiently. The switching frequency is a key design parameter, and it directly influences the electromagnetic emissions, size of passive components, and power efficiency.

Typical Switching Frequency Ranges of SMPS

  1. Low-Frequency SMPS

    • Frequency: 20 kHz – 100 kHz.

    • Applications: Older designs, high-power industrial supplies, and unfiltered consumer devices.

    • Notes: Easier to design but requires larger inductors and capacitors; more audible noise risk (e.g., transformer whine near 20–25 kHz).

  2. Mid-Frequency SMPS

    • Frequency: 100 kHz – 500 kHz.

    • Applications: General-purpose power adapters, desktop power supplies, embedded systems.

    • Notes: Common in modern devices due to the tradeoff between size and efficiency; easier EMI compliance than low-frequency systems.

  3. High-Frequency SMPS

    • Frequency: 500 kHz – 2 MHz.

    • Applications: Laptop adapters, PoE (Power over Ethernet) injectors, compact DC-DC converters.

    • Notes: Enables tiny transformers and fast transient response; requires advanced EMI suppression.

  4. Very High-Frequency SMPS

    • Frequency: 2 MHz – 20 MHz+

    • Applications: RF front ends, space- and weight-critical applications, GaN-based and SiC-based converters.

    • Notes: EMI is a significant challenge in cutting-edge telecom, aerospace, and some LED drivers.

Examples by Application

  • ATX PC Power Supply: 50 – 150 kHz.

  • Laptop Charger: 100 – 500 kHz.

  • Telecom DC-DC Module: 300 kHz – 1.2 MHz.

  • LED Driver (High Quality): 40 kHz – 1 MHz.

  • GaN USB-C PD Adapter: 1 MHz – 2.4 MHz.

  • Class-D Amplifier Power Supply: ~400 kHz.

  • PoE Injector/PD: 150 kHz – 600 kHz.

Engineering Considerations

  • Lower frequencies: Larger magnetic components, lower EMI frequency content (easier shielding), but more audible noise.

  • Higher frequencies: Smaller magnetics, faster transient response, higher efficiency potential, but more difficult EMC compliance.

SMPS Frequency Ranges and EMI/Compliance Considerations

  • 20–50 kHz (Low-Frequency SMPS)

    • Typical Use: Industrial supplies, legacy designs, large equipment.

    • EMI Concern: Emissions fall in the low-frequency conducted EMI band.

    • Compliance Impact: More likely to exceed CISPR 11/22 Class A conducted limits without filtering.

    • Other Note: May produce audible transformer noise.

  • 50–150 kHz (Standard Consumer/Desktop Supplies)

    • Typical Use: ATX power supplies, low-cost chargers.

    • EMI Concern: High conducted EMI in the 150 kHz–30 MHz range.

    • Compliance Impact: Requires input filtering to meet FCC Part 15 Class B or IEC 61000-6-3.

    • Other Note: Large common-mode chokes or Pi filters are common mitigation.

  • 150–500 kHz (Modern Mid-Frequency Designs)

    • Typical Use: LED drivers, laptop adapters, embedded DC-DC converters.

    • EMI Concern: Emissions shift into the mid-to-upper conducted range, near 30 MHz radiated transition.

    • Compliance Impact: Balancing act—requires careful PCB layout and shielding to pass CISPR 32 Class B.

    • Other Note: Often employs spread-spectrum switching for EMI reduction

  • 500 kHz–2 MHz (High-Frequency Switching)

    • Typical Use: Telecom DC-DC modules, PoE, USB-C PD chargers.

    • EMI Concern: Emissions enter the radiated EMI region (30–300 MHz).

    • Compliance Impact: Requires careful enclosure design, ferrites, and shielding.

    • Other Note: Smaller inductors and capacitors reduce product size.

  • 2–20 MHz+ (Very High-Frequency SMPS)

    • Typical Use: RF front ends, aerospace, GaN/SiC fast chargers.

    • EMI Concern: Strong radiated EMI, broadband emissions into the VHF spectrum.

    • Compliance Impact: Difficulty meeting FCC, CISPR 25/32, often requires multi-stage filtering.

    • Other Note: High-efficiency, small form factor, but very demanding EMC design.

EMC Mitigation Checklist by Switching Frequency Range

20–50 kHz (Low-Frequency SMPS)

  • Use common-mode and differential-mode filters on the AC/DC input.

  • Add large-value X and Y capacitors for low-frequency attenuation.

  • Ensure proper transformer shielding and grounding.

  • Avoid audible range: confirm switching frequency > 20 kHz.

  • Check compliance with CISPR 11 or FCC Part 15 Class A.

  • Metal enclosures or bonded shields are used for E-field containment.

  • Ensure tight input/output cable routing to minimize loop area.

50–150 kHz (Standard Consumer/Desktop SMPS)

  • Design a Pi-filter (C-L-C) on the input to suppress 150 kHz–1 MHz harmonics.

  • Minimize trace lengths from MOSFETs and rectifiers to reduce ringing.

  • Use snubber circuits (RC or RCD) across switching nodes.

  • Ground heatsinks properly to prevent capacitive E-field coupling.

  • Evaluate EMI with LISN and EMI receiver up to 30 MHz.

  • Avoid using a floating secondary without a Y-capacitor to chassis ground.

  • Optimize loop areas in switching and rectification sections.

150–500 kHz (Modern Mid-Frequency SMPS)

  • Use multi-stage filtering with both CM and DM attenuation.

  • Add shielded inductors to suppress near-field radiation.

  • Implement spread-spectrum modulation to distribute emissions.

  • Using ferrite beads on input/output leads to suppressing high-frequency noise.

  • Improve PCB return path integrity with uninterrupted ground planes.

  • Validate PCB layout with simulation tools (e.g., HFSS or EMC Studio.)

  • Test compliance with CISPR 32 Class B and IEC 61000-6-3.

500 kHz–2 MHz (High-Frequency SMPS)

  • Use shielded transformers and opto-isolated feedback loops

  • Implement EMI shields around high-speed switching components

  • Add coaxial or twisted pair cabling where long leads are unavoidable

  • Ensure continuous chassis bonding and single-point ground referencing

  • Use low-ESR MLCCs for fast edge decoupling at power switch nodes

  • Apply layout rules to reduce radiated emissions (short traces, orthogonal signal routing)

  • Confirm radiated compliance to FCC Part 15 Subpart B and CISPR 22/32

2–20 MHz+ (Very High-Frequency / GaN/SiC SMPS)

  • Employ fully enclosed metal shielding canisters around the power stage.

  • Use 6-layer or 8-layer PCBs with ground/power stitching vias.

  • Design filters with GHz-range ferrite beads and nanoHenry inductors.

  • Include common-mode chokes on both the AC input and the DC output.

  • Test with RF absorbers and spectrum analyzers up to 1 GHz+.

  • Apply microstrip or stripline transmission line practices.

  • Use active snubbers or resonant topologies to reduce switching noise.

©2025 All Rights are Reserved.