Electromagnetic interference (EMI) is the most common reason switching power supplies fail regulatory testing (CISPR 32, FCC Part 15). Conducted noise on the input lines and radiated EMI from the power loop can delay product launch by weeks.
This practical guide covers the main sources of EMI in SMPS, and the most effective mitigation techniques—from component selection to PCB layout.
Shaanxi SiRui Technology Co., Ltd. manufactures common mode chokes, X2/Y2 capacitors and EMI filter inductors for power supply applications. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. Where EMI Comes From
- Differential mode noise: pulsating input current, input cap charges/discharges at switching frequency
- Common mode noise: high dv/dt nodes coupling through parasitic capacitance to ground
- Radiated EMI: high-frequency current loops acting as antennas
2. Key Mitigation Techniques
Input Filter
- X capacitors: across lines, attenuate differential mode
- Y capacitors: line to ground, attenuate common mode
- Common mode choke: high impedance to CM noise—the single most effective component
- Differential mode inductor: additional DM attenuation
PCB Layout
- Minimize power loop area (input cap → switch → diode)
- Keep high dv/dt nodes small (switch node / drain)
- Separate analog and power grounds (star ground)
- Place filter components closest to input entry
Edge Slowing
- Add snubber across switch or diode to reduce dv/dt
- Use gate resistor to slow switching edge
- Trade some efficiency for lower EMI
3. Typical Filter Topology
Input → Fuse → X Cap → CM Choke → DM Inductor → Y Cap(s) → Bridge/Stage. For high-noise applications, a two-stage filter provides better attenuation.
4. Common Mistakes
- Undersized CM choke that saturates at peak current
- Poor PCB layout with large power loop
-
- Not testing EMI early in design cycle
Missing Y capacitor for ground-referenced noise
5. Sirui-Tech EMI Components
- Common mode chokes: nanocrystalline and ferrite, 1–100 A
- Differential mode inductors: powder core for high current
- Custom EMI filter modules: complete filter designs
Conclusion
EMI control is a combination of good filter design, proper PCB layout and thoughtful component selection. The common mode choke is the most effective single component for conducted noise. Minimize power loop area and pre-test EMI early.
Need EMI filter components? Contact Sirui-Tech at sales@sirui-tech.com with your power level and switching frequency.