EMI filters use two types of chokes: common mode (CM) chokes and differential mode (DM) chokes. They look similar but serve completely different purposes. Understanding the difference—and when to use each—is critical for effective EMI filtering.
This guide compares CM and DM chokes, how they work, and how to combine them in a complete EMI filter.
Shaanxi SiRui Technology Co., Ltd. manufactures both common mode and differential mode chokes for power supply EMI filters. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. The Two Types of EMI Noise
- Differential mode (DM): noise flowing in opposite directions on live and neutral; caused by pulsating input current
- Common mode (CM): noise flowing in the same direction on both lines, returning through ground; caused by high dv/dt nodes coupling through parasitic capacitance
2. Common Mode Choke (CMC)
Two windings on the same core, wound in opposite directions. Normal current produces opposing fields that cancel. CM noise produces adding fields that create high impedance. Core material: high-μ ferrite or nanocrystalline. Best for attenuating CM noise above 1 MHz, meeting CISPR 32 and FCC limits.
3. Differential Mode Choke (DMC)
An inductor in series with the line. High impedance to all current. Must handle full load current without saturation. Core material: powder core or gapped ferrite. Best for attenuating DM noise below 1 MHz.
4. Side-by-Side Comparison
| Parameter | CM Choke | DM Choke |
|---|---|---|
| Attenuates | Common mode noise | Differential mode noise |
| Effect on normal current | Minimal (fields cancel) | Significant (series impedance) |
| Core material | High-μ ferrite / nanocrystalline | Powder core / gapped ferrite |
| Windings | 2+ windings, opposite phase | 1 winding per line |
| Saturation risk | Low (normal current cancels) | High (full load current) |
| Frequency range | 100 kHz–30 MHz | 10 kHz–1 MHz |
5. A Complete EMI Filter
Input → Fuse → X Cap → CM Choke → DM Inductor → Y Cap(s) → Bridge. X cap attenuates DM, CM choke attenuates CM, DM inductor adds DM attenuation, Y caps provide CM path to ground.
6. When Each Is Needed
- CM choke alone: low power (<50W), cost-sensitive, small DM noise
- Both CM + DM: higher power (>100W), strict EMI limits (medical, industrial)
- DM alone: seldom sufficient—can’t handle CM noise to ground
7. Sirui-Tech Filter Components
- CM chokes: nanocrystalline and ferrite, 1–100 A
- DM inductors: powder core, high current
- Complete filter modules: custom designs for specific power levels
Conclusion
CM and DM chokes are complementary, not interchangeable. The CM choke is the single most effective component for conducted EMI. Most commercial EMI filters combine both, plus X and Y capacitors, for complete attenuation.
Need EMI filter components? Contact Sirui-Tech at sales@sirui-tech.com with your power level and attenuation requirement.