What Is a Common Mode Choke?
A common mode choke (CMC) is a coil that suppresses the common-mode noise flowing in the same direction on two conductors, while passing the wanted signal (differential), which flows in opposite directions, almost unaffected.
What Are Common Mode and Differential?
A differential signal flows in opposite directions on the two lines (wanted data/power). Common-mode noise flows in the same direction on both lines (usually external interference). A CMC distinguishes the two.
How Does It Work?
Two windings are wound on the same core in opposite phase. The magnetic fields of the differential currents cancel each other → the coil has no effect. The fields of the common-mode currents add up → a high impedance forms and the noise is suppressed.
| Signal | Flow | CMC effect |
|---|---|---|
| Differential (wanted) | Opposite direction | Passes (low impedance) |
| Common-mode (noise) | Same direction | Suppressed (high impedance) |
Difference From a Choke
A normal choke resists all current on the line; a CMC is selective: it blocks only common-mode noise and doesn't disturb the wanted signal. So it's preferred on data and power lines.
Where Is It Used?
USB/HDMI/Ethernet data lines, AC mains-input filters (for EMI compliance), motor drives and switch-mode power supplies.
Common Mistakes
- Wrong impedance: a CMC matched to the noise frequency to be suppressed must be chosen.
- Unbalanced lines: if the line impedances are unbalanced, common-mode converts to differential and degrades performance.
- Current rating: if the rated current is exceeded, the core saturates.
Frequently Asked Questions
Why doesn't a CMC disturb the wanted signal?+
Why is it needed on data lines?+
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