08 januari 2026
A common-mode line filter is used in AC-DC power supplies mainly to control electromagnetic interference (EMI), both to meet regulations and to ensure reliable operation.
1. Suppresses conducted EMI (noise on the mains)
Switch-mode AC-DC power supplies generate high-frequency switching noise (from tens of kHz up into MHz). This noise can travel back onto the AC mains.
- Common-mode noise: The same noise voltage appears on both Line and Neutral with respect to earth
- Without filtering, this noise:
o Interferes with nearby equipment
o Causes radio/communication interference
o Fails EMC standards (CISPR, FCC, EN55032, etc.)
A common-mode choke presents high impedance to common-mode noise, strongly attenuating it.
2. Passes normal AC power with minimal loss
The key advantage of a common-mode filter:
- Normal AC current flows in opposite directions in Line and Neutral
- Magnetic fields cancel → very low impedance to 50/60 Hz power
- Minimal voltage drop and power loss
So:
- ✔ Filters noise
- ✔ Does not affect power delivery
3. Works with Y-capacitors to reduce EMI
In a typical AC-DC input filter:
- Common-mode choke blocks common-mode noise
- Y-capacitors provide a return path for common-mode noise to earth
- Together, they form an effective EMI suppression network
Without the choke, Y-capacitors alone would:
- Be less effective
- Increase leakage current beyond safety limits
More information can be found on the datasheets:
If you want more information or if you have any questions, please contact Nijkerk Electronics!