Why should we use a common mode line filter in ACDC power supplies

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 data sheets:

251212 Common mode TLV Series

251212 Common mode TUC15CL

251212 common mode TUK Series

If you want more information or if you have any questions, please contact Nijkerk Electronics!