EMC Question of the Week: June 15, 2026

A poor common-mode filter design

A schematic for a DC power line filter is shown in the figure. The device is located in a metal enclosure. In order to filter the common-mode power bus noise effectively, which of the components shown in the schematic must be eliminated?  

  1. L2
  2. L3
  3. CX1 and CX2
  4. CX4 and CX5

Answer

The best answer is “a.” Generally, common-mode chokes should never be used in unbalanced circuits. In this example, the common-mode choke converts part of the noise current coming from the SMPS into a common-mode voltage that drives the chassis relative to the circuit board ground. The circuit board ground is likely to be reference voltage for any signals entering or leaving the system. It's generally important to ensure that the board ground and chassis ground have the same high-frequency potential at all external interfaces.

Note that without the common-mode choke, there are no longer two independent modes that require filtering. A simple pi-filter attenuates the noise coming from the SMPS, which reduces both common-mode and differential-mode currents on the power cable. 

If this question seems strangely familiar, it may be because it is almost identical to the February 9, 2026 EMC Question of the Week. The answer to that question (about an AC power line filter) was very different. 

For more information on common-mode filtering and the use of common-mode chokes, see this month's EMC Blog post on the LearnEMC website.

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