EMC Question of the Week: March 18, 2024

DC Power Input

A 24-VDC component has a two-wire power cord and fails to meet its conducted emissions requirement at 360 kHz (the third harmonic of the buck converter on the power input). Two LISNs are employed in the test set-up, and it is determined that the common-mode and differential-mode components of the emissions are approximately equal. To meet emissions requirements, this device needs

  1. better filtering of the 24-V input
  2. better filtering of 24-V and 0-V inputs 
  3. a common-mode choke
  4. a shielded enclosure


The best answer is “a.” The fact that the input connects to a buck converter suggests that the power input is not balanced (i.e., the 0-V input connects to the PCB ground). The reason that the measured emissions exhibit equal CM and DM components is because one LISN (the +24 V side) is driven strongly. The other LISN (the 0-V side) is connected to the DUT ground and is not driven strongly relative to the LISN ground. The most effective way to deal with this is to filter the 24-V input better.   

The general guideline that applies in this situation is, "Never intentionally introduce an impedance in the return side of an unbalanced power or signal current path." That includes the use of common-mode chokes, which should only be employed to filter balanced power or signal paths.

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