I have one question which I encountered during usage of common mode choke.

Question : "If a common mode choke is used on the main power / ground input, ensure that no I/O circuits effectively bypass the ground side of the common mode choke. For example, the shield of screened cables or I/O signal grounds may present a better AC ground to the module than the main power ground. Common mode chokes may be needed on some I/O signals to prevent this."

Answer : "AC ground current may find an easier path to ground in the vehicle through IO circuits etc. rather than through the ground side of the common mode choke. If this occurs, the function of the common mode choke is effectively neutralised, potentially causing conducted emissions concerns. Normally signal grounds, cable shields would be connected to the "vehicle side" ground of the common mode choke, unless this causes a functional problem (e.g. accuracy of analog signals), in which case a common mode choke on analog / ground signals may also be needed."

I have the answer to the question also. But I am unable to understand it clearly. Can someone explain the question and answer in simple terms please. What the question is asking me to check and how will I check in my design?

Thank you.


1 Answer 1


For a common mode choke to be effective all the supply current it passes through one limb of the 2 coils must return through the other limb of the coils. If your load also makes a connection to ground then you have, in effect, short circuited the 2nd limb of the choke and the common mode choke is no longer effective.

For this reason, it is better to isolate the load circuit from ground either by using a DC-DC converter or other methods such as opto-isolation.

  • \$\begingroup\$ Anyone care to share with me the reason for the downvote? \$\endgroup\$
    – Andy aka
    Jul 29, 2019 at 11:28
  • \$\begingroup\$ How does the DC-DC converter help in isolation ? Can you please explain \$\endgroup\$
    – Newbie
    Nov 7, 2019 at 14:29
  • \$\begingroup\$ Having a non-isolated DC converter can mean you have to be extra careful about grounding the load on the output. Your load ground should be on one specific node and that node has to be also fed via the common mode choke. This is why, when using an isolated DC converter, things are much simpler because the load is isolated and, even if you connected one side of your load to earth, you wouldn't be inadvertently shorting out one limb of the CM choke. \$\endgroup\$
    – Andy aka
    Nov 7, 2019 at 15:02
  • \$\begingroup\$ I am using an IC. TPS54260 Buck converter. The load is grounded. How to identifying whether the DC-DC Converters are isolated or non-isolated? \$\endgroup\$
    – Newbie
    Nov 8, 2019 at 5:45
  • 1
    \$\begingroup\$ Then, in all probability, it is non isolating. \$\endgroup\$
    – Andy aka
    Nov 8, 2019 at 8:29

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