One of the toughest EMC issues you are likely to encounter when designing printed circuit boards is managing radiations from common mode currents.
What makes this so difficult is that the actual parts generating the common mode current, which are the parasitics, are not visible in the schematics.
This is why designing the PCB is not intuitive.
One way to understand and mitigate radiations from common mode currents is to model our system as a dipole (or monopole) antenna.
In this model, the source of the common mode noise is the source of the antenna.
What also becomes clearer is that we have very few ways to mitigate these issues.
Basically, we have to do everything in our control to make this antenna as inefficient as possible!
Here are a few steps we can take:
- Reduce the source of voltage drop in the return reference path: This requires designing the return path very carefully.
- Filter and/or shield the cables: Sometimes easier said than done, especially with high power applications.
- Completely isolate the cables from the return reference path: This solution can sometimes be a bit bulky.
My favorite approach is to design the return reference path with as much or more dedication than the signal forward path.
By doing so, you aim to control the parasitics as much as possible, reducing the common mode voltage source to a minimum.
Another important point to remember is that common mode currents are much more efficient at radiating than differential mode currents.
In fact, only a few microamps of common mode currents are necessary to fail some radiated emissions tests!
I hope this post brings a bit more clarity on the topic.
Dario
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