his article discusses the common-mode and differential-mode radiation from cables and presents the measurement results from the SMPS connecting wires.
If the fields generated by the forward current cancel the fields of the return current and no other circuits, or sources, or coupling paths are present, then the forward current equals the return current. In virtually any practical circuit, a different scenario takes place, as shown in Figure 2.
ÎD is referred to as the differential-mode (DM) current while ÎC is referred to as the common-mode (CM) current. The DM currents are usually the functional currents. They are equal in magnitude and of opposite directions. The CM (unwanted) currents are equal in magnitude and of the same direction (See [1] for the discussion of the CM current creation).
Treating each antenna as a linear dipole of length l, the magnitude of the total field at a distance d from the antennas is, [2],
Now, consider the scenario shown in Figure 4, where two linear antennas carry the common-mode currents.
The radiated fields due to both conductors are of same directions, thus reinforcing each other to give the total radiated field as shown. The magnitude of the total field at a distance d from the antennas is
For instance, it takes only 8 µA of the CM current to exceed the FCC Class B limit of 100 µV/m at a distance of 3m, as the following calculations show. From Eq. (2) we can calculate the expression for the CM current in terms of the maximum allowable field strength, [3].
It is, therefore, no surprise that the CM current is of great interest (or fear) to the EMC engineers. Next, we will discuss the DM and CM current measurements from the cables connecting a SMPS.
The current probe used is shown in Figure 6.
The SMPS used in this experiment is a step-down (buck), 12V to 5V DC, switching at 420 kHz.
With both wires inside the probe, the differential current fields (ideally) cancel each other, and the current probe measures only the common-mode currents. To be precise, it (ideally) measures twice the value of the CM current, i.e., 2IC. The measurement results are shown in Figure 8 and summarized in Table 1. The measurement values are the relative values in the units of dBµV.
Note that in these setups, we measure the magnitudes of the total currents Î1 and Î2, given by Eqs. (1a) and (1b) and shown in Figure 2. As we shall see, in this experiment, the CM currents exist at the different frequencies than the DM currents, and thus, the total current measurements can be used to extract the DM currents. That’s why we measured the CM currents first.
- Bogdan Adamczyk, Common-Mode Current Creation and Suppression, In Compliance Magazine, October 2019.
- Bogdan Adamczyk, Foundations of Electromagnetic Compatibility with Practical Applications, Wiley, 2017.
- Henry W. Ott, Electromagnetic Compatibility Engineering, Wiley, 2009.
- Bogdan Adamczyk, Common-Mode Current Creation and Suppression, In Compliance Magazine, October 2019.
- Bogdan Adamczyk, Foundations of Electromagnetic Compatibility with Practical Applications, Wiley, 2017.
- Henry W. Ott, Electromagnetic Compatibility Engineering, Wiley, 2009.