Measuring Differential- and Common-Mode Current Radiation from Cables

EMC concepts explained

his article discusses the common-mode and differential-mode radiation from cables and presents the measurement results from the SMPS connecting wires.

Differential-Mode and Common-Mode Circuit Model

Consider a typical circuit model shown in Figure 1.

If the fields generated by the forward current cancel the fields of the return currents 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.

Î_D is referred to as the differential-mode (DM) current while Î_C 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).

Figure 1: Typical circuit model

In the analysis of the DM and CM currents, we often utilize the total currents Î₁ and Î₂ flowing in the same direction. The reason for this is that it is easier to apply the classical circuit theory to the total currents than it is to the individual currents. Once the equations are developed for the total currents, we simply substitute the differential or common-mode currents for the total currents in the derived expressions. This approach will be demonstrated in the next section.

The total currents Î₁ and Î₂ flowing are related to the DM and CM currents by

Radiation from Differential- and Common-Mode Radiation

Differential- and common-mode radiation can be modeled as the radiation from two Hertzian dipoles driven by a noise voltage.

Let’s begin with the DM radiation. Consider the scenario shown in Figure 3. where two linear antennas (conductor 1 and conductor 2) placed along the x-axis, carry the differential-mode currents along the z-direction.

Graphic of Circuit model showing the CM, DM, and total currents

Figure 2: Circuit model showing the CM, DM, and total currents

Graphic of DM currents and the associated fields

Figure 3: DM currents and the associated fields

Graphic of CM currents and the associated fields

Figure 4: CM currents and the associated fields

The maximum radiated field is broadside to the antenna (in the xy-plane, where θ = 90° and in the z-direction, as shown. Note that the radiated fields due to both conductors are of opposite directions, giving a small total radiated field as shown. This total radiated field at the observation point in the far field can be obtained by superimposing the fields due to each antenna.

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],

where f is the frequency of the current carried by the antennas.

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

It should be noted that the CM currents could be several orders of magnitude smaller than the DM currents, yet the radiation from them could exceed the regulatory limits.

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].

Letting, l = 1m, d = 3m, f = 30 MHz, E_θ = 100 μV⁄m, we obtain I_C = μV⁄m.

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.

Differential-Mode and Common-Mode Current Measurement

Figure 5 shows the test setup to measure the differential- and common-mode currents.

The current probe used is shown in Figure 6.

Figure 5: Measurement setup

Closeup current probe for DM- and CM- measurements

Figure 6: Current probe for DM- and CM- measurements

The SMPS used in this experiment is a step-down (buck), 12V to 5V DC, switching at 420 kHz.

The CM currents were measured with the current probe, where both the power and ground wires were placed inside the current probe, as shown in Figure 7.

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., 2I_C. The measurement results are shown in Figure 8 and summarized in Table 1.

Next, let’s measure the differential-mode currents. The DM currents were measured with two different setups: current probe over the ground wire and the current probe over the power wire, as shown in Figure 9.

The measurement results with the probe over the ground line are shown in Figure 10, while the results for the power line are shown in Figure 11. Both results are summarized in Table 2.

Figure 7: CM-current measurement setup

Figure 8: CM-current measurement results

Table 1: CM-current measurement results

Figure 9: DM-current measurements

Observations

The magnitudes of the differential-mode currents on the ground and power wires are very close (within 2 dBµV), as they are supposed to be. Both the ground and the power wire differential-mode measurements also capture the common-mode currents. These currents and their magnitudes are not as predictable as the DM currents. Note that the ground-wire CM-current is present at point A in Figure 10, but it is not present at that frequency on the power wire in Figure 11. Another CM current at a lower frequency, at point K, appears in Figure 11, and it was not present at that frequency in Figure 10.

Graphic of DM-current measurement results – ground wire

Figure 10: DM-current measurement results – ground wire

Graphic of DM-current measurement results – power wire with point K

Figure 11: DM-current measurement results – power wire

Table 2: DM-current measurement results

References

Bogdan Adamczyk, Common-Mode Current Creation and Suppression, In Compliance Magazine, August 2019.
Bogdan Adamczyk, Foundations of Electromagnetic Compatibility with Practical Applications, Wiley, 2017.
Henry W. Ott, Electromagnetic Compatibility Engineering, Wiley, 2009.

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Dr. Bogdan Adamczyk is professor and director of the EMC Center at Grand Valley State University (http://www.gvsu.edu/emccenter) where he regularly teaches EMC certificate courses for industry. He is an iNARTE certified EMC Master Design Engineer. Prof. Adamczyk is the author of the textbook “Foundations of Electromagnetic Compatibility with Practical Applications” (Wiley, 2017) and the upcoming textbook “Principles of Electromagnetic Compatibility with Laboratory Exercises” (Wiley 2022). He can be reached at adamczyb@gvsu.edu.

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