Measuring Common Mode Versus Differential Mode Conducted Emissions

Introduction

When faced with excessive conducted emissions from switching power supplies, one of the first things to investigate is the adequacy of the power line filter. Line-powered switching supplies generally have both common mode (CM) and differential mode (DM) sections of the filter, as shown in the generalized schematic in Figure 1.

Figure 2 shows the flow of differential mode (DM) and common mode (CM) currents through the filter network. The DM current is the current supplied to the switching circuit, so it is sourced by the Line and returned via the Neutral (shown in blue). The CM current is generated by switching transients or “ground bounce”, coupled with parasitic capacitance to chassis structure and flows into the Line/Neutral wires and returns via the chassis ground (shown in red). This current loop is fairly uncontrolled and can result in unwanted emissions. In order to design an effective filter, we need to measure the two currents versus frequency.

Making the Measurement

When using a line impedance stabilization network (LISN), we are actually measuring the sum of the common mode and differential mode conducted voltages. An example measurement is shown in Figure 3. We’re measuring a Velleman 12V / 2A switching supply (model PSIN02512N). The overall emissions weren’t too bad, so I zoomed in to measure from zero to 2 MHz, so I could observe the noise better.

The result may be seen in Figure 4. Note the very little difference in measured values between line and neutral. Well-designed line-operated supplies usually have fairly equal measurements.

One method to separately break out the DM and CM emissions is to use a current probe. Note that this is not directly comparable to the LISN measurement, which is a voltage measurement, but it will still provide some useful information on which mode is noisiest.

We’re all probably familiar with the procedure for measuring CM currents. We clamp the current probe around both line and neutral wires and make the measurement (Figure 5). Remember, CM currents flow out both wires in the same direction and normally end up radiating out and conducting back into the mains. This is shown in the violet trace of Figure 7.

On the other hand, to measure DM currents and cancel out the CM currents, we need to configure the wires such that they feed through the current probe in opposite directions (Figure 6). Note that when we do this, the voltage reading will be twice the actual DM current (6 dB higher). This is shown in the blue trace in Figure 7. We must also be careful not to exceed the maximum current rating for the probe.

Note that in Figure 7, we see that the DM currents are substantially larger than the CM currents. This suggests the CM section of the line filter is adequate, while the DM filtering could use some additional work.

For example, a small series inductor in the line side would provide more impedance for the X-capacitor (C_X) to work with. An alternative would be to increase the differential mode inductance of the common mode choke. Increasing C_Y will not likely be an option because this will increase the leakage current to chassis, which could exceed the 3.5 mA maximum as dictated by product safety standards.

This technique of isolating the common mode and differential mode components of conducted noise currents may be a valuable tool for your troubleshooting toolbox.

By using the board in Figure 8, I was able to select an “X” capacitor value that reduced the DM current.

Summary

Switch-mode power supplies can produce large amounts of both DM and CM emissions. An RF current probe can help the designer by displaying either the DM or CM emissions. Depending on which appears dominant, designers can determine whether the filtering requires DM or CM filters or a combination of the two.

References

1. Tekbox Digital Solutions, https://www.tekbox.com
2. Wyatt, Review: Tekbox LISNs, EDN 2015, https://www.edn.com/review-tekbox-lisns/
3. Nave, Power Line Design for Switched-Mode Power Supplies, Springer, 2010 (2nd Ed)
4. Mediano, Review of the EMC Filters Kit from Würth Elektronik, Interference Technology 2018, https://interferencetechnology.com/review-emc-filters-kit-wurth-elektronik/