Differential-Mode Current and Common-Mode Current

The key to understanding and solving EMC problems lies in understanding current flow. Current flows in loops, but it is not uncommon for digital circuit designers to forget this critical fact. They often work with voltages, typically in scenarios like one gate driving another. Circuit schematics use one or more ground symbols to represent signal or power return paths. This is often referred to as the "hidden schematic." Guidance or consideration is rarely given to how these return paths are routed, defined, or how they return to the power source. Problems often arise when the board's return layer is manually routed by the designer or autorouted by CAD software, leading to many boardrelated EMI issues. However, understanding how return currents find their way back to their source and ensuring the return path is low impedance can significantly mitigate problems, though it's still a long journey to completely resolving EMI issues.

First, consider how current flows. At low frequencies (below 50kHz), return current typically follows the path of least resistance. At high frequencies (above 50kHz), return current generally follows the path of least impedance. Note that these two concepts are not the same. The path of least resistance depends on the material properties of the conductor in the return path and the distance from source to loadmeaning the return current takes the most direct path back to the source. The path of least impedance depends on the inductive and capacitive effects between the trace and the return path, which causes the return current to flow directly beneath the signal (or power) trace. This occurs because, at higher frequencies, the self inductance of the signal current path is minimized, resulting in minimal stored magnetic energy and path impedance, which generally minimizes the physical space (or loop area) between the outgoing and return currents. This concept is discussed in detail in Chapter 2 of this book.

This concept is easier to understand by observing how a transformer works, as shown in Figure 1.2. When alternating current (AC) or high-frequency current flows through one winding of a transformer, it induces a current in the opposite direction in an adjacent winding through magnetic coupling. When currents flow through conductors or PCB layers, the principle is similar to currents flowing in circuit traces. A detailed discussion on this follows in Chapter 2.

Next, consider the concept of what is commonly termed Differential-Mode (DM) current or differential current. These are the signal or power currents that flow from the source to the load and then back from the load to the source. DM current flows in a loop: outgoing current from source to load and return current from load back to source. The closer these two paths are, the smaller the self-induced magnetic field, which reduces coupling with other wires, traces, or circuits.

Problems arise when the power/signal trace or wire is forced away from its return path, creating a larger loop, as shown in Figure 1.3. A larger loop creates a stronger radiating magnetic field and, conversely, makes the circuit more susceptible to magnetic fields from other sources, potentially introducing interference into these circuits.

Fortunately, return layers or reference layers (often mistakenly called ground planes) are commonly used. As noted above, a trace over a return/reference layer creates direct inductive coupling with the return path beneath it. A trace with a locally coupled return path minimizes the closed loop area between itself and the return path, thereby minimizing emissions from the trace and reducing the circuit's susceptibility to external interference. However, if during PCB layout the signal current path and return current path are separated by multiple layers, or if there are gaps or splits in the return/reference layer, the benefits of the reference layer can be completely lost.

DM current flows differently than Common-Mode (CM) current. The key difference for CM current is that it flows in the same directionon both the signal path and the return path. Its magnitude is usually very small, on the order of microamperes. A good way to understand DM and CM currents is: each DM current (e.g., a digital signal) requires its own dedicated return path to function, whereas CM currents can share a common return path. If a specific return path is not provided for CM signals, they will find their own path, which will likely form a large loop area, resulting in strong radiated emissions!

Consider the scenario with two subsystems (Circuit 1 and Circuit 2), as shown in Figure 1.4. The subsystems could be two Integrated Circuits (ICs) or two circuit boards sharing a common return/reference path. Even if the return path is a solid plane, the impedance between two points on it, while small (milliohms), is not zero. A small voltage difference will exist between any two points on the return path due to various currents from electromagnetic fields flowing through this impedance. Where there is a voltage drop between two points, a current must flow. It is this small current that constitutes CM current, which flows in the same directionon both the signal path and the signal return path. This situation often occurs with I/O cables connected to the product and is a very common cause of excessive radiated emissions.

Other ways CM current can be generated include coupling via voltage sources, which might capacitively couple to the chassis, such as through a power supply heat sink. A voltage between a switching device and the chassis can generate CM current throughout the circuit. In this case, a connection to the chassis must be found to allow the CM current to return to the power source. For isolated power supplies without any AC connection to the chassis, this can cause radiation from all power and interconnect cables; effort should be made to allow this energy to return to the chassis and then back to the power source. CM currents can also be generated if neglected slots or gaps in the return layer force return currents to take long detours, or if there is poor impedance matching between the source and load of a high-frequency signal circuit that has been overlooked.

Because the distance between the CM signal and its return path is typically large, CM noise can radiate much more effectively compared to DM noise. Some simulation models suggest that, depending on the frequency and loop geometry, CM current can radiate with an efficiency up to 10^4 times greater than DM radiation. Put simply, 1µA of CM current can radiate as effectively as 1mA of DM current.