We will stick to the conventional descriptions and definitions, before we talk about the differences that arise when applying these concepts to power conversion. Conducted emissions fall into two basic categories:
Differential mode (DM), also called symmetric mode or normal mode
Common mode (CM), also called asymmetric mode or ground leakage mode.
Looking at Figure 2-1, 'L' stands for Live (or Phase), 'N' for Neutral, and 'E' is the safety ground or 'Earth' wire. 'EUT' stands for Equipment Under Test. Note that the Earth is shown represented by the IEC symbol for Protective Earth (ground with a circle around it) and is occasionally labeled 'PE' in literature. The DM noise generator is across the L and N pair. It tries to push and pull a current Idm through these two wires. No current flows through the Earth connection on account of the DM noise source.
Note: There is nothing sacrosanct about showing the DM noise current as coming out of the equipment (and going in through the N wire). It could very well the other way around. It could also slosh back and forth depending on what part of the incoming AC (line) cycle we are on.
Caution: The designer will note that the basic AC (operating) current of the power supply is also differential in nature (by definition), since it flows into one of the L or N wires and leaves by the other. However, the Idm shown in Figure 1 obviously does not include this component. That is because the operating current, though differential, is not considered noise. For one, its basic frequency is twice the line frequency, and at 100 or 120 Hz it is virtually considered DC. Even its harmonics are well outside the range of standard conducted EMI limit curves (150 kHz to 30 MHz). However, it must not be forgotten that the operating current still retains a capacity to DC bias the filter choke, and thereby adversely affect the performance of the EMI filter, and also of any current probes being used to gather data. So we can ignore the line AC component, but only thus far.
The CM noise source connects on one side to the Earth, and has equal impedances to each of the L and N lines. It will therefore drive equal noise currents into the L and N wires. But that is assuming equal line impedances. We can easily visualize that if the impedances are unbalanced, we will get some sort of an asymmetrical common mode current distribution in the L and N wires. This sounds like a misnomer (and is) because CM is itself called an asymmetric mode. To avoid confusion, this mode should preferably be called "nonsymmetric" rather than 'asymmetric'. It happens to be the dominant emission mode in most power supplies.
We will see in the worked examples to follow, that by applying unequal load or line impedances, we effectively convert the CM current into part CM and part DM. This becomes of concern when for example, a DC-DC converter is providing power to a subsystem that typically has unequal (unbalanced) impedances looking into its input. Now, any hitherto 'disregarded' common mode noise existing on the output rails of the DC-DC converter, becomes a very real (differential) input voltage ripple applied to the subsystem. No amount of built-in subsystem CMRR (common mode rejection ratio) will help, as this noise is not completely common mode. So eventually the subsystem may start behaving erratically. Therefore, reducing common mode currents at their point of creation is always the highest priority. Equalizing the impedances is the next. In addition, by the nature of their creation, common mode currents have much higher frequency content than differential mode currents. Therefore, they have the capacity to cause intense radiation, besides also causing inductive and capacitive coupling to nearby components and circuits. It is often said that the thumb rule is that a mere 5uA of common mode current in a 1m length of wire can cause FCC Class B radiation limits to be exceeded. For FCC Class A limits this number is 15uA. Note also that the shortest standard AC power cord is 1m in length.
To avoid confusion when comparing the formulae within related literature, note that that the current through the Earth connection is often called 2?Icm instead of Icm.
Note: There is neither anything sacrosanct about showing the CM noise current in Figure 1 as coming out of the equipment (through both the L and N wires). It could very well be going the other direction too. And like DM noise, it too could also be sloshing back and forth depending on what part of the incoming AC (line) cycle we are on.
Note: We will see that in an actual power supply, differential mode noise is initiated by a swinging (pulsating) current, but the DM noise generator is itself closer to a voltage source. On the other hand, current mode noise is initiated by a swinging voltage, but the generator itself behaves more like a current source. That is what makes common mode noise more stubborn. Like all current sources, it demands a path to flow through. Since its path can include the chassis, the enclosure itself that can become a large high-frequency antenna.
Q&A: What is the actual galvanic path by which the noise currents return?
The physical electrical route between the lines, is at best, a very great distance away. So (if no EMI filter is present), some of the noise current would manage to return through various distributed parasitic capacitances along the way. The rest of will go through the air. This is simply radiation. Fields thus generated will impinge on the adjacent conductors, producing tiny current flows in them. Eventually, these tiny return currents try to add up to keep their algebraic sum at the input of the power supply exactly zero, thus keeping Kirchhoff relatively undisturbed.
Let us do some simple math to split the measured currents in the L and N lines into CM and DM components. But to avoid algebraic errors, we must establish a convention for what is a 'positive direction' for the current flow. Let us assume that the direction right to left is a positive direction (left to right being negative), and also we keep in mind that a current 'I' flowing in one direction on any wire is equivalent to '"I' in the other direction.