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Common mode interference vs. differential mode interference

In the process of debugging EMC problems, engineers always habitually divide the interference into differential-mode interference and common-mode interference. However, it is difficult to completely separate differential-mode interference and common-mode interference in the actual circuit, because differential-mode interference and common-mode interference will be converted into each other. To distinguish the causes of differential-mode interference and common-mode interference, it is mainly to facilitate the analysis, debugging and countermeasures of electromagnetic compatibility problems.

1. Differential mode and common mode:

When the change of voltage and current is transmitted through the wire, there are two forms, one is that the two wires are used as the signal transmission path and the return path respectively, which we call "differential mode"; The other is that two wires do the signal transmission path at the same time, and the reflow path is returned through the reference plane (ground plane & power plane), which we call "common mode". If the currents flowing through a pair of wires are equal in magnitude and in opposite directions, it is called differential mode current. When a differential-mode current flows through a wire loop in a circuit, it causes differential-mode radiation.

As shown in the figure above, the blue signal is transmitted back and forth between the two wires, which we call "differential mode"; The yellow signal, on the other hand, is transmitted between the signal and the reference ground plane, which we call "common mode".

2. Differential mode interference and common mode interference:

Interference on any two power lines or signal lines can be expressed as differential-mode interference and common-mode interference: differential-mode interference is transmitted between two wires and is symmetrical interference. Common-mode interference is when two lines return to the source through a common reference plane, which is asymmetrical interference.

The currents of differential mode interference are equal in magnitude and opposite in direction (phase). Differential-mode currents are converted to common-mode currents due to discontinuities in distributed capacitance, parasitic inductance, trace impedance, and signal return paths flowing through non-designed paths.

The magnitude of the currents of common-mode interference is not necessarily equal, and the direction (phase) is the same. The external interference of electrical equipment is mainly based on common mode interference, and the external interference is mainly based on common mode interference (such as lightning surge and EFT test), and common mode interference usually does not affect the equipment, but after the common mode interference is converted into differential mode interference, it will seriously affect the equipment.

3. Causes of common mode interference:

The current generated by the ground reference difference is driven by the potential difference between the devices at both ends of the circuit routing due to the different ground reference potentials.

The potential difference between the potential of the circuit reference ground and the earth, and the current generated by the potential difference between the two.

The voltage induced by an external electromagnetic field on the signal wiring of all circuits. (This voltage is relatively in phase with the earth), and the current generated by this voltage.

The common-mode interference voltage is connected to the power grid in series.

Potential difference between different devices, different circuits, and potential difference from earth.

Discontinuities in distributed capacitance, parasitic inductance, and trace impedance can convert differential-mode signals to common-mode signals.

The discontinuity of the circuit reference plane and the change of the reflow path lead to the conversion of differential-mode signals into common-mode interference.

When the cable is routed in the vicinity of the switching power supply magnetics, the cable is highly susceptible to common-mode interference.

4. How to identify common mode interference:

4.1. Judging from the type of interference source:

The interference generated by the arc of the welding machine, the nearby radio station, and the high-power radiating device such as the high-power generator on the cable is usually common-mode interference. Electrostatic discharge, lightning surges, and electrical fast burst tests are also common-mode interferences generated by the interference generated by the electrical fast burst test on the cable.

4.2. With the help of current calipers and spectrum analyzer tools, judge:

the current caliper is clamped on the signal line or the ground wire (live wire or neutral wire) respectively, the interference intensity of a certain induced frequency (f1) is recorded, the current caliper is clamped with the signal wire and the ground wire at the same time, if the interference at (f1) can be observed, the interference at (f1) contains a common-mode interference component, to judge whether only the common-mode interference component is contained, the next step of the step is carried out, the caliper is clamped with the signal line and the ground wire respectively, and if the amplitude of the (f1) interference measured on the two wires is the same, then the (f1) interference only contains the common-mode interference component; If not, (f1) interference also contains a differential-mode interference component.

4.3. Use magnetic ring and common mode inductance to judge:

By adding magnetic rings and common-mode inductors of different filter frequency bands to the cable or signal, if the effect is very obvious, it is basically judged that the common-mode interference component is the main factor, but it cannot be completely ruled out that there is no differential-mode component, because the common-mode inductance itself has a leakage inductance, and the differential-mode inductance effect will also be formed after the magnetic circumference.

4.4. Make a hand judgment:

Some devices use a 2-core power cord, which is floating relative to the ground, and when the device under test is touched by hand, the interference improvement is very obvious, indicating that there is a potential difference between the equipment and the ground, which is basically judged to be common-mode interference.

5. How to suppress common mode interference:

EMC engineers know that the main idea of solving EMI problems is to suppress the source of interference and cut off the coupling path. The same is true for the suppression of common-mode interference. Previously, we analyzed the mechanism of common-mode interference, and we know that the way of common-mode interference is mainly caused by the ground potential difference, so reducing the ground potential difference is one of the killer solutions to the common mode interference.

We know that the main reason for the occurrence of ground potential difference is the existence of parasitic inductance in the ground plane, and the potential difference will occur when the high-frequency current flows through the two ends of the parasitic inductance, so the essence of reducing the ground potential difference is to reduce the parasitic inductance at high frequency, and the following measures can be taken:

The signal transmission uses a complete reference ground plane as the reflow path.

The module circuit is connected to the reference ground plane by multi-point grounding.

The device is connected to the ground.

Capacitive jumpers are used between different signal types.

The ground routing width is as wide as possible.

The density of ground vias.

Common-mode interference generally exists in the form of current, and from the perspective of the coupling path of common-mode interference, the coupling path can also be cut off by common-mode inductance. When a common-mode interference current flows through the coil, due to the isotropy of the common-mode interference current, a co-directional magnetic field will be generated in the coil to increase the inductance of the coil, so that the coil behaves as a high impedance and produces a strong damping effect, so as to attenuate the common-mode interference current and achieve the purpose of filtering; When the normal differential-mode current in the circuit flows through the common-mode inductor, the current generates a reverse magnetic field in the common-mode inductance coil wound in the same phase and cancels each other out, so there is basically no attenuation effect on the normal differential-mode current.