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How to interpret common mode interference and differential mode interference of EMC electromagnetic compatibility

2025-01-31 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > Internet Technology >

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How to interpret the common mode interference and differential mode interference of EMC electromagnetic compatibility, I believe that many inexperienced people do not know what to do about it. Therefore, this paper summarizes the causes and solutions of the problem. Through this article, I hope you can solve this problem.

1. What is common mode and differential mode

The power line of electrical equipment, the communication line of telephone, etc., and the communication line exchanged with other equipment or peripheral equipment, there are at least two wires, which are used as round-trip lines to transmit power or signals. in addition to these two wires, there is usually a third conductor, which is the "ground wire". When the changes of voltage and current are transmitted through wires, there are two forms, one is that two wires are used as round-trip lines, which we call "differential mode", and the other is that two wires are used as paths and ground wires as return transmissions. we call it "common mode".

As pictured above, the blue signal is transmitted back and forth between the two wires, which we call "differential mode", while the yellow signal is transmitted between the signal and the ground wire, which we call "common mode".

The interference on any two power lines or communication lines can be expressed by common mode interference and differential mode interference:

1. Common mode interference

The common mode interference propagates between the wire and the ground (housing), which is an asymmetric interference, which is defined as the unwanted potential difference between any current-carrying conductor and the reference ground.

The current magnitude of the common-mode interference is not necessarily equal, but the direction (phase) is the same. The external interference of electrical equipment is mainly common-mode interference, and the external interference is mainly common-mode interference. Common-mode interference itself generally does not harm the equipment, but if common-mode interference is transformed into differential-mode interference, the interference will be serious, because useful signals are differential-mode signals.

2. Differential mode interference

Differential mode interference, which is transmitted between two wires, belongs to symmetrical interference, which is defined as the unwanted potential difference between any two current-carrying conductors. In general, the common mode interference amplitude is large, the frequency is high, and radiation can be generated through the wire, resulting in greater interference. The amplitude of differential mode interference is small, the frequency is low, and the interference is small.

The magnitude of the current of differential mode interference is equal, and the direction (phase) is opposite. Due to the discontinuity of the distributed capacitance, inductance and signal impedance of the line, and the flow of the signal return path through the unexpected path, the differential mode current will be converted into common mode current.

Second, the causes of common mode interference

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

2. Radiation interference (such as lightning, equipment arc, nearby radio station, high-power radiation source) induces common-mode interference on the signal line, because the alternating magnetic field produces alternating current, the area of the ground wire-zero wire loop is different from that of the ground wire-fire wire loop, and the impedance of the two loops is different.

3. The grounding voltage is different. To put it simply, the common mode interference is caused by the potential difference.

4. The common mode interference caused by the line inside the equipment to the power line.

Common mode interference current

Common-mode interference generally occurs in the form of common-mode interference current. in general, there are three reasons for common-mode interference current.

1. The external electromagnetic field induces a voltage on all the wires in the circuit (the voltage is equal to the amplitude and phase relative to the earth), and the current generated by this voltage.

2. Because the devices at both ends of the circuit are connected to different ground potentials, the current generated by the ground potential difference.

3. There is a potential difference between the circuit wiring on the device and the earth, so the common mode interference current will be produced on the circuit wiring.

If the common mode interference current is produced on the circuit wiring of the device, the circuit wiring will produce strong electromagnetic radiation, which will produce electromagnetic interference to the components of electronic and electrical products, and affect the performance index of the product; in addition, when the circuit is unbalanced, the common mode interference current will be transformed into differential mode interference current, and the differential mode interference current will have a direct interference effect on the circuit. For the signal lines and their loops in electronic and electrical product circuits: when the differential mode interference current flows through the wire loop in the circuit, it will cause differential mode interference radiation, which is equivalent to a small loop antenna, which can radiate magnetic field into space or receive magnetic field.

Fourth, how to identify common mode interference

1. Judging from the interference source: the interference on the cable caused by lightning, nearby arc, nearby radio station or other high-power radiation device is common-mode interference.

2. Judging from the frequency, the common mode interference is mainly concentrated above 1MHz. This is because common-mode interference is sensed to the cable through space, which is easy to occur only at higher frequencies. With one exception, low-frequency common-mode interference is also felt when the cable passes next to a strong magnetic field radiation source (for example, a switching power supply).

3. Measure with the instrument: as long as there is a spectrum analyzer and a current caliper, you can measure and judge. The steps of judgment are as follows:

a. Clamp the current caliper on the signal wire or ground wire (hot wire or zero wire) respectively, and record the interference intensity of a certain induction frequency (F1).

b. If the current caliper clamps the signal line and the ground wire at the same time, if the interference at (F1) can be observed, the interference (F1) contains the common mode interference component. To judge whether it contains only the common mode interference component, proceed the discrimination of step c.

c. If the calipers are stuck on the signal wire and the ground wire respectively, if the amplitude of the (F1) interference measured on the two lines is the same, then the (F1) interference contains only the common mode interference component; if not, the (F1) interference also contains the differential mode interference component.

5. How to suppress common-mode interference

Common-mode interference is the most common and harmful interference in EMC interference. Filtering is the most direct way to suppress common-mode interference, which is an important measure to suppress and prevent common-mode interference. * * the function of the filter is to allow the signal of a particular frequency to pass smoothly, while the signal of other frequency is greatly suppressed. It is essentially a frequency selection circuit, which cuts off the path of electromagnetic interference propagation along the signal line or power line. In addition, it is an effective method to compress the interference spectrum. When the interference spectrum is different from the frequency band of the useful signal, the useless interference signal can be filtered out by the filter. Therefore, the proper selection and correct use of the filter is very important to suppress common-mode interference.

If the useful signal is a differential mode signal and the interference signal is a common mode signal, the common mode inductor can be used to suppress the interference signal.

VI. Principle and interference suppression of common-mode inductors

When the common mode interference current flows through the coil, the inductance of the coil will be enlarged due to the homotropy of the common mode interference current, which will increase the inductance of the coil due to the isotropy of the common mode interference current, which will make the coil show high impedance and produce 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 produces a reverse magnetic field in the common mode inductor coil wound in the same phase and cancels each other, so it basically has no attenuation effect on the normal differential mode current.

1. Common mode interference suppression method on USB signal.

The signal on the USB transmission line is a differential signal and the interference source is a common-mode interference signal. The common-mode inductor on the transmission line can better suppress the common-mode interference without any attenuation to the useful differential signal. The common mode interference signal is effectively suppressed by adding filter-common mode inductor to the circuit.

If the common mode interference source is in the power supply circuit, the common mode capacitor can be used to suppress the interference signal.

If the common-mode capacitor is introduced into the circuit, the common-mode capacitor provides the shortest path to bypass the common-mode interference signal, thus suppressing the generation of common-mode interference.

If there is also differential mode interference in the power supply circuit, use differential mode capacitance to suppress the interference.

If the differential mode capacitor is introduced into the circuit, the differential mode capacitor provides the shortest path to bypass the differential mode interference signal, thus suppressing the generation of differential mode interference.

Common-mode interference is the most common and harmful interference in EMC interference. In addition to filtering, common-mode interference can also be reduced by shielding the signal line and reducing the ground impedance on the PCB board to reduce the common-mode signal strength.

After reading the above, have you mastered how to interpret the common mode interference and differential mode interference of EMC electromagnetic compatibility? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel, thank you for reading!

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