Power line noise: common mode interference, differential mode interference
Time:2022-10-17
Views:1700
1. Power line noise is caused by the electromagnetic disturbance generated by various electrical equipment in the power grid propagating along the power line. Power line noise is divided into two categories: common mode interference and differential mode interference.
Common mode interference is defined as the unwanted potential difference between any current carrying conductor and the reference ground; Differential mode interference is defined as the unwanted potential difference between any two current carrying conductors.
As shown in the figure above, the blue signal is transmitted back and forth between two wires, which we call "differential mode"; The yellow signal is transmitted between the signal and the ground wire, which is called "common mode". 2.1 Common mode interference signal The current magnitude of 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 also mainly common mode interference. Common mode interference itself generally does not harm the equipment. However, if the common mode interference turns into differential mode interference, the interference will be serious, because the useful signals are differential mode signals.
2.2 Differential mode interference signal
The differential mode interference current is equal in magnitude and opposite in direction (phase). Due to the discontinuous distributed capacitance, inductance and signal line impedance of the routing, and the unexpected path of the signal return path, the differential mode current will be converted into the common mode current.
2.3 Common mode interference causes
2.3.1. Common mode interference voltage is connected to the power grid in series.
2.3.2 Radiation interference (such as lightning, equipment arc, nearby radio station, high-power radiation source) induces common mode interference on the signal line. The reason is that the alternating magnetic field generates alternating current, the area of ground wire neutral wire loop is different from that of ground wire live wire loop, and the resistance of the two loops is different.
2.3. 3. The grounding voltage is different. Simply speaking, the common mode interference is caused by the potential difference. 4. Common mode interference caused by lines inside the equipment to power lines.
2.4 Common mode interference current
Common mode interference generally occurs in the form of common mode interference current. Generally, common mode interference current is caused by three aspects:
2.4.1. The external electromagnetic field induces voltage on all wires in the circuit routing (the voltage is equal amplitude and in-phase with respect to the earth), and the current generated by this voltage.
2.4.2. The current generated under the drive of the ground potential difference due to the different ground potentials connected to the devices at both ends of the circuit wiring.
2.4.1. The external electromagnetic field induces voltage on all wires in the circuit routing (the voltage is equal amplitude and in-phase with respect to the earth), and the current generated by this voltage.
2.4.2. The current generated under the drive of the ground potential difference due to the different ground potentials connected to the devices at both ends of the circuit wiring.
2.4.3. There is a potential difference between the circuit wiring on the device and the ground, so the circuit wiring will produce common mode interference current.
2.5 Precautions
2.5.1 If the device generates common mode interference current on its circuit wiring, the circuit wiring will generate strong electromagnetic radiation, which will cause electromagnetic interference to electronic and electrical components and affect the performance indicators of the product;
2.5.2. When the circuit is unbalanced, the common mode interference current will be converted into differential mode interference current, which will directly interfere with the circuit. For the signal lines and circuits in the circuits of electronic and electrical products, when the differential mode interference current flows through the wire loop in the circuit, it will cause differential mode interference radiation. This loop is equivalent to a small loop antenna, which can radiate or receive magnetic fields into space.
2.5.3 Common mode interference is mainly concentrated above 1MHz. This is because common mode interference is induced to the cable through space, which is easy to occur only at higher frequencies. However, there is an exception. When the cable passes by a strong magnetic field radiation source (such as switching power supply), it will also sense the common mode interference with low frequency.
3、 How to suppress common mode interference
Common mode interference is the most common and harmful interference in EMC interference. The most direct way to suppress it is filtering.
Common mode inductors are connected in series in the circuit. When a common mode interference current flows through the coil, due to the directivity of the common mode interference current, a magnetic field in the same direction will be generated in the coil to increase the inductance of the coil, making the coil show high impedance and 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 will generate a reverse magnetic field in the common mode inductor coil wound in the same phase to offset each other
Case Common mode interference suppression method on USB signal
Filter processing of USB port - use common mode inductance The signal on USB transmission line is a differential signal and the interference source is a common mode interference signal. The common mode inductance on the transmission line can better suppress the common mode interference without any attenuation of the useful differential signal.
USB high-speed operation will generate strong common mode interference on the DM/DP signal line
The common mode interference signal is effectively suppressed by adding a filter common mode inductor to the circuit. If the common mode interference source is in the power circuit, the common mode capacitor can be used to suppress the interference signal.
If a common mode capacitor is introduced into the circuit, the common mode capacitor provides the shortest path to bypass the common mode interference signal, so as to suppress the generation of common mode interference. If differential mode interference also exists in the power circuit, differential mode capacitor is used to suppress the interference.
If differential mode capacitor is introduced into the circuit, the differential mode capacitor will provide the shortest path to bypass the differential mode interference signal, so as to suppress the generation of differential mode interference. Common mode interference is the most common and harmful interference in EMC interference. In addition to filtering, the common mode signal strength can also be reduced by shielding the signal circuit and laying the ground on the PCB in a large area to reduce the ground wire impedance.
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