Introduction to common mode testing and differential mode testing_Century Wisdom (Suzhou) Testing Technology Co., Ltd

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Introduction to common mode testing and differential mode testing

Release time:

2022-10-28 00:00

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　　1. Common mode signal and differential mode signal

　　Usually the power line has three wires (single phase), live wire L, neutral wire N and ground wire PE.

　　There are two forms of voltage and current transmission through wires. One is that two wires are used as a round-trip transmission, which we call "differential mode"; the other is that two wires are used as an outgoing path, and the ground wire is used as a return transmission, we call it "common mode".　

　　As shown in the figure above, the blue signal is transmitted back and forth inside the two wires, which we call "differential mode"; the yellow signal is transmitted between the signal and the ground wire, which we call "common mode".

　　2. Common mode interference and differential mode interference

　　The interference existing on any two power lines can be represented by common mode interference and differential mode interference.

　　Common mode interference: transmitted between the wire and the ground (chassis), it is an asymmetric interference, which is defined as an undesired potential difference between any current-carrying conductor and the reference ground;

　　Differential mode interference: transmitted between two wires, it is a symmetrical interference, which is defined as an undesired potential difference between any two current-carrying conductors.

　　In general, the common-mode interference has a large amplitude and high frequency, and can also generate radiation through wires, causing relatively large interference. The differential mode interference has small amplitude and low frequency, and the interference caused by it is small.

　　2.1 Common mode interference signal

　　The magnitude of the current of common mode interference is not necessarily equal, but the direction (phase) is the same. The external interference of electrical equipment is mostly common-mode interference, and the external interference is mostly common-mode interference. Common-mode interference itself generally does not cause harm to the equipment, but if the common-mode interference changes to differential mode interference, the interference will be Seriously, because the useful signals are all differential mode signals.

　　2.2 Differential mode interference signal

　　The currents of differential mode interference are equal in size and opposite in direction (phase). Differential-mode currents are converted to common-mode currents due to trace distributed capacitance, inductance, signal trace impedance discontinuities, and signal return paths that flow through unintended paths.

　　2.3 Causes of Common Mode Interference

　　1. Common mode interference voltage is connected in series to the power grid.

　　2. Radiation interference (such as lightning, equipment arcing, nearby radio stations, high-power radiation sources) induces common-mode interference on the signal line. The area of the wire-live wire loop is different, and the impedance of the two loops is different, which causes the current magnitude to be different.

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

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

　　2.4 Common mode interference current

　　Common-mode interference generally appears in the form of common-mode interference current. Generally, there are three reasons for common-mode interference current:

　　1. The external electromagnetic field induces a voltage on all the wires in the circuit trace (this voltage is equal in magnitude and in phase with respect to the ground), and the current generated by this voltage.

　　2. Due to the different ground potentials connected to the devices at both ends of the circuit trace, the current generated under the drive of this ground potential difference.

　　3. There is a potential difference between the circuit traces on the device and the ground, so that common-mode interference currents will be generated on the circuit traces.

　　2.5 Precautions

　　1. If the device generates common-mode interference current on its circuit traces, the circuit traces will generate strong electromagnetic radiation, which will cause electromagnetic interference to electronic and electrical product components and affect product performance indicators;

　　2. When the circuit is unbalanced, the common-mode interference current will be converted into a differential-mode interference current, and the differential-mode interference current will directly interfere with the circuit. For the signal line and its loop in the circuit 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 spread to space Radiate magnetic fields, or receive magnetic fields.

　　3. Common mode interference is mainly concentrated above 1MHz. This is due to the fact that common-mode interference is induced onto the cable through space, and this induction is only prone to occur at higher frequencies. But there is an exception. When the cable passes by a strong magnetic field radiation source (for example, a switching power supply), common-mode interference with a lower frequency will also be induced.

　　3. How to suppress common mode interference

　　Common mode interference is the most common and harmful interference in EMC interference. The most direct way for us to suppress it is to filter.

　　A common-mode inductor is connected in series in the circuit. When there is a common-mode interference current flowing through the coil, due to the isotropy of the common-mode interference current, a magnetic field in the same direction will be generated in the coil to increase the inductive reactance of the coil, so that the coil behaves It is 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 inductor coil wound in the same phase to cancel each other out, so there is basically no attenuation effect on the normal differential-mode current.

　　Common Mode Interference Suppression Method on Example USB Signals

　　Filtering for USB Ports - Using Common Mode Inductors

　　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 connected in series 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 after adding a filter-common mode inductor to the circuit

　　If the source of common mode interference is in the power loop, common mode capacitors can be used to suppress the interference signal.

　　When a common mode capacitor is introduced into the circuit, the common mode capacitor provides the shortest path to bypass the common mode interference signal, thereby suppressing the generation of common mode interference.

　　If there is differential mode interference in the power circuit at the same time, use a differential mode capacitor to suppress the interference.

　　Introducing a differential-mode capacitor into the circuit, the differential-mode capacitor provides the shortest path to bypass the differential-mode interference signal, thereby suppressing the generation of differential-mode interference.

　　Common mode interference is the most common and harmful interference in EMC interference. In addition to filtering, the method of suppressing it can also reduce the common mode signal by shielding the signal line and laying a large area on the PCB to reduce the ground impedance. Strength etc.

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