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# A Brief Tutorial on Electromagnetic Compatibility (4) Common Mode Interference and Differential Mode Interference

Posted by: yiernuolashes 2022-11-09 Comments Off on A Brief Tutorial on Electromagnetic Compatibility (4) Common Mode Interference and Differential Mode Interference

Common mode refers to the existence of two or more wires, and the current flowing through all wires is of the same polarity, and differential mode refers to the opposite polarity of the current on the wire pair. .

Common Mode Interference and Differential Mode Interference

Common-mode interference (Common-mode): The interference currents on two wires have the same amplitude and the same direction as common-mode interference. Differential-mode interference (Differential-mode): The interference currents on two wires with equal amplitude and opposite directions are called differential-mode interference.  Common mode refers to the existence of two or more wires, and the current flowing through all wires is of the same polarity, and differential mode refers to the opposite polarity of the current on the wire pair. .

The interference current of common mode interference has the same amplitude/phase on all wires in the cable, and it forms a loop flow between the cable and the ground, as shown in Figure (a). The interference current of differential mode interference flows between the signal line and the signal ground line, as shown in Figure (b).

Since the interference currents of common mode interference and differential mode interference flow in different ways on the cable, the filtering methods for these two interference currents are also different. Therefore, it is necessary to understand the type of interference current faced before designing the filter. There are two forms when the change of voltage and current is transmitted through the wire, which we call “common mode” and “differential mode”. The power cord of the equipment, the communication line of the telephone, etc., and the communication line exchanged with other equipment or peripheral equipment, have at least two wires, and these two wires are used as a round-trip line to transmit power or signals. But there is usually a third conductor in addition to these two wires, which is the “ground”. The interference voltage and current are divided into two types: one is that two wires are used for round-trip transmission; the other is that two wires are used for outgoing, and the ground wire is used for return transmission. The former is called “differential mode” and the latter is called “common mode”.

Series mode interference (differential mode interference) and common mode interference (ground interference). Taking the two PCB traces on the motherboard (the wires connecting the components of the motherboard) as an example, the so-called series-mode interference refers to the interference between the two traces; while the common-mode interference refers to the two traces and the PCB ground. Interference caused by potential differences between lines.

choke coil

The main method to suppress common mode interference is to apply common mode choke coils. The common mode choke is the inductive component that dominates the common mode insertion loss. It is wound on the upper and lower half rings of a magnetic ring/closed magnetic circuit with the same number of turns but opposite coils. Differential mode interference means that the interference voltage exists between the signal line and its return line. The interference loop flows in the loop formed by the wire and the return wire.

The main method to suppress differential mode interference is to apply common differential choke coils. Differential mode choke coils are the inductive components that play a dominant role in differential mode insertion loss. It is wound with a single winding structure instead of two identical windings on one core like a common mode choke. The common mode Inductor is a common mode interference suppression device with a ferrite core. It consists of two coils of the same size and the same number of turns wound symmetrically on the same ferrite toroidal core to form a four-terminal The device has a suppressing effect on the common-mode signal with a large inductance, but has little effect on the differential-mode signal with a small leakage inductance. The principle is that when the common mode current flows through the magnetic flux in the magnetic ring, the magnetic flux in the magnetic ring is superimposed on each other, which has a considerable inductance, which inhibits the common mode current. When the differential mode current flows through the two coils, the magnetic flux in the magnetic ring The channels cancel each other out, and there is almost no inductance, so the differential mode current can pass through without attenuation. Power supply noise is a kind of electromagnetic interference, and the frequency spectrum of its conducted noise is roughly 10kHz~30MHz, up to 150MHz. According to the different propagation directions, power noise can be divided into two categories: one is the external interference introduced from the power supply line, and the other is the noise generated by the Electronic equipment and conducted through the power line. This shows that the noise is a two-way interference signal, and the electronic device is both an object of noise interference and a noise source.

From the perspective of formation characteristics, noise interference is divided into two types: series mode interference and common mode interference. Differential mode interference is the noise between two power lines (referred to as line-to-line), while common mode interference is the noise between the two power lines to the ground (referred to as line-to-ground). Therefore, the electromagnetic interference filter should meet the requirements of electromagnetic compatibility (EMC), and must also be a two-way radio frequency filter. Noise interference is emitted from the outside, so as not to affect the normal operation of other electronic devices in the same electromagnetic environment. In addition, the electromagnetic interference filter should suppress both series mode and common mode interference.

In most circuit designs, the value of the common mode inductance is usually 5-33mH, and the typical value is 10-33mH. The minimum value of the inductance can also be calculated according to the calculation formula of the common mode inductance. X1 is the impedance value when the frequency is f. This time, the common mode inductor of 33mH/3A is selected.

What is common mode residual voltage

common mode voltage: The average value of the phasor voltage that occurs between each conductor and a specified reference point (often earth or frame). In other words, the part of the input voltage added between the two measuring terminals of the voltmeter and the specified common terminal at the same time.

Differential voltage (symmetrical voltage): The voltage between any two of a specified set of live conductors. Therefore, the differential mode voltage is also called the symmetrical voltage.

In the specified waveform, the nominal discharge current impinges on the zinc oxide valve, and the peak voltage measured at both ends of the valve is called residual voltage. The ratio of the residual voltage to the varistor voltage, the residual voltage ratio. Lightning strikes and lightning will generate instantaneous high voltage and large current on the input/output power lines, which will affect the stable operation of user equipment, and may cause equipment damage in severe cases. The arrester can be divided into common mode connection method and differential mode connection method according to the connection method: the arrester is connected between the phase lines or between the phase line and the zero line, which is called the differential mode connection method, which is the so-called lateral protection. The arrester is connected between the phase wire and the ground wire or between the neutral wire and the ground wire, which is called the common mode connection method, which is the so-called longitudinal protection.

Eliminate common mode interference

(1) Use shielded twisted pair and effectively ground
(2) Keep away from high-voltage lines when wiring, and do not bundle high-voltage power lines and signal lines together.
(3) Use linear regulated power supply or high-quality switching power supply (ripple interference is less than 50mV)
(4) Using a differential circuit
(5) Connect a common mode choke coil in series in the signal line or power line, connect a capacitor in parallel between the ground and the wire, and form an LC filter for filtering to filter out common mode conduction noise.

Eliminate differential mode interference

(1) The premise is to reduce common mode interference, otherwise common mode interference may be converted into differential mode interference
(2) Using differential mode choke coils.