Elaborate on common mode interference and differential mode interference

One 、 Common mode signal and differential mode signal

Usually the power cord has three wires , Fire line L, Zero line N And ground PE.

There are two types of changes in voltage and current as they travel through the wire , One is that the two wires are used as the back and forth line transmission , We call it " Differential mode "; The other is two wires to make the way , Return transmission of ground wire , We call it " Common mode ".

 

Pictured above ,  The blue signal is transmitted back and forth between the two wires , We call it " Differential mode "; The yellow signal is transmitted between the signal and the ground , We call it " Common mode ".

Two 、 Common mode interference and differential mode interference

Interference on any two power lines , Both of them can be represented by common mode interference and differential mode interference .

Common mode interference is between conductor and ground （ Casing ） Transferred between , It's asymmetric interference , It is defined as the unwanted potential difference between any current carrying conductor and the reference ground ;

Differential mode interference is transmitted between two wires , It's symmetrical interference , It is defined as the unwanted potential difference between any two current carrying conductors .

In general , Large common mode interference 、 High frequency , Radiation can also be generated through wires , The interference is greater . The amplitude of differential mode interference is small 、 Low frequency 、 The interference is small .

2.1 Common mode interference signal

      

The current magnitude of common mode interference is not necessarily equal , But the direction （ phase ） same . The external interference of electrical equipment is mainly common mode interference , The external interference is mainly common mode interference , Common mode interference itself generally does not harm the equipment , But if common mode interference turns into differential mode interference , The interference is serious , Because useful signals are differential mode signals .

2.2 Differential mode interference signal

The current of differential mode interference is equal , Direction （ phase ） contrary . Because of the distributed capacitance of the wiring 、 inductance 、 Signal line impedance is not continuous , And the signal return path flows through an unexpected path , Differential mode current will be converted into common mode current .

2.3 Common mode interference causes

1. The power grid is connected with common mode interference voltage .

2. Radiation interference （ Like thunder and lightning , Equipment arc , A nearby radio station , High power radiation source ） Common mode interference is induced on the signal line , The reason is that the alternating magnetic field produces alternating current , Ground wire － Zero line loop area and ground wire － The area of live wire circuit is different , Different impedance of two circuits and other reasons cause different current .

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

4. Common mode interference caused by the circuit inside the equipment to the power line .

2.4 Common mode interference current

Common mode interference usually occurs in the form of common mode interference current , Generally, there are three reasons for the common mode interference current ：

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

 2. Because the ground potential of the devices at both ends of the circuit is different , The current generated by this ground potential difference .

 3. There is a potential difference between the circuit line on the device and the earth , In this way, common mode interference current will be generated on the circuit line .

2.5 matters needing attention

1. If the device produces common mode interference current on its circuit line , Then the circuit will produce strong electromagnetic radiation , For electrons 、 Electrical components produce electromagnetic interference , Affect the performance index of the product ;

2. When the circuit is unbalanced , Common mode interference current will change into differential mode interference current , The differential mode interference current directly affects the circuit . For electrons 、 In terms of signal line and circuit in electrical product circuit ： When the differential mode interference current flows through the wire loop in the circuit , Will cause differential mode interference radiation , This kind of loop is equivalent to a small loop antenna , Can radiate magnetic fields into space , Or receive a magnetic field .

3. Common mode interference is mainly concentrated in 1MHz above . This is because common mode interference is sensed to the cable through space , This kind of induction is only easy to occur at a higher frequency . But there is an exception , When the cable radiates from a strong magnetic field ( for example , Switching Mode Power Supply ) When passing by , The common mode interference with lower frequency will also be sensed .

3、 ... and 、 How to suppress common mode interference

Common mode interference as EMC The most common and harmful interference , The most direct way we can suppress it is filtering .

Insert a common mode inductor in series in the circuit , When there is a common mode interference current flowing through the coil , Because of the common mode interference current in the same direction , It will generate the same direction magnetic field in the coil and increase the inductive reactance of the coil , Make the coil behave as high impedance , Produce strong damping effect , So as to attenuate the common mode interference current , To achieve the purpose of filtering ;

When the normal differential mode current in the circuit flows through the common mode inductance , The current in the common mode inductance coil of the in-phase winding produces the reverse magnetic field and cancels each other , Therefore, it has no attenuation effect on the normal differential mode current .

Case study  USB Common mode interference suppression method on signal

USB Filter processing of port － Use common mode inductors

USB The signal on the transmission line is differential signal and the interference source is common mode interference signal , The common mode inductor on the transmission line can suppress the common mode interference , There is no attenuation for the useful differential signals .

USB High speed operation will be in DM/DP Strong common mode interference is generated on the signal line

Add a filter to the circuit － The common mode interference signal is effectively suppressed after the common mode inductance

If the common mode interference source is in the power circuit , Common mode capacitors can be used to suppress interference signals .

Introduce a common mode capacitor into the circuit , The common mode capacitor provides the shortest path for the common mode interference signal to be bypassed , So as to suppress the generation of common mode interference .

If the power circuit also has differential mode interference , Use differential mode capacitance to suppress interference .

Introduce a differential mode capacitor into the circuit , The differential mode capacitor provides the shortest path for the differential mode interference signal to be bypassed , So as to suppress the generation of differential mode interference .

Common mode interference as EMC The most common and harmful interference , Besides filtering, the way to suppress it , It can also shield the signal line , stay PCB To reduce the common mode signal strength by laying a large area of ground wire on the board .