Common mode interference of EMC

1. What are common mode and difference mode

Power cord of electrical equipment , A communication line for a telephone, etc , A communication line that exchanges with other devices or peripherals , At least two wires , These two wires act as a back and forth line to transmit power or signals , There is usually a third conductor besides these two wires , This is it. " Ground wire ". 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 ".

2. Common mode interference and differential mode interference

Interference in any two power or communication 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 .

3. 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 .

4. 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 .

5. 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 the alternating magnetic field 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 .

6. 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 .

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 ; in addition , 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 .

7. How to identify common mode interference

1. Judging from the interference source : Thunder and lightning 、 An electric arc happened nearby 、 The interference of nearby radio station or other high-power radiation device on the cable is common mode interference .
2. Judging from the frequency : 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. Measure with instruments : As long as there is a spectrum analyzer and a current caliper can be measured 、 Determine the , The steps of judgment are as follows ：
   a. Clamp the current calipers on the signal wire or ground wire respectively ( A live or neutral line ) On , Record a certain frequency (f1) The interference intensity of .
   b. Clamp the current clamp on the signal wire and ground wire at the same time , If you can observe (f1) Interference at , be (f1) Interference contains common mode interference components , To determine if there is only common mode Interference components , Go ahead with the steps c Discrimination of .
   c. Clamp the caliper on the signal wire and ground wire respectively , If it's measured on two lines (f1) The magnitude of the interference is the same , be (f1) The interference contains only common mode interference components ; If not , be (f1) The interference also includes the differential mode interference component .

8. 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 , This is an important measure to suppress and prevent common mode interference . The function of the filter is to allow a certain frequency signal to pass smoothly , The signals of other frequencies are greatly suppressed , It is essentially a frequency selection circuit , It cuts off the path of electromagnetic interference along the signal line or power line , In addition, it is also an effective way to compress the interference spectrum , When the interference spectrum is different from the frequency band of the useful signal , You can use a filter to filter out unwanted interference signals . therefore , It is very important to select and use the filter properly to suppress the common mode interference .

If the useful signal is a differential mode signal and the interference signal is a common mode signal , Common mode inductors can be used to suppress interference signals ：

9. Principle of common mode inductance and interference suppression

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 .

10. summary

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 .

The article is from the official account , Reference to the original