Eight ways to solve EMC and EMI conducted interference

solve EMI Eight methods of conducting interference

Solve the problems in circuit design EMI Conducted interference is one of the difficulties faced by many electronic engineers , So how to solve the conducted interference ？ This paper gives a solution EMI Eight methods of conducting interference , It is hoped that these eight methods can help engineers solve the problem of conducted interference , Do a good job in circuit design .
electromagnetic interference EMI The interference signal generated by the electronic equipment in is transmitted through wires or public power lines , Mutual interference is called conduction interference . How to solve conducted interference ？ Find the right way , You'll find that , Conduction interference is actually very easy to solve , Just add the power input circuit EMC The number of filter sections , And adjust the parameters of each filter , Basically meet the requirements , Here are eight strategies , To solve the problem of conduction interference .

Method 1 to solve conducted interference ： Minimize the effective area of each circuit

Conducted interference differential mode interference DI And common mode interference CI Two kinds of . First, let's see how conduction interference is generated . Pictured 1 Shown , The circuit current produces conducted interference . There are several loop currents in it , We can think of each circuit as an induction coil , Or the beginning of the transformer coil 、 secondary , When there is current flowing through a circuit , In another circuit, induced electromotive force will be generated , To interfere with . The most effective way to reduce interference is to minimize the effective area of each circuit .

Method 2 to solve conducted interference ： shielding 、 Reduce the area of each current circuit and the area and length of the live conductor

One end of the common mode signal is the whole circuit board , The other end is the earth . The common end in the board cannot be counted as ground , Do not connect the common end to the housing , Unless the case is grounded , otherwise , The common end is connected to the housing , Will increase the effective area of the radiation antenna , Common mode radiation interference is more serious . Ways to reduce radiation interference , One is shielding , The other is to reduce the area of each current circuit （ Magnetic interference ）, And the area and length of the live conductor （ Electric field interference ）.

Method 3 to solve conducted interference ： Magnetic shielding of the transformer 、 Minimize the effective area of each current circuit

Among all the electromagnetic interference , The interference caused by transformer leakage inductance is the most serious . If the leakage inductance of the transformer is regarded as the primary of the induction coil of the transformer , Then other circuits can be regarded as the secondary circuit of the transformer , therefore , In the circuit around the transformer , Will be induced to produce interference signals . The way to reduce interference , On the one hand, the transformer is magnetically shielded , The other is to minimize the effective area of each current circuit

Method 4 for solving conducted interference ： Shield the transformer with copper foil

Shield the transformer , It is mainly to reduce the electromagnetic induction interference caused by the leakage flux of transformer to the surrounding circuit , And external electromagnetic radiation interference . In principle , Non magnetic materials can not directly shield the magnetic flux leakage , But copper foil is a good conductor , The alternating leakage flux will produce eddy current when it passes through the copper foil , And the direction of the magnetic field produced by the eddy current is just opposite to the direction of the leakage flux , Part of the leakage flux can be cancelled , therefore , Copper foil can also play a good role in shielding magnetic flux .

Five methods to solve conducted interference ： Two wire transmission and impedance matching

Two adjacent wires , If the current is the same , The current is in the opposite direction , Then the lines of force they produce can cancel each other . For circuits with serious interference or easy to be interfered , Try to use two-wire transmission , Don't use the public to transmit signals , The smaller the public ground current, the smaller the interference . When the length of the conductor is equal to or greater than one quarter of the wave length , Impedance matching must be considered in the transmission line , Mismatched transmission lines produce standing waves , And produce a strong radiation interference to the surrounding circuit .

Six methods to solve conducted interference ： Reduce the area of the current circuit

Pictured 6 Shown , The interference of magnetic field radiation is mainly caused by the flux flowing through the high frequency current circuit to the receiving circuit , therefore , Try to reduce the area of the high frequency current circuit and the area of the receiving circuit . In style ：e1、Φ1、S1、B1 They are respectively the electromotive force generated in the radiation current circuit 、 Flux 、 area 、 Flux density ;e2、Φ2、S2、B2 They are respectively the electromotive force generated in the radiation current circuit 、 Flux 、 area 、 Flux density .

Here's a picture 7 schematic , Explain the radiation of current circuit in detail . Pictured ,S1 Output filter circuit for rectifier ,C1 Filter capacitance for energy storage ,i1 For loop high frequency current , This current is maximum in all current circuits , It also produces the most serious magnetic interference , It should be minimized S1 The area of .
　　 stay S2 In the loop , Basically, there is no high-frequency loop current ∆I2 Mainly power ripple current , High frequency components are relatively small , therefore S2 The size of the area does not need to be considered .C2 Filter capacitance for energy storage , Specially for the load R1 Provide energy ,R1、R2 It's not just load resistance , It's a high frequency circuit load , high-frequency current i3 Basically by C2 Provide ,C2 Location is relatively important , Its connection position should consider making S3 The smallest area ,S3 One more ∆I3, It's mainly power ripple current , There are also a small amount of high-frequency current components . stay S4 In the loop , Basically there's no high-frequency loop current ,∆I4 Mainly power ripple current , High frequency components are relatively small , therefore S4 The size of the area does not need to be considered .S5 The situation of the loop is basically the same as S3 The circuit is the same ,i5 The area of the current circuit should also be as small as possible .

Seven methods to solve conducted interference ： Do not use multiple circuits to supply power in series

chart 7 Several current circuits in , Power is supplied in series with each other , It's very easy to produce current common mode interference , Especially in high frequency amplifier circuit , It can produce high frequency noise . The reason for the current common mode interference is ：∆I2 = ∆I3+ ∆I4+ ∆I5.

Eight methods to solve conducted interference ： Avoid the interference signal to produce resonance in the circuit

One pole of the common mode antenna is the whole circuit board , The other pole is to connect the ground wire in the cable . The most effective way to reduce radiation interference is to shield the whole circuit board , And the case is grounded . The reason of electric field radiation interference is that the conductor or lead is charged by high-frequency signal , The length and surface area of the conductor should be minimized . The reason for the magnetic field interference is that there is a high frequency current flowing through the conductor or circuit , The length and area of the current loop in the circuit board should be minimized . The higher the frequency is. , The more serious the electromagnetic interference ; When the length of the carrier fluid is comparable to the wavelength of the signal , Interference signal radiation will increase .

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