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EMC isolation design method in electronic products

2020-12-06 11:08:23 Procedural ape without hair loss

Catalog

1、 Isolation classification

2、EMC In isolation analysis

2.1、 Optocoupler isolation

2.2、 Relay isolation

2.3、 Common mode choke ( Common mode inductance ) Isolation


Design in electronics , In order to cut off the internal circuit of the product and the external interference transmission channel , Or for security reasons , Usually in I/O Port or internal circuit signal transmission process using isolation mode , This isolation technology is EMC One of the important technologies in , Its main purpose is to try to cut off the path of noise interference through isolation elements , So as to achieve the effect of suppressing noise interference . At low frequencies , After taking measures of isolation , Most of the circuits can achieve good noise suppression effect , Make the device low frequency EMC The requirements of .

1、 Isolation classification

Common circuit isolation is commonly used in the following situations :

  • Isolation in analog circuits

For analog signal measurement system , The isolation circuit is relatively complex , Both the accuracy and the accuracy should be considered 、 The bandwidth factor , Also consider the price factor .

At the same time, there is high voltage 、 High current signal , And there's Micro voltage 、 Micro current signal , These signals need to be isolated , Achieve isolation at a certain frequency .

  • Isolation in digital circuits

The digital input system mainly adopts pulse isolation transformer 、 Optocoupler isolation ; And the digital output system mainly uses optocoupler isolation 、 Relay isolation , In some cases, high frequency isolation transformer can be used for isolation .

  • Isolation between analog and digital circuits

Generally speaking , The conversion between analog circuit and digital circuit is through mode / Data converter (A/D) Or number / A / D converter (D/A) To achieve . however , If we don't take some measures , High frequency periodic signal in digital circuit will bring certain interference to analog circuit , Affect the accuracy of measurement .

In order to suppress the interference of digital circuit to analog circuit , Generally, analog circuits and digital circuits must be wired separately , But this kind of wiring method can not completely eliminate the interference from digital circuits . To eliminate interference from digital circuits , Digital circuits can be isolated from analog circuits . The common method of isolation is in A/D A photoelectric coupler is added between the converter and the digital circuit , Separate digital circuits from analog circuits . If this kind of circuit can not fundamentally solve the interference problem in analog circuit , The signal receiving part is also isolated from the analog processing part .

for example , In the pre-processing stage with analog-to-digital converter (A/D) Add a linear isolation amplifier between , In the mold / Data converter (A/D) It is separated from digital circuit by optocoupler , Separate analog from digital . thus , It not only prevents the interference of the digital system into the analog part , It also blocks the common mode interference and differential mode interference from the front-end circuit . Digital to analog (D/A) Circuit isolation and analog to digital conversion (A/D) The isolation of circuits is similar to , So the technical measures adopted are similar .

2、EMC In isolation analysis

The next step is to isolate by optocoupler 、 Relay isolation and common mode choke ( Common mode inductance ) Isolation cases , understand EMC In isolation design method .

2.1、 Optocoupler isolation

The optocoupler has a small size 、 Long service life 、 Wide operating temperature range 、 No contact, etc , So it is widely used in various electronic equipment . Optocouplers can be used to isolate circuits 、 Load interface and various industrial products 、 In electrical circuits such as household appliances . In hardware, photoelectric coupler is often used to realize " electric — light — electric " The isolation , To some extent, it can destroy the entry of common mode current , Reliable isolation of signals , It is easy to form various functional states .

Using optocouplers ( Optocoupler for short ) You can cut off the direct connection to the electrical signal , The transmission of signals by light . Photoelectric isolation is accomplished by optocoupler . Optoelectronic coupling devices are light-emitting devices ( Such as LED ) And photosensitive devices ( Such as photosensitive triode ) Put it together , Electricity formed by photoelectric coupling — Light and light — Electrical conversion devices . The following figure is the schematic diagram of the commonly used triode optocoupler , Work with light as a medium to transmit information , So the input and output are electrically isolated .

There is parasitic capacitance between the two ends of the optocoupler isolated by optical signal , It's usually 2pF, An optocoupler can provide good isolation at very high frequencies . When the electrical signal is sent to the input of the optocoupler , Light emitting diodes emit light through an electric current , When a photosensitive element is illuminated, it produces an electric current , The photosensitive triode is on ; When there is no signal at the input , The LED doesn't work , Photosensitive triode cut off . For digital quantities , When the input is low “0” when , Photosensitive triode cut off ; When the input is high “1” when , Saturation conduction of photosensitive triode .

The optocoupler has a series of characteristics mentioned above , It has been widely used in electronic products , Especially in the measurement control system , It has become a very important isolation device in interface technology .

  • Photoelectric isolation in MCU interface circuit

MCU has multiple input ports , Receive status signals from remote field devices , The single chip microcomputer processes these signals , Output various control signals to perform the corresponding operation . When the site environment is bad , There will be large noise interference , If these interferences enter the microcomputer system along with the input signal , It will reduce the accuracy of control , There is a malfunction .

therefore , It can be used in the input and output of single chip microcomputer , Using optocoupler as interface , Isolation of signal and noise . If in "A/D converter " Digital signal output of the photoelectric isolation , Realize the isolation of analog circuit and digital circuit , It can suppress the common mode interference . For linear analog circuit channels , It is required that the optocoupler must have the characteristics of linear conversion and transmission .

  • Photoelectric isolation of power drive circuits

In the control system , Photoelectric isolation is widely used in switch control , These switching values are generally passed through the SCM I/O Output , and I/O The driving power of the company is limited , Generally not enough to drive some electromagnetic actuators , Need to add the driver interface circuit , To avoid interference , Isolation measures must be taken . For example, the main circuit where the thyristor is located is generally a strong AC circuit , The voltage is high , The current is high , It's not easy to connect with MCU directly , Optical coupler can be used to isolate the microcomputer control signal from the thyristor trigger circuit , The schematic diagram of bidirectional thyristor isolation drive is shown below :

  • Long distance isolated transmission

In the measurement and control system , Due to the long-term transmission between the measurement and control system and the equipment under test and controlled, it is inevitable to carry out long-term transmission , The signal is easily disturbed in the transmission process , Causing distortion or distortion of the transmitted signal . in addition , Between distant devices connected by longer cables , It is often due to the ground potential difference between the equipment ( At low frequencies ), Common mode ground loop current resulting in low frequency , The differential mode interference voltage is formed on the circuit . To ensure the reliability of long-distance transmission , Optocoupler isolation can be used , Improve the anti-interference performance of the circuit system . If the transmission line is long 、 There's a lot of interference on the scene , The long line can be completely " Floating " get up , Optical coupling floating processing of long transmission line , As shown in the figure below :

Long line transmission “ Floating ” Is to remove the common ground wire at both ends of the long line , It not only effectively eliminates the noise generated by the current of each circuit passing through the common ground wire, but also forms mutual crosstalk , It also solves the problem of long line driving and impedance matching . meanwhile , When the controlled equipment is short circuited , And protect the system from damage . But this kind of “ Floating ” Only for low frequency , At high frequencies ,“ Floating ” There will be serious EMC problem . Many people think that optocoupler is the best way to cut off the interference path , However, although the optocoupler has the above advantages EMC The characteristics and advantages of , When using optocoupler for isolation , Also note the following :

1、 Independent and isolated power supply must be used in the input part and output part of the optocoupler respectively , If both ends share the same power supply , Then the isolation effect of the optocoupler will lose its significance . Even isolated power , To ensure high frequency isolation , It is necessary to ensure that the isolation of the power supply is equivalent to that of the optocoupler , That is, the parasitic capacitance between two groups of independent power supply is equivalent to the parasitic capacitance at both ends of the optocoupler , otherwise , The high frequency signal will pass through the power supply to the other side of the isolation , Failure of isolation at high frequencies . actually , The parasitic capacitance between the primary stages of power isolation transformer is relatively large , At high frequencies , The power supply is not well isolated , This reduces the overall isolation effect .

2、 When optocouplers are used to isolate the input / Output channel , It has to be on all signals ( Including digital signals 、 Control signal 、 Status signals ) All quarantined , So that there is no electrical connection between the isolated sides , Otherwise, the isolation is meaningless .

3、 When multiple signals are isolated , Multiple optical couplers are used in parallel , This will reduce the high frequency isolation of the whole circuit , Because the parallel connection of multiple optocouplers increases the total parasitic capacitance between the two ends of the optocoupler , Leading to poor isolation effect of high frequency .

4、 Because the optocoupler is not completely isolated in the sense of high frequency , So in product design , When interference is applied to one end of the optocoupler , The signal at the other end of the optocoupler should also be filtered .

There are two ways of optocoupler filtering :

  • For optocouplers with base terminals , The filter capacitor is paralleled to the base terminal , The value of filter capacitance is in 100pF above , The specific value depends on the working frequency of the optocoupler , As shown in the figure below :

  • For optocouplers without base terminals , The filter capacitor is paralleled to the collector terminal , The value of filter capacitance is also in 100pF above , The specific value depends on the working frequency of the optocoupler . As shown in the figure below :

2.2、 Relay isolation

Electromagnetic relay generally consists of iron core 、 coil 、 Armature 、 Contact spring, etc . As long as a certain voltage is applied to both ends of the coil , A certain amount of current will flow through the coil , And then the electromagnetic effect , The armature will overcome the pulling force of the return spring to the iron core under the action of electromagnetic attraction , So as to drive the moving contact and the static contact of the armature ( Normally open contact ) Imbibition . When the coil is de energized , The electromagnetic attraction disappears , The armature will return to its original position under the reaction of the spring , Make the moving contact and the original static contact ( Normally closed contacts ) Imbibition . This sucking in 、 Release , To achieve the conduction in the circuit 、 The purpose of the cut is . For relays “ Normally open 、 Normally closed ” contact , It can be distinguished in this way : When the relay is not energized, the static contact in the open state is called “ Normally open contact ”, A static contact in the on state is called “ Normally closed contacts ”.

A relay is actually an electronic control device , It has a control system ( Also called input loop ) And the controlled system ( Also called output loop ), Usually used in automatic control circuit , It's actually a way of controlling a larger current with a smaller current “ Automatic switch ”, It plays an automatic adjustment in the circuit 、 safeguard 、 Switching circuit, etc . As shown in the figure below , When A Point in high level , Crystal triode T Saturated conduction , Relay J Imbibition ; When A The point is low level ,T end , Relay J Then release , Completed the signal transmission process .D It's a protective diode . When T From conduction to cutoff , High back EMF is generated at both ends of the relay coil , To continue to maintain the current I. Since the back EMF is usually very high , Easy to cause T The breakdown of . Add diodes D after , It provides a discharge circuit for the back EMF , Thus protecting the triode T.

Extended learning : How to design the transistor control relay circuit

In applications, the isolation of relays is usually used with optocouplers , The schematic diagram is as follows :

2.3、 Common mode choke ( Common mode inductance ) Isolation

In the context of discussing common mode interference , There is an inductance device used to suppress common mode interference called common mode inductance (Common modeChoke) Also called common mode choke . Common mode inductors are not like optocoupler isolation 、 Relay isolation is a kind of isolation device , The isolated ends of these devices , To transmit by magnetic or optical signals . The common mode inductance is in EMC When applied in the field , The main function is to isolate common mode interference from common mode inductor input / Both ends of the output .

The schematic diagram of the common mode inductor is shown in the figure below . In the figure La and Lb It's a common mode inductance coil . These two coils are wound on the same core , Turns and phases are the same ( Reverse winding ). such , When the normal current in the circuit flows through the common mode inductor , The current in the same phase winding inductance coil produces the reverse magnetic field and cancels each other , At this time, the normal signal current is only affected by the coil resistance ( And a small amount of damping due to leakage ); When there is a common mode current flowing through the coil , Because of the isotropy of the common mode current , 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 , In this way Decaying common mode current , To filter Purpose . Another characteristic of common mode inductors is , To prevent magnetic saturation , The common mode inductor can use a high permeability core and a relatively high inductance can be obtained when the core is relatively small , This is why one common mode inductor cannot be replaced by two differential mode inductors connected in series on the signal line and the signal return line respectively .

Common mode choke is usually used in series between balanced transmission signals such as differential signals , Cannot be used in unbalanced signal circuits . In industry RS485、CAN Common mode choke is often used in the design of communication bus , Industry RS485 An example of a bus schematic is shown below :

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