Shulou(Shulou.com)06/01 Report--

The cause of electromagnetic compatibility EMC and simulation test, aiming at this problem, this article introduces the corresponding analysis and solution in detail, hoping to help more partners who want to solve this problem to find a more simple and feasible method.

At present, the worsening electromagnetic environment makes us pay more and more attention to the working environment of the equipment and the impact of the electromagnetic environment on the electronic equipment. Starting from the design, we integrate into the electromagnetic compatibility design to make the electronic equipment work more reliably.

Electromagnetic compatibility design mainly includes surge (shock) immunity, ringing wave surge immunity, electrical fast transient burst immunity, voltage sag, short-term interruption and voltage change immunity, power frequency power supply harmonic immunity, electrostatic immunity, radio frequency electromagnetic field radiation immunity, power frequency magnetic field immunity, pulsed magnetic field immunity, conducted disturbance, radiated disturbance, conducted immunity of radio frequency field induction and so on.

The main forms of electromagnetic interference

Electromagnetic interference mainly enters the system through conduction and radiation, affecting the work of the system, and other ways include co-impedance coupling and inductive coupling.

Conduction: conductive coupling means that disturbance on one electric network is coupled to another through a conductive medium, which belongs to the part of lower frequency (lower than 30MHz). The ways of conducting coupling in our products usually include power lines, signal lines, interconnection lines, grounding conductors and so on.

Radiation: the disturbance on one electric network is coupled to another through space, which belongs to the part with higher frequency (higher than 30MHz). The way of radiation is transmitted through space, and the radiation interference introduced and produced in our circuit is mainly the antenna effect formed by various wires.

Common impedance coupling: mutual interference when the currents of more than two different circuits flow through the common impedance. The disturbance current conducted on the power cord and grounding conductor is often introduced into the sensitive circuit in this way.

Inductive coupling: through the principle of mutual inductance, the electrical signal transmitted in one loop is sensed to another circuit to cause interference. It is divided into two types: electrical induction and magnetic induction.

We should adopt the corresponding countermeasures to the interference caused by these ways: filtering the conduction (for example, the head capacitance of each IC in our design plays a filtering role), radiating interference by reducing the antenna effect (such as signal close to the ground wire), shielding and grounding and other measures, can greatly improve the product's ability to resist electromagnetic interference, but also effectively reduce the electromagnetic interference to the outside world.

Electromagnetic compatibility design

For the R & D and design process of a new project, the electromagnetic compatibility design needs to run through the whole process. Only by considering the electromagnetic compatibility design in the design, can we avoid rework, avoid repeated research and development, and shorten the time to market of the whole product. Improve the efficiency of the enterprise.

From R & D to market, a project needs to go through several stages, such as demand analysis, project establishment, project outline design, project detailed design, sample trial production, function test, electromagnetic compatibility test, project production, market and so on.

In the stage of demand analysis, it is necessary to carry out product market analysis, on-site research, excavate useful information for the project, integrate the development prospects of the project, sort out the working environment of the project products in detail, inspect the installation location, and whether there is any restricted space for installation. Whether the working environment is special, whether there is corrosion, humidity, high temperature, etc., the working conditions of the surrounding equipment, whether there is a bad electromagnetic environment, whether it is limited with other equipment Whether the successful development of the product can greatly improve the production efficiency, or whether it can bring great convenience to people's living or working environment, and whether the mode of operation and use can be easily accepted by people, this requires that the project products should meet the functional needs of the site, easy to operate and so on. Finally, it is necessary to sort out a detailed demand analysis report for demand review.

After the review of the relevant responsible persons within the enterprise, improve the requirements analysis report, and then the project establishment, the project needs to set up a project team, software, hardware, structure, testing and other personnel to the project team, assign their respective responsibilities. The next stage of the project development is the project outline design, which divides the project into multiple functional modules, uses the WBS decomposition structure to decompose and refine the function of the project, and arranges time and personnel according to the workload. Sort out the summary design report of the project, evaluate the project as a whole, determine the type of power supply, power distribution, power isolation filter mode, system grounding mode, product shielding, product structure adopts shielding design, adopt shielding chassis case, analyze signal types, and take protective measures against lightning, static electricity, group pulse and other interference.

After the product outline design report comes out, it will be reviewed by the relevant personnel to analyze whether the implementation method is reasonable and whether the implementation plan is feasible. The review report shall be given by the reviewer. After the summary design is modified by the project team combined with the review report, the product will enter the detailed design stage, which includes schematic design, PCB design, PCB procurement and welding, software writing, function debugging and so on. The schematic design should take into account the influence of electromagnetic compatibility, add the filter capacitor to the board-level power supply and the filter circuit to the interface part of the signal, and select the appropriate filter circuit according to the signal type. If the signal is a low frequency type, the low-pass filter circuit should be selected, the appropriate cutoff frequency should be calculated, and the corresponding resistance and capacitance should be selected. In addition, a large current discharge circuit is designed for the interface part, and lightning protection devices are set up to achieve the third level of lightning protection.

I. selection of components

Our commonly used electronic devices include active devices and passive devices, active devices mainly refer to IC and module circuits and other devices, passive devices mainly refer to resistors, capacitors, inductors and other components. Below, the selection of these two types of components and the issues to be considered in terms of electromagnetic compatibility are introduced.

Selection of active device EMC

The EMC with wide operating voltage has good characteristics, the EMC with low working voltage has good characteristics, the characteristics with large delay (usually referred to as slow speed) are better within the allowable range of design, the characteristics with low static current and low power consumption are better than those with large power consumption, and the EMC performance of patch packaged devices is better than that of plug-in devices.

Passive component selection

Passive devices usually include resistors, capacitors and inductors in our applications. For the selection of passive devices, we should pay attention to the frequency characteristics and distribution parameters of these components.

Passive devices show different characteristics at some frequencies, some resistors have inductance characteristics at high frequency, such as wire-wound resistance, electrolytic capacitors have good low-frequency characteristics, poor high-frequency characteristics, while thin-film capacitors and ceramic capacitors have better high-frequency characteristics. but it's usually small. Considering the influence of temperature on components, devices with various temperature characteristics are selected according to the design principle.

II. PCB design

When designing the printed board, we should take into account the influence of interference on the system, strictly separate the analog part of the circuit from the digital part of the circuit, focus on the protection of the core circuit, surround the ground wire of the system, and wire as thick as possible, increase the filter circuit of the power supply, adopt DC-DC isolation, adopt photoelectric isolation of the signal, and design the isolation power supply. Analyze the parts that are easy to produce interference (such as clock circuit, communication circuit, etc.) and those that are easy to be interfered (such as analog sampling circuit, etc.), and take measures for these two types of circuits respectively. Suppression measures are taken for interference elements, isolation and protection measures are taken for sensitive elements, and they are spaced and electrically separated. In the board-level design, we should also pay attention to the placement of components away from the edge of the printed board, which is beneficial to the protection of air discharge.

The schematic design of the sampling circuit is shown in figure 1:

Figure 1: sampling circuit design.

The reasonable layout of the circuit can reduce the interference and improve the electromagnetic compatibility. According to the function of the circuit, several functional modules are divided, and the interference sources and sensitive signals of each module are analyzed for special processing.

When wiring the PCB, you need to pay attention to the following aspects:

1. Keeping the loop area minimum, such as the loop formed between the power supply and the ground, reducing the loop area will reduce the induced current of electromagnetic interference on this loop, and the power line will be as close to the ground wire as possible to reduce the loop area of differential mode radiation. reduce the impact of interference on the system and improve the anti-jamming performance of the system. The parallel wires are tightly held together, connected by a thick wire, and the signal lines are routed close to the ground plane to reduce interference. A high frequency filter capacitor is added between the power supply and the ground.

2. Shorten the length of the wire as much as possible, reduce the area of the PCB and reduce the interference on the wire.

3. The complete ground plane design and multi-layer board design are adopted to lay the stratum to facilitate the release of interference signals.

4. Keep the electronic components away from the plane where discharge may occur, such as chassis panels, handles, screws, etc., to keep the shell in good contact with the ground and provide a good discharge channel for interference. Packet processing of sensitive signals to reduce interference.

5. Use chip components as far as possible, and the EMC performance of chip devices is much better than that of plug-in devices.

6. Analog and digital grounding at the connection between PCB and the outside world.

7. The high-speed logic circuit should be close to the edge of the connector, the low-speed logic circuit and memory should be arranged away from the connector, and the medium-speed logic circuit should be arranged between the high-speed logic circuit and the low-speed logic circuit.

8. The width of the printed line on the circuit board should not change suddenly, and the corners should be arc-shaped, not right or sharp corners.

9. The clock line and signal line are also as close to the ground line as possible, and the wiring is not too long to reduce the loop area of the loop.

Third, system wiring design

After the PCB is designed, trial production, welding debugging, system installation, electromagnetic compatibility design factors are considered, cabinet structure and cable design need to pay attention to the following aspects:

1. The cabinet uses electromagnetic shielding cabinet, which has good shielding performance, shielding the system well and reducing the influence of external electromagnetic interference on the system.

2. Shielded power cord and magnetic ring are selected for the input line of the total power supply, and the shielding layer is grounded 360 degrees at the place where it enters the cabinet.

3. The shielding wire is selected for the external signal line of the system, and the entrance of the cabinet in the shielding layer is well grounded.

4. The outer shell of the equipment is close to the cabinet to avoid crossing.

5. The system is equipped with isolation transformer and ups to ensure the pure power supply of the system.

6. strictly separate the power cord from the signal line, make good contact between the surfaces of the equipment shell and between the boards and panels, and the contact resistance should be less than 0.4

Oh, the smaller the better, to ensure that the equipment shell is well connected to the earth, so that when there is electrostatic discharge, it will not affect the normal operation of the system.

IV. System grounding design

Grounding is the most effective way to suppress the source of disturbance, which can solve 50% of the EMC problem. The system is datum connected to the earth and can restrain electromagnetic disturbance. The metal parts of the shell are directly connected to the earth and can also provide a leakage path of electrostatic charge to prevent the accumulation of static electricity.

1. The concept of ground wire

Safe grounding includes protective grounding and lightning protection grounding.

Protective grounding provides a low impedance channel for the fault current of the product to enter the earth.

Lightning protection grounding provides a way to release large current.

The reference grounding provides the reference level for the stable and reliable operation of the product and the reference potential for the power supply and signal.

Safe grounding is to provide a discharge circuit for high current and high voltage when there are some electrical abnormalities, which is mainly a protective measure for the circuit. The reference ground is mainly the signal place and the power source place, which is the basis to ensure the function of the circuit.

2. Grounding mode

Suspension grounding is generally not a problem for an independent system with no interface with the outside, but if there are interfaces such as communication ports and sampling lines between the system and other systems, then suspension grounding is easily affected by static electricity and lightning, so most of the general electronic products do not use suspension grounding.

Single-point grounding can be selected when f "1MHz, which can be divided into parallel single-point grounding and multi-stage circuit series single-point grounding.

Parallel single point grounding: each circuit module is connected to a single point ground, and each unit is connected to the reference point at the same point.

Series single point grounding of multistage circuits: connect the ground of circuits with similar characteristics to form a common point, and then connect each common point to a single point.

Multi-point grounding will be used when f "10MHz. All the circuits in the equipment take the grounding bus as the reference point.

Single-point grounding circuits are connected at the same point to provide a common potential reference point, there is no co-impedance coupling and low-frequency ground loop, but there is a large ground impedance to high-frequency signals. Multi-point grounding is the nearest grounding, each ground wire can be very short, providing a lower grounding impedance. Which grounding method 1MHz~10MHz can choose according to actual needs.

Hybrid grounding combines the advantages of single-point grounding and multi-point grounding. Single-point grounding is used for the low-frequency part of the system and multi-point grounding is used for the high-frequency part of the system.

The shielding grounding of signal line can be divided into high frequency and low frequency, high frequency adopts multi-point grounding, and low frequency cable adopts single point grounding. Low-frequency electric field shielding requires single-point grounding at the receiving end, and low-frequency magnetic field shielding requires grounding at both ends. Multi-point grounding, in addition to grounding at both ends, and grounding at the interval of 3 beat, 20 or 10 working wavelengths.

The system can be well grounded in order to effectively restrain electromagnetic interference. A large system cabinet should first ensure that each surface has good contact and compact contact, and secondly, the internal equipment of the cabinet should be grounded nearby to avoid secondary interference and release electromagnetic interference nearby. The interface shielding line should be looped and then connected to the nearest rack. The grounding copper bar is set under the cabinet, and the copper belt is better for the total ground wire of the system, which can discharge the electromagnetic interference well and ensure the normal operation of the system.

Electromagnetic compatibility test

After meeting the functional needs of the field, the system function test is carried out for electromagnetic compatibility test. the problems in electromagnetic compatibility test are static electricity, group pulse, surge, radio frequency field conduction and so on.

1. Detection of electrostatic immunity

Participated in the electrostatic immunity test of several projects, have a certain understanding of static electricity. Static electricity is divided into contact discharge and air discharge. Static electricity is the accumulated high voltage, which will discharge instantly when it comes into contact with the metal shell of the equipment, which will affect the normal work of the electronic equipment and may cause equipment failure or restart. This is not allowed in situations with good safety requirements.

Static electricity will affect the display effect, may appear display flicker or black screen, affecting the normal display and operation. Static electricity may also cause abnormal CPU operation, program crash or restart.

If the relevant design of electromagnetic compatibility is adopted in the detailed design stage of the product, there is no need to worry too much about the electrostatic test. through the design, the good discharge of the accumulated electrostatic charge will not affect the normal operation of the system.

2. Immunity detection of electrical fast transient burst.

The electrical fast transient pulse group is a series of high frequency and high voltage transient pulses applied to the equipment to observe whether the equipment is affected by it. The main method to protect the group pulse is "dredging" and "blocking". "dredging" is to provide a release loop, which is that the interference is released to the earth before entering the system, and a good shield is grounded, which can release most of the dynamic interference. "blocking" is to filter the group pulse outside the equipment and add the magnetic ring, the effect is obvious, the effect of the closed magnetic ring is better than that of the buckle magnetic ring, and the magnetic ring can also be added to the board level and fixed in the printed board. This makes the equipment more reliable.

Adding magnetic rings to both ends of power lines, signal lines and communication lines can protect against group pulse interference.

3. Lightning surge detection

Lightning surge mainly includes two aspects, one is power supply lightning protection, the other is signal lightning protection.

The power supply lightning protection is mainly aimed at the system level, the system level design should be in accordance with the three-level lightning protection design, the total power supply entry end is equipped with power supply lightning protection (such as V20-C/3-PH 385 of OBO Company), which can protect the power supply of the system at the first level. After the power supply lightning protection, the power supply enters the isolation transformer. The isolation transformer can better protect the electromagnetic interference signal and restrain its impact on the system. After entering the UPS,UPS, part of the interference signal can be filtered, so the power supply enters the system equipment again. The power supply is a kind of pure power supply, which can make the system work better and more reliably.

Signal lightning protection is to protect the signal path of the system, which mainly involves board-level design. Lightning protection devices such as gas discharge tube and TVS discharge circuit are added in the board-level design. When there is a large current, it is discharged through matching resistance, TVS and gas discharge tube to protect the later circuit. Then the signal is optically isolated and then enters the system, and the system can collect a stable signal, so that the system can analyze and judge normally, issue instructions normally, and work normally. On the other hand, a wide signal range is designed. When the signal fluctuates normally, the system works normally.

4. Immunity detection of RF field induction conduction.

The radiation sensitivity test may affect the display signal, acquisition driver, etc., may make the display flicker or black screen, affect the operation of the equipment, may make the acquisition driver work abnormally, can not collect the required signal, and can not drive the field equipment.

The radio frequency test is to interfere with the signal line and power supply in the range of 0.15k~80M frequency, and the third level strength is 10V/m.

The principle of radiation protection is to shield the power supply and signal line, ground the shield layer well, select the appropriate frequency to filter and filter the interference.

5. radiation emission detection, radio frequency field radiation immunity detection

The main purpose of this test is to test the anti-radio frequency signal and the overall shielding performance of the system. As long as the system is well shielded and the ground wire of the system is well grounded, the system can pass the test.

Through the relevant electromagnetic compatibility testing, the product can be put into the market for trial operation, and summarize the problems in the trial operation in order to prepare for the improvement of the product.

Electronic products meet the relevant electromagnetic compatibility testing standards, through the test, can be introduced to the market, users can rest assured to use, greatly reduce the accidents caused by electromagnetic interference, and play a positive role in the benefit of enterprises and the promotion of products.

The reasons for electromagnetic compatibility EMC and the answers to the simulation test questions are shared here. I hope the above content can be of some help to you. If you still have a lot of doubts to be solved, you can follow the industry information channel for more related knowledge.

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