The cause of electromagnetic compatibility EMC and simulation test

At present, the deteriorating electromagnetic environment makes us pay more and more attention to the working environment of equipment and the influence of electromagnetic environment on electronic equipment. Starting from the design, electromagnetic compatibility design is integrated to make electronic equipment work more reliably.

EMC design mainly includes surge (impact) immunity, ringing wave surge immunity, electrical fast transient pulse group 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, pulse magnetic field immunity, conducted disturbances, radiated disturbances, radio frequency field induced conducted disturbances and other related designs.

Main forms of electromagnetic interference

Electromagnetic interference enters the system mainly through conduction and radiation, and affects the operation of the system. Other methods include common impedance coupling and inductive coupling.
Conduction: Conduction coupling refers to coupling the disturbance on one electrical network to another electrical network through a conductive medium, which belongs to the lower frequency part (below 30MHz). The ways of conduction coupling in our products usually include power lines, signal lines, interconnecting lines, grounding conductors, etc.
Radiation: The harassment on one electrical network is coupled to another electrical network through space, which belongs to the higher frequency part (above 30MHz). The path of radiation is transmitted through space, and the radiation interference introduced and generated in our circuit is mainly the antenna effect formed by various wires.
Common impedance coupling: mutual interference that occurs when the currents of two or more different circuits flow through the common impedance. The harassment current conducted on the power line and the grounding conductor is often introduced into the sensitive circuit in this way.
Inductive coupling: Through the principle of mutual inductance, the electric signal transmitted in one loop is induced to interfere with the other loop. Divided into two kinds of electric induction and magnetic induction.
Corresponding countermeasures should be adopted for the interference caused by these ways: conduction filtering (such as the capacitor of each IC in our design is filtering), radiation interference is used to reduce the antenna effect (such as the signal walking close to the ground), Measures such as shielding and grounding can greatly improve the product's ability to resist electromagnetic interference, and can also effectively reduce electromagnetic interference to the outside world.

EMC design

For the R&D and design process of a new project, the electromagnetic compatibility design needs to run through the entire process. The electromagnetic compatibility design is taken into account in the design to avoid rework and avoid repeated R&D, which can shorten the time to market for the entire product and improve the efficiency of the enterprise.
From research and development to market launch, a project needs to go through several stages such as demand analysis, project establishment, project outline design, project detailed design, sample trial production, functional testing, electromagnetic compatibility testing, project production, and market launch.
In the demand analysis stage, it is necessary to conduct product market analysis, on-site research, dig out useful information for the project, integrate the development prospects of the project, organize the working environment of the project products in detail, and inspect the installation location on the spot, whether there is a limited space for installation, and 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 harsh electromagnetic environment, whether it is restricted to other equipment, whether the successful development of the product can greatly improve production efficiency, or can it give people a living or working environment Bring great convenience, whether the operation and use method can be easily accepted by people, which requires the project products to meet the on-site functional needs, easy to operate, etc., to organize detailed demand analysis for demand review.
After the review by the relevant person in charge of the enterprise, the requirements analysis is improved, and then the project is established. The project establishment requires the formation of a project team, and arranges software, hardware, structure, and testing personnel into the project team and assigns their respective responsibilities. The next stage of project development is the outline design of the project. The project is broken down into multiple functional modules, and the WBS breakdown structure is used to decompose and refine the function of the project. Time is arranged according to the workload and specific personnel are arranged. Organize the project outline design, evaluate the project as a whole, determine the type of power supply used, power distribution, power isolation filtering method, system grounding method, product shielding, product structure using shielding design, shielding chassis, analysis of signal types, and lightning protection , Static electricity, group pulse and other interference to take protective measures.
After the product outline is designed, it must be reviewed by relevant personnel to analyze whether the implementation method is reasonable and whether the implementation plan is feasible. The reviewer will give the review. After the project team revises the outline design in conjunction with the review, it enters the detailed product design stage. Including the process of schematic design, PCB design, PCB procurement and welding, software writing, functional debugging, etc. The schematic design should take into account the influence of electromagnetic compatibility, add filter capacitors to the board-level power supply, and add filter circuits to the interface part of the signal. According to the signal type, choose the appropriate filter circuit. If the signal is a low-frequency model, you should choose a low-pass filter circuit, calculate the appropriate cut-off frequency, and select the corresponding resistance, capacitor, etc. In addition, a large current discharge circuit is designed for the interface part, and a lightning protection device is installed to achieve the third level of lightning protection.

1. Component selection

Our commonly used electronic devices mainly include active devices and passive devices. Active devices mainly refer to devices such as ICs and module circuits, and passive devices mainly refer to components such as resistors, capacitors, and inductors. The following is an introduction to the selection of these two types of components and the issues to be considered in terms of electromagnetic compatibility.
Active device EMC selection
Good EMC characteristics for wide working voltage, good EMC characteristics for low working voltage, longer delay within the design allowable range (usually slower), better characteristics, lower quiescent current and lower power consumption than higher characteristics , The EMC performance of chip packaged devices is better than that of plug-in devices.

Passive component selection

Passive devices usually include resistance, capacitance, inductance, etc. in our applications. For the selection of passive devices, we must pay attention to the frequency characteristics and distribution parameters of these components.
Passive devices exhibit different characteristics at certain frequencies. Some resistors have inductance characteristics at high frequencies, such as wire-wound resistors. Electrolytic capacitors have good low-frequency characteristics and poor high-frequency characteristics, while film capacitors and ceramic capacitors The high frequency characteristic is better, but the capacity is usually smaller. Consider the influence of temperature on components, and select components with various temperature characteristics according to design principles.

2. Printed board design

When designing the printed board, consider the impact of interference on the system. The analog part of the circuit and the digital part of the circuit should be strictly separated, and the circuit should be protected. The system ground wire should be surrounded and the wiring should be as thick as possible. The power supply should be increased with a filter circuit. Adopt DC-DC isolation, the signal adopts photoelectric isolation, design the isolated power supply, analyze the parts that are prone to interference (such as clock circuits, communication circuits, etc.) and the parts that are easily interfered (such as analog sampling circuits, etc.). The circuit takes measures separately. Take suppression measures for interfering components, take isolation and protection measures for sensitive components, and distance them in space and electricity. In the board-level design, pay attention to the placement of components away from the edge of the printed board, which is beneficial to protect against 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 interference and improve electromagnetic compatibility. Divide several functional modules according to the function of the circuit, analyze the interference source and sensitive signal of each module for special processing.

When wiring the printed board, pay attention to the following aspects:
1. Keep the loop area, such as the loop formed between the power supply and the ground, and reduce the loop area, which will reduce the electromagnetic interference induced current on this loop. The power line should be as close to the ground as possible to reduce the differential mode The radiated ring area reduces the impact of interference on the system and improves the anti-interference performance of the system. The parallel wires are tightly placed together, and a thick wire is used for connection, and the signal wire is routed close to the ground plane to reduce interference. Add a high-frequency filter capacitor between the power supply and the ground.
2. Make the length of the wire as short as possible, reduce the area of the printed board, and reduce the interference on the wire.
3. Adopt a complete ground plane design, adopt a multi-layer board design, and lay the ground to facilitate the release of interference signals.
4. Keep the electronic components away from the surface where discharge may occur, such as chassis panels, handles, screws, etc., and keep the chassis in good contact with the ground, and provide a good discharge channel for interference. Deal with sensitive signal packets to reduce interference.
5. Use SMD components as much as possible. SMD devices have much better electromagnetic compatibility than in-line devices.
6. The analog ground and digital ground are grounded at one point where the PCB connects to 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 far 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. Do not change the width of the printed lines on the circuit board. The corners should be arc-shaped, not right or sharp.
9. The clock line and signal line should be as close as possible to the ground line, and the trace should not be too long to reduce the loop area of the loop.

Three, system wiring design

After the printed board is designed, carry out trial production, welding and debugging, system installation, considering the electromagnetic compatibility design factors, the cabinet structure and cable design need to pay attention to the following aspects:
1. Electromagnetic shielding cabinet is selected for the cabinet, which has good shielding performance, shields the system well, and reduces the influence of external electromagnetic interference on the system.
2. Use shielded power cords for the main power supply inlet and add a magnetic ring. The shielding layer is 360-degree grounded at the point where it enters the cabinet.
3. Use shielded wires for the external signal lines of the system, and the shielding layer is well grounded at the entrance of the cabinet.
4. The equipment shell is connected to the cabinet nearby to avoid crossing.
5. The system is equipped with isolation transformer and UPS to ensure that the system supplies pure power.
6. Strictly separate the power cord and the signal line, and make good contact between each surface of the equipment shell and each board panel. The contact resistance must be less than 0.4 ohms. The smaller the better, ensure that the equipment shell is well connected to the ground. When static electricity is discharged, it will not affect the normal operation of the system.

Four, system grounding design

Grounding is an effective way to suppress the source of harassment and can solve 50% of EMC problems. The reference ground of the system is connected to the ground, which can suppress electromagnetic disturbance. The metal parts of the shell are directly connected to the ground, which can also provide a leakage path for electrostatic charge to prevent static electricity accumulation.
1. The concept of ground wire
Safety grounding includes protective grounding and lightning protection grounding.
Protective grounding provides a low-impedance channel for product fault current to enter the earth;
Lightning protection grounding provides a path for leaking and amplifying current;
Reference ground Provides a reference level for stable and reliable operation of the product, and provides a reference potential for power and signals.
Safety grounding is to provide a discharge loop for large current and high voltage when some electrical abnormalities occur, and it is mainly a protective measure for the circuit. The reference ground is mainly the signal ground and the power ground, which is the basis for ensuring the function of the circuit.
2. Grounding method
Floating grounding is generally not a problem for an independent system with no external interface, but if there are interfaces between the system and other systems, such as communication ports and sampling lines, then the floating grounding is vulnerable to static electricity and lightning strikes. Therefore, most general electronic products do not use floating grounding.
Single-point grounding When f<1MHz, single-point grounding can be selected, which can be divided into parallel single-point grounding and multi-level 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 multi-level circuits: Connect the grounds of circuits with similar characteristics to form a common point, and then connect each common point to a single-point ground.
Multi-point grounding When f>10MHz, multi-point grounding will be used. The circuits in the equipment all take the grounding bus as the reference point nearby.
Single-point grounding. Each circuit is connected at the same point to provide a common potential reference point. There is no common impedance coupling and low-frequency ground loop, but there is a large ground impedance for high-frequency signals. Multi-point grounding is the nearest grounding, and each ground wire can be very short to provide lower ground impedance. 1MHz~10MHz can choose which grounding method according to actual needs.
Hybrid grounding is a combination of 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.
Signal wire shielding grounding is 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 a 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 intervals of 3/20 or 1/10 operating wavelength.
The system must be well grounded to effectively suppress electromagnetic interference. A large system cabinet must first ensure that each surface is in good contact and compact contact, and secondly, the equipment inside the cabinet must be grounded nearby to avoid secondary interference and discharge electromagnetic interference nearby. The shielded wire of the interface should be looped, and then connected to the nearest rack. A grounding copper bar is set under the cabinet, and it is better to use copper tape for the total ground wire of the system, which can vent the electromagnetic interference well and ensure the normal operation of the system.

Electromagnetic compatibility test

System function test, after meeting the on-site function needs, conduct electromagnetic compatibility test. Electromagnetic compatibility test is prone to problems such as static electricity, group pulse, surge, radio frequency field conduction, etc.
1. Electrostatic immunity test
Participated in the electrostatic immunity test of several projects, and have a certain understanding of static electricity. Static electricity is divided into contact discharge and air discharge. Static electricity is an accumulated high voltage. When it touches the metal shell of the device, it will be discharged instantly, which will affect the normal operation of electronic equipment, and may cause equipment failure or restart. This is not allowed on occasion.
Static electricity will affect the display effect, and the display may flicker or black screen, which will affect the normal display and operation. Static electricity may also cause the CPU to work abnormally, and program crashes or restarts.
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 static electricity test. Through the design, the charge accumulated by static electricity can be discharged well, and the normal operation of the system will not be affected.
2. Detection of immunity to electrical fast transient pulse groups
Electrical fast transient pulse group is a series of high-frequency and high-voltage transient pulses applied to the equipment, and observe whether the equipment is affected by it. The main method of protecting group pulses is to "drain" and "block". "Drain" is to provide a venting circuit. Before the interference enters the system, it is vented to the ground. A good shielding layer is grounded, which can vent and amplify part of the dynamic interference. "It is to filter the group pulses in the equipment and add a magnetic ring, the effect is obvious, the effect of the closed magnetic ring is better than the magnetic ring buckle, and the magnetic ring can also be added to the board level and fixed in the printed board. Make the equipment more reliable.
Adding magnetic rings to both ends of the power line, signal line, and communication line can protect against group pulse interference.
3. Lightning surge detection
Lightning surge mainly includes two aspects, one is power supply lightning protection, and the other is signal lightning protection.
The power supply lightning protection is mainly aimed at the system level. The system level design should be designed in accordance with the three-level lightning protection design. The power supply lightning protection (such as OBO's V20-C/3-PH 385) is set at the main power input end, which can be used to protect the system. The power supply is protected. After the power supply is protected from lightning, the power supply enters the isolation transformer. The isolation transformer can better protect the electromagnetic interference signal and restrain its influence on the system. After entering the UPS, the UPS can filter out a part of the interference signal, so that the power supply enters the system equipment. The power supply is a pure power supply, which can make the system work better and more reliable.
The cause of electromagnetic compatibility EMC and simulation test
Figure 2: Design example of system power supply part.

Signal lightning protection is to protect the signal path of the system.

What is involved is board-level design. In the board-level design, add lightning protection devices, such as gas discharge tubes, and increase TVS discharge circuits. When there is a large current, it is discharged through matching resistors and TVS and gas discharge tubes. Play a protective role. Then the signal is photoelectrically isolated, and then enters the system. The system can collect a stable signal so that the system can analyze and judge normally, issue instructions normally, and work normally. The other is to design a wider signal range. When the signal fluctuates normally, the system works normally.
4. Immunity detection of radio frequency field induction conduction
The radiation sensing test may affect the display signal, acquisition drive, etc., may cause the display to flicker or black screen, affect the operation of the device, may cause the acquisition and drive to work abnormally, collect the required signal, and cannot drive the field device.
The radio frequency test is to interfere with the signal line and power supply in the frequency range of 0.15k to 80M, and the level 3 intensity is 10V/m.
The principle of radiation protection is to do a good job of shielding the power supply and signal lines, the shielding layer is well grounded, and the appropriate frequency is selected for filtering to filter out interference.
5. Radiated emission detection, radio frequency field radiation immunity detection
The test is mainly to test the anti-RF signal and overall shielding performance of the system. As long as the system is well shielded and the system ground is well grounded, the system can pass the test.
Through the relevant electromagnetic compatibility test, the product can be brought to the market for trial operation, and the problems arising during the trial operation will be summarized in preparation for product improvement.
Electronic products meet the relevant electromagnetic compatibility test standards. Only after passing the test can they be put on the market, and users can use it with confidence, greatly reducing accidents caused by electromagnetic interference, and playing a positive role in the benefit of enterprises and the promotion of products.