Electromagnetic compatibility design and testing

In view of the current severe electromagnetic environment, the sources of electromagnetic interference are analyzed, and electromagnetic compatibility design is integrated into the product development process through decomposition. The schematic design, PCB design, component selection, system wiring, system grounding and other aspects are gradually analyzed, and the key points of electromagnetic compatibility design are summarized. Finally, the relevant content of electromagnetic compatibility testing is introduced.

At present, the increasingly deteriorating electromagnetic environment has made us gradually pay attention to the working environment of the equipment and the impact of the electromagnetic environment on electronic equipment. Starting from the design, we should integrate electromagnetic compatibility design to make electronic equipment work more reliably.

Electromagnetic compatibility design mainly includes surge (impact) immunity, ringing wave surge immunity, electrical fast transient pulse group immunity, voltage sag, short 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 disturbance, radiated disturbance, radio frequency field induced conducted disturbance and other related designs.

The main forms of electromagnetic interference

Electromagnetic interference mainly enters the system through conduction and radiation, affecting system operation. Other ways include common impedance coupling and inductive coupling.

Conduction: Conduction coupling is the coupling of disturbances on one electrical network to another electrical network through a conductive medium. It is a relatively low frequency part (below 30MHz). In our products, the channels of conduction coupling usually include power lines, signal lines, interconnects, ground conductors, etc.

Radiation: coupling the disturbance on one electrical network to another electrical network through space, which belongs to the higher frequency part (above 30MHz). The radiation path 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 occurs when currents from two or more different circuits flow through a common impedance. Disturbance currents conducted on power lines and ground conductors are often introduced into sensitive circuits in this way.

Inductive coupling: Through the principle of mutual induction, the electrical signal transmitted in one circuit is induced into another circuit to interfere with it. It is divided into electric induction and magnetic induction.

We should adopt corresponding countermeasures for the interference generated by these channels: conduction filtering (such as the header capacitor of each IC in our design plays a filtering role), radiation interference adopts measures such as reducing antenna effect (such as signal close to the ground line), shielding and grounding, which can greatly improve the product's ability to resist electromagnetic interference and effectively reduce electromagnetic interference to the outside world.

Electromagnetic compatibility design

For the R&D and design process of a new project, electromagnetic compatibility design needs to run through the entire process. Taking electromagnetic compatibility into consideration in the design can avoid rework and repeated R&D, shorten the time to market of 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, including demand analysis, project approval, project outline design, project detailed design, sample trial production, functional testing, electromagnetic compatibility testing, project commissioning, and market launch.

During the demand analysis stage, it is necessary to conduct product market analysis and on-site research, dig out useful information for the project, integrate project development prospects, organize the working environment of the project products in detail, inspect the installation location on site, whether there are any space restrictions for installation, whether the working environment is special, whether there is corrosion, moisture, high temperature, etc., the working conditions of surrounding equipment, whether there is a harsh electromagnetic environment, whether it is restricted by other equipment, whether the successful development of the product can greatly improve production efficiency, or whether it can bring great convenience to people's lives or working environment, whether the operation and use methods can be easily accepted by people. This requires the project product to meet the functional needs of the site, be easy to operate, etc. Finally, a detailed demand analysis report should be compiled for demand review.

After the review by the relevant person in charge within the enterprise, the demand analysis report is improved, and then the project is established. The project establishment requires the establishment of a project team, and the personnel of software, hardware, structure, testing, etc. are assigned to the project team and their respective responsibilities are assigned. The next stage of project development is the project outline design, which decomposes the project into multiple functional modules, uses the WBS decomposition structure to perform functional decomposition and refinement of the project, arranges time according to the workload, and arranges specific personnel. The project outline design report is sorted out, and the project is evaluated overall to determine the type of power supply to be used, the power distribution, the power isolation and filtering method, the system grounding method, the product shielding, the product structure adopts a shielding design, and uses a shielded chassis and casing. The signal type is analyzed, and protective measures are taken against interference such as lightning, static electricity, and group pulses.

After the product outline design report is released, 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 a review report. After the project team modifies the outline design based on the review report, it will enter the product detailed design stage. The content of this stage includes schematic design, PCB design, PCB procurement and welding, software writing, function debugging and other processes. The schematic design should take into account the impact of electromagnetic compatibility, add filter capacitors to the board-level power supply, and add filter circuits to the signal interface . According to the signal type, select the appropriate filter circuit. If the signal is a low-frequency model, a low-pass filter circuit should be selected, and the appropriate cut-off frequency should be calculated to select the corresponding resistors , capacitors, etc. In addition, a large current discharge circuit is designed for the interface part, and lightning protection devices are set to achieve the third level of lightning protection.

1. Component Selection

The electronic devices we commonly use mainly include active devices and passive devices. Active devices mainly refer to devices such as ICs and module circuits, while passive devices mainly refer to components such as resistors, capacitors, and inductors. The following will introduce the selection of these two types of components and the issues that need to be considered in terms of electromagnetic compatibility.

Active Component EMC Selection

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

Passive Component Selection

Passive components in our applications usually include resistors, capacitors, inductors, etc. When selecting passive components, we must pay attention to the frequency characteristics and distribution parameters of these components.

Passive components will show different characteristics at certain frequencies. Some resistors have inductive characteristics at high frequencies, such as wirewound resistors. Electrolytic capacitors have good low-frequency characteristics but poor high-frequency characteristics, while film capacitors and ceramic capacitors have good high-frequency characteristics, but usually have smaller capacitance. Considering the impact of temperature on components, select components with various temperature characteristics according to design principles.

2. PCB design

When designing a printed circuit board, the impact of interference on the system should be taken into consideration. The analog and digital parts of the circuit should be strictly separated. The core 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 equipped with a filter circuit, DC-DC isolation should be used, and the signal should be isolated by LED photoelectric isolation. An isolated power supply should be designed. The parts that are prone to interference (such as clock circuits, communication circuits, etc.) and the parts that are easily interfered with (such as analog sampling circuits, etc.) should be analyzed, and measures should be taken for these two types of circuits. Suppression measures should be taken for interfering components, isolation and protection measures should be taken for sensitive components, and they should be spaced and electrically separated. When designing at the board level, it should also be noted that the components should be placed away from the edge of the printed circuit board, which is beneficial for protecting against air discharge.

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

Figure 1: Sampling circuit design.

Reasonable layout of the circuit can reduce interference and improve electromagnetic compatibility. According to the function of the circuit, it is divided into several functional modules, and the interference source and sensitive signal of each module are analyzed for special processing.

1. Keep the loop area to a minimum. For example, the loop formed between the power supply and the ground. Reducing the loop area will reduce the induced current of electromagnetic interference in this loop. The power line should be as close to the ground line as possible to reduce the loop area of ​​differential mode radiation, reduce the impact of interference on the system, and improve the anti-interference performance of the system. Put the parallel wires tightly together and use a thick wire for connection. The signal line can be routed close to the ground plane to reduce interference. Add high-frequency filter capacitors between the power supply and the ground.

2. Shorten the wire length as much as possible to reduce the area of ​​the printed circuit board and reduce interference on the wire.

3. Adopt a complete ground plane design, a multi-layer board design, and lay the ground layer to facilitate the discharge of interference signals.

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

5. Try to use surface mount components as much as possible. The electromagnetic compatibility performance of surface mount components is much better than that of direct insertion components.

6. The analog ground and digital ground are grounded at one point where the PCB is connected to the outside world.

7. High-speed logic circuits should be placed close to the edge of the connector, low-speed logic circuits and memory should be placed away from the connector, and medium-speed logic circuits should be placed between high-speed logic circuits and low-speed logic circuits.

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

9. The clock line and signal line should be as close to the ground line as possible, and the routing should not be too long to reduce the loop area.

3. System wiring design

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

1. The cabinet uses an electromagnetic shielding cabinet, which has good shielding performance, can shield the system well and reduce the impact of external electromagnetic interference on the system.

2. Use a shielded power cable for the main power supply and add a magnetic ring. The shielding layer should be grounded 360 degrees when entering the cabinet.

3. Use shielded cables for the system's external signal cables, and ensure that the shielding layer is well grounded at the cabinet entrance.

4. Connect the equipment casing to the cabinet as close as possible to avoid crossing.

5. The system is equipped with isolation transformer and UPS to ensure the system supplies pure power.

6. Strictly separate the power line and the signal line. There should be good contact between the various surfaces of the device casing and the various board panels. The contact resistance should be less than 0.4 ohms, the smaller the better. Ensure that the device casing is well connected to the ground so that when static electricity is released, it will not affect the normal operation of the system.

4. System grounding design

Grounding is the most effective way to suppress interference sources and can solve 50% of EMC problems. The system reference ground is connected to the earth to suppress electromagnetic interference. The metal parts of the shell are directly connected to the earth, which can also provide a leakage path for electrostatic charges and 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 path for the product's fault current to enter the earth;

Lightning protection grounding provides a path for discharging large currents;

The reference ground provides a reference level for the stable and reliable operation of the product and a reference potential for power supply and signals.

Safety grounding is to provide a discharge loop for high current and high voltage when some electrical abnormalities occur. It is mainly a protection 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 that has no external interface. However, if there is an interface between the system and other systems, such as a communication port and a sampling line, the floating grounding is easily affected by static electricity and lightning strikes. Therefore, most electronic products do not use floating 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 multi-stage circuits: Connect the grounds of circuits with similar characteristics together to form a common point, and then connect each common point to the single-point ground.

When f>10MHz, multi-point grounding is used. The circuits in the equipment are all based on the nearest ground bus as the reference point.

Single-point grounding connects all circuits to the same point, providing a common potential reference point, without common impedance coupling and low-frequency ground loops, but with a large ground impedance for high-frequency signals. Multi-point grounding is grounding at the nearest point, and each ground line can be very short, providing a lower ground impedance. 1MHz~10MHz can choose which grounding method is based on 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.

Signal line shielding grounding is divided into high frequency and low frequency. High frequency uses multi-point grounding, and low frequency cables use 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, is also grounded at intervals of 3/20 or 1/10 of the working wavelength.

The system must be well grounded to effectively suppress electromagnetic interference. A large system cabinet must first ensure that each surface has good contact and compact contact. Secondly, the equipment inside the cabinet must be grounded nearby to avoid secondary interference and discharge electromagnetic interference nearby. The interface shielding wire must be looped and then connected to the rack nearby. A grounding copper bus is set under the cabinet, and copper tape is better for the system's total ground wire, which can discharge electromagnetic interference well and ensure the normal operation of the system.

Electromagnetic compatibility testing

After the system function test meets the on-site functional requirements, the electromagnetic compatibility test is carried out. Problems that are prone to electromagnetic compatibility testing include static electricity, group pulses, surges, radio frequency field conduction, etc.

1. Electrostatic immunity test

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

Static electricity can affect the display effect, causing the display to flicker or go black, affecting normal display and operation. Static electricity can also cause the CPU to work abnormally, causing the program to freeze or restart.

If electromagnetic compatibility-related designs are adopted in the detailed design stage of the product, there is no need to worry too much about electrostatic testing. Through design, the accumulated static electricity charges can be well discharged and will not affect the normal operation of the system.

2. Electrical fast transient pulse group immunity test

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 methods of protecting group pulses are "drainage" and "blocking". "Drainage" is to provide a discharge circuit to discharge the interference to the ground before entering the system. A good shielding layer grounding can discharge most of the dynamic interference. "Blocking" is to filter the group pulses out of the equipment. Adding magnetic rings has obvious effects. The effect of closed magnetic rings is better than that of buckled magnetic rings. Magnetic rings can also be added to the board level and fixed in the printed circuit board, which makes the equipment more reliable.

Adding magnetic rings at 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, the other is signal lightning protection.

Power lightning protection is mainly for the system level. The system level design should be based on the three-level lightning protection design. The power lightning protection (such as OBO's V20-C/3-PH 385) is set at the main power input end to provide the first-level protection for the system power supply. After the power lightning protection, the power enters the isolation transformer, which can provide better protection for electromagnetic interference signals and suppress their impact on the system. After entering the UPS, the UPS can filter out some interference signals, so that the power enters the system equipment again. The power is a pure power supply, which can make the system work better and more reliably.

Figure 2: Example of designing the power supply portion of a system.

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 tubes, and TVS discharge circuits are added to the board-level design. When there is a large current, it is discharged through the matching resistors, TVS, and gas discharge tubes to protect the subsequent circuits. The signal is then photoelectrically isolated and then enters the system. The system can collect a stable signal, allowing the system to analyze and judge normally, issue instructions normally, and work normally. On the other hand, a wider signal range is designed so that the system can work normally when the signal fluctuates normally.

4. Immunity test of RF field induction conduction

The radio sensing test may affect the display signal, acquisition driver , etc., which may cause the display to flicker or go black, affecting the operation of the equipment, and may cause the acquisition driver to work abnormally, fail to collect the required signal, and fail to drive the field equipment.

The radio frequency test interferes 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 radio frequency protection is to shield the power supply and signal lines well, ground the shielding layer well, select the appropriate frequency for filtering, and filter out the interference.

5. Radiated emission detection, RF field radiation immunity detection

This test is mainly to test the system's resistance to RF signals and overall shielding performance. As long as the system is well shielded and the system ground is well grounded, the system can pass the test.

After passing the relevant electromagnetic compatibility tests, the product can be launched on the market for trial operation. Problems encountered during the trial operation can be summarized in preparation for product improvement.

Electronic products must meet relevant electromagnetic compatibility test standards and pass the tests before they can be put on the market and users can use them with confidence, which greatly reduces accidents caused by electromagnetic interference and plays a positive role in the company's benefits and product promotion.