Analysis of Several Forms and Testing Techniques of Electromagnetic Compatibility Design

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

Electromagnetic compatibility design mainly includes surge (impact) immunity, ringing wave immunity, electrical fast transient pulse group immunity, voltage sag, short-term interruption and voltage variation immunity, power frequency power supply Harmonic immunity, electrostatic immunity, RF electromagnetic field immunity, power frequency magnetic field immunity, pulse magnetic field immunity, conducted disturbance, radiated disturbance, and conducted immunity of RF field induction.

Main form of electromagnetic interference

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

Conduction: Conductive coupling is the coupling of disturbances on one electrical network to another via a conductive medium, which is part of the lower frequency (below 30 MHz). Conductive coupling in our products typically includes power lines, signal lines, interconnects, ground conductors, and the like.

Radiation: The coupling of disturbances on one electrical network to another via a space, which is a higher frequency part (above 30 MHz). The path of radiation is transmitted through space, and the radiation interference introduced and generated in our circuits is mainly the antenna effect formed by various wires.

Common impedance coupling: Mutual interference that occurs when currents of two or more different circuits flow through a common impedance. The disturbance current conducted on the power line and the ground conductor is mostly introduced into the sensitive circuit in this way.

Inductive coupling: Through the principle of mutual inductance, the electrical signal transmitted in one loop is induced to interfere with another loop. Divided into electric induction and magnetic induction.

The corresponding countermeasures should be adopted for the interference caused by these several ways: the transmission adopts filtering (such as the chip head capacitor of each IC in our design is the filtering effect), and the radiation interference adopts the reduction of the antenna effect (such as the signal is close to the ground), Shielding and grounding measures can greatly improve the product's ability to resist electromagnetic interference, and can also effectively reduce electromagnetic interference to the outside world.

Electromagnetic compatibility design

For the R&D design process of a new project, the EMC design needs to run through the whole process. In the design, considering the design of electromagnetic compatibility, it will not be reworked, avoiding repeated research and development, which can shorten the time-to-market of the whole product and improve the efficiency of the enterprise.

From the research and development to the market, a project needs to undergo several stages such as demand analysis, project establishment, project summary design, project detailed design, sample trial production, functional testing, electromagnetic compatibility testing, project commissioning, and investment in the market.

In the demand analysis stage, product market analysis, on-site investigation, mining of useful information for the project, integration of project development prospects, detailed work environment of the project product, site inspection of installation location, whether there is limited space for installation, and whether the working environment is special, Is there corrosion, humidity, high temperature, etc., the working condition 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 give people a living or working environment. It brings great convenience, and the operation and use method can be easily accepted by people. This requires the project product to meet the needs of the field function, easy to operate, etc. Finally, the detailed demand analysis report should be compiled for the needs review.

After reviewing the relevant internal responsible persons, the requirements analysis report is completed, and then the project is established. The project project needs to set up a project team, and arrange software, hardware, structure, test and other personnel into the project team to assign their respective responsibilities. The next stage of project development is the project outline design, which breaks down the project into multiple functional modules, uses the WBS decomposition structure to decompose the project, and arranges the time according to the workload to arrange specific personnel. Organize the project summary design report, evaluate the project as a whole, determine the power type, power distribution, power isolation filter, system grounding, product shielding, product structure using shield design, shielded chassis, analysis signal type, Thunder, static electricity, group pulse and other interferences take protective measures.

After the product summary design report comes out, it must be reviewed by relevant personnel, whether the implementation method is reasonable, and whether the implementation plan is feasible. The review report is given by the reviewer. The project team and the review report modify the summary design and enter the detailed design stage of the product. The content of the stage includes schematic design, PCB design, PCB procurement and soldering, software writing, function debugging and other processes. The schematic design should take into account the influence of electromagnetic compatibility, increase the filter capacitance to the board-level power supply, and increase the interface part of the signal. The filter circuit selects a suitable filter circuit according to the signal type. If the signal is a low frequency model, the low pass filter circuit should be selected to calculate the appropriate cutoff frequency and select the corresponding resistor and capacitor. In addition, a large current bleeder circuit is designed for the interface part, and a lightning protection device is provided to achieve the third level of lightning protection.

First, component selection

Our commonly used electronic devices mainly include active devices and passive devices. Active devices mainly refer to ICs and module circuits. Passive devices mainly refer to resistors, capacitors, inductors and other components. The following is a brief introduction to the selection of these two types of components and the issues to be considered in electromagnetic compatibility.

Active device EMC selection

The EMC characteristic of the working voltage is good, the EMC characteristic of the working voltage is good, the delay is large in the range allowed by the design (so-called slow speed), the quiescent current is small, the power consumption is small, and the characteristic is good. The EMC performance of the SMD packaged devices is better than that of the interposer devices.

Passive device selection

Passive devices usually include resistors, capacitors, inductors, etc. in our applications. For the selection of passive components, 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 wirewound resistors, low frequency characteristics of electrolytic capacitors, poor high frequency characteristics, and thin film capacitors and ceramic capacitors. High frequency characteristics are good, but usually the capacity is small. Consider the influence of temperature on components, and select devices with various temperature characteristics according to the design principle.

Second, the printed board design

When designing the printed board, the influence of the interference on the system should be taken into consideration. The analog part of the circuit and the circuit of the digital part should be strictly separated. The core circuit should be protected, the system ground should be surrounded, and the wiring should be as thick as possible. DC-DC isolation, signal isolation, design of isolated power supply, analysis of parts that are prone to interference (such as clock circuits, communication circuits, etc.) and parts that are easily interfered (such as analog sampling circuits, etc.) The circuit takes separate measures. Inhibition measures are taken for the interference components, isolation and protection measures are taken for the sensitive components, and they are spatially and electrically separated. When designing at the board level, it is also important to note that the components are placed away from the edge of the printed board, which is advantageous for shielding against air discharge.

See Figure 1 for the schematic design of the sampling circuit:

Figure 1: Sampling circuit design.

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

When printing printed circuit boards, you need to pay attention to the following aspects:

1. Keep the loop area to a minimum, such as the loop formed between the power supply and the ground, reduce the loop area, and reduce the induced current of electromagnetic interference on the loop. The power line is as close as possible to the ground line to reduce the difference. The ring area of ​​the mode radiation reduces the influence of interference on the system and improves the anti-jamming performance of the system. The parallel wires are placed tightly together, and a thick wire is used for connection. The signal line is close to the ground plane to reduce interference. Add high frequency filter capacitors between the power supply and 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, using a complete ground plane design, the use of multi-layer board design, laying the ground layer, easy to interfere with signal release.

4. Keep the electronic components away from planes that may discharge, such as chassis panels, handles, screws, etc., to keep the casing in good contact with the ground, providing a good venting channel for interference. Handling sensitive signals to reduce interference.

5, try to use the chip components, the chip device is much better than the electromagnetic compatibility of the in-line device.

6. Ground the analog ground and the digital ground at the connection between the 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 placed away from the connector, and the medium-speed logic circuit should be placed between the high-speed logic circuit and the low-speed logic circuit.

8, the printed circuit board width should not be abrupt, the corner should be rounded, not right angle or sharp corner.

9. The clock line and signal line should also 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.

Third, the system wiring design

After the printed board is designed, trial production, welding debugging, system installation, considering electromagnetic compatibility design factors, cabinet structure, cable design need to pay attention to the following aspects:

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

2. The shielded power cable is used for the main power inlet, and the magnetic ring is added. The shield is grounded 360 degrees into the cabinet.

3. Use shielded wires for the external signal lines of the system, and the ground at the entrance of the shielded cabinet is well grounded.

4. The equipment casing is close to the cabinet to avoid crossover.

5. The system sets the isolation transformer and ups to ensure that the system supplies pure power.

6. Strictly separate the power cable and the signal cable. The surface of the device should be in good contact with each panel and the panel. The contact resistance should be less than 0.4 ohms. The smaller the better, the better the device casing is connected to the earth. When the static electricity is released, it will not affect the normal operation of the system.

Fourth, system grounding design

Grounding is the most effective way to suppress sources of disturbance and solve 50% of EMC problems. The system ground is connected to the ground to suppress electromagnetic disturbances. The metal parts of the case are directly connected to the ground, and can also provide an electrostatic charge leakage path to prevent static electricity from accumulating.

1, the concept of ground wire

Safety grounding includes protective earthing 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 amplified current;

Reference Ground Provides a reference level for stable and reliable operation of the product, providing a reference potential for the power supply and signal.

The safety grounding is to provide a bleed circuit for high current and high voltage when some electrical anomalies occur, mainly as 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

Suspension grounding is generally not a problem for a system that has no external interface to the outside, but if the system has interfaces such as communication ports and sampling lines with other systems, the floating ground is susceptible to static electricity and lightning strikes. Impact, so most of the 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-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: Ground the circuits of similar characteristics to form a common point, and then connect each common point to a single point.

Multi-point grounding Multi-point grounding is used when f>10MHz. The circuits in the device are all close to the ground bus as a reference point.

Single-point grounding circuits are connected at the same point, providing a common potential reference point, without common impedance coupling and low frequency ground loops, but with large ground impedance for high frequency signals. Multi-point grounding is grounded nearby, and each ground wire can be short and provides a lower ground impedance. 1MHz~10MHz can choose which grounding method to use according to actual needs.

Hybrid grounding is the advantage of integrated 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 shield grounding There are high frequency and low frequency points, high frequency adopts multi-point grounding, and low frequency cable adopts single point grounding. The low-frequency electric field shielding requires a single point grounding at the receiving end, and the low-frequency magnetic field shielding is required to be grounded at both ends. Multi-point grounding, except grounded at both ends, and grounded at 3/20 or 1/10 operating wavelength intervals.

The system can be well grounded to effectively suppress electromagnetic interference. A large system cabinet must first ensure good contact with each surface and compact contact. Secondly, the internal equipment of the cabinet should be grounded nearby to avoid secondary interference and discharge electromagnetic interference. The interface shielded cable should be looped and then connected to the rack. A grounding copper bar is arranged under the cabinet. The copper strip is better for the system ground wire, and the electromagnetic interference is well discharged to ensure the normal operation of the system.

Electromagnetic compatibility test

System function test, after meeting the needs of the field function, carry out electromagnetic compatibility test, electromagnetic compatibility test is easy to cause problems are static electricity, group pulse, surge, RF field conduction, etc.

1, static immunity test

Participated in the static immunity test of several projects, and had a certain understanding of static electricity. Static electricity is divided into contact discharge and air discharge. Static electricity is the accumulated high voltage. When it comes into contact with the metal casing of the equipment, it will discharge instantaneously, which will affect the normal operation of the electronic equipment, may cause equipment failure or restart, and have better safety requirements. This is not allowed on occasions.

Static electricity can affect the display, and flashing or black screens may appear, affecting normal display and operation. Static electricity can also cause the CPU to work abnormally, and the program may crash or restart.

If the electromagnetic compatibility related design is adopted in the detailed design stage of the product, it is not necessary to worry too much about the electrostatic test. By design, the charge accumulated by the static electricity is well discharged, and the normal operation of the system is not affected.

2. Electrical fast transient pulse group immunity detection

An electrical fast transient burst is a series of high frequency, high voltage transient pulses applied to the device to see if the device is affected. The main method of protection group pulse is “sparing” and “blocking”. “Sparging” means providing bleed circuit. It is venting to the earth before entering the system. Good shielding layer is grounded, and it can vent a part of dynamic interference. "It is to filter the group pulse out of the device, increase the magnetic ring, the effect is obvious, the effect of closing the magnetic ring is better than that of the magnetic ring, or the magnetic ring can be added to the board level and fixed in the printed board. Make the device more reliable.

Adding a magnetic ring to both ends of the power line, signal line, and communication line can protect the group pulse interference.

3, lightning surge detection

Lightning surges mainly include two aspects, one is power supply lightning protection, and the other is signal lightning protection.

The lightning protection of the power supply is mainly for the system level. The system level design should be designed according to the three-level lightning protection design. The main power input terminal is set to the power supply lightning protection (such as OBO's V20-C/3-PH 385), which can be used for the system. The power supply performs the first-level protection. After the power supply is lightning-proof, it enters the isolation transformer. The isolation transformer can better protect the electromagnetic interference signal and suppress its influence on the system. After entering the UPS, the UPS can filter out some of the interference signals, so that the power supply enters the system equipment again. The power supply is a pure power source, which can make the system work better and more reliably.

Figure 2: Example of a system power supply design.

Signal lightning protection is to protect the signal path of the system. It mainly involves the board level design. In the board level design, lightning protection devices, such as gas discharge tubes, are added to increase the TVS bleeder circuit. When there is a large current, the matching resistor and TVS and gas discharge tubes are vented to protect the rear stage circuit. 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, and the command is issued normally and works normally. On the other hand, the signal range is designed to be wide, and the system works normally when the signal fluctuates normally.

4. Anti-interference detection of RF field induced conduction

The sensation test may affect the display signal, acquisition drive, etc., which may cause the display to flicker or black screen, affecting the operation of the device, may cause the acquisition drive to work abnormally, and may not collect the required signal, and cannot drive the field device.

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

The principle of radiation protection is to shield the power supply and signal lines, and the shielding layer is well grounded. Select the appropriate frequency for filtering and filter the interference.

5, radiation emission detection, RF field radiation immunity detection

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

Through the relevant electromagnetic compatibility test, the product can be put on the market, the test run is carried out, and the problems occurring in the trial run are summarized for the improvement of the product.

The electronic products meet the relevant electromagnetic compatibility test standards, and can be pushed to the market through testing, so that users can use them with confidence, greatly reducing accidents caused by electromagnetic interference, and playing a positive role in the efficiency of enterprises and the promotion of products.