Design and test of the hottest electromagnetic com

2022-07-23
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EMC design and testing

Abstract: in view of the current severe electromagnetic environment, this paper analyzes the sources of electromagnetic interference, integrates EMC design through the decomposition of product development process, and gradually analyzes from the aspects of schematic design, PCB design, component selection, system wiring, system grounding, etc., summarizes the key points of EMC design, and finally introduces the relevant contents of EMC testing

key words: electromagnetic interference, conduction, radiation Ningbo Institute of materials has successfully developed 8 series of anti-corrosion and wear-resistant coatings, including graphene (white graphene) based heavy-duty anti-corrosion coatings for coastal oil storage tanks, electrostatic conductive anti-corrosion coatings, power grid tower anti-corrosion coatings, photovoltaic tower anti-corrosion coatings, seawater resistant anti-corrosion coatings and aerospace National anti-corrosion special coatings, such as radiation, electromagnetic compatibility, wiring and grounding

at present, the deteriorating electromagnetic environment, So that we gradually pay attention to the working environment of equipment and the impact of electromagnetic environment on electronic equipment. From the design, we 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-time interruption and voltage change immunity, power frequency power harmonic immunity, electrostatic immunity, RF electromagnetic field radiation immunity, power frequency magnetic field immunity, pulsed magnetic field immunity, conducted disturbance, radiated disturbance, conducted immunity induced by RF field, etc

main forms of electromagnetic interference

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

conduction: conduction coupling refers to coupling the disturbance on one electrical network to another through a conductive medium, which belongs to the part with low frequency (lower than 30MHz). In our products, the ways of conducting coupling usually include power line, signal line, interconnection line, grounding conductor, etc

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

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

inductive coupling: through the mutual inductance principle, the electrical signal transmitted in one circuit is induced to another circuit to cause interference. It is divided into electric induction and magnetic induction

the corresponding countermeasures we should adopt for the interference generated by these ways: filtering is adopted for conduction (for example, the chip head capacitance of each IC in our design plays a filtering role), and measures such as reducing antenna effect (for example, the signal is close to the ground), shielding and grounding are adopted for radiated interference, which can greatly improve the product's ability to resist electromagnetic interference and 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. The design of electromagnetic compatibility should be considered in the design so as not to rework and avoid repeated R & D, which can shorten the time to market of the whole product and improve the benefits of the enterprise

a project needs to go through several stages from R & D to market investment, such as demand analysis, project initiation, project outline design, project detailed design, sample trial production, function test, electromagnetic compatibility test, project production and market investment

in the demand analysis stage, it is necessary to conduct product market analysis and on-site investigation, excavate useful information for the project, integrate the development prospect of the project, sort out the working environment of the project product in detail, and conduct on-site investigation on the installation location, whether there is limited 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 harsh electromagnetic environment, and whether it is restricted to 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 operation mode can be easily accepted by people, this requires that the project products should meet the functional needs of the site and be easy to operate. Finally, a detailed demand analysis report should be sorted out for demand review

after the review of the relevant responsible persons within the enterprise, the needs analysis report will be improved, and then the project initiation. The project initiation requires the establishment of a project team, and the software, hardware, structure, testing and other personnel will be assigned to the project team to assign their respective responsibilities. The next stage of 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 project functions, and arranges time and specific personnel according to the workload. Sort out the project outline design report, evaluate the project as a whole, determine the type of power supply used, power distribution, power isolation and filtering mode, system grounding mode, product shielding, product structure adopts shielding design, adopts shielding chassis, analyzes the signal type, and takes protective measures against lightning, static electricity, group pulse and other interferences

after the product outline design report is issued, it shall be reviewed by relevant personnel to analyze whether the implementation method is reasonable and whether the implementation scheme is feasible. The reviewer shall provide the review report on the steel bar zigzag testing machine developed in accordance with the provisions of the national standard metal zigzag testing method (gb232 (8)). The project team shall modify the outline design in combination with the review report and enter the product detailed design stage. The contents of this stage include schematic design In the process of PCB design, PCB procurement and welding, software writing, function debugging, etc., the influence of electromagnetic compatibility shall be taken into account in the schematic design. The filter capacitor shall be added to the board level power supply, and the filter circuit shall be added to the interface part of the signal. The appropriate filter circuit shall be selected according to the signal type. If the signal is a low-frequency model, the low-pass filter circuit shall be selected, the appropriate cut-off frequency shall be calculated, and the corresponding resistance and capacitance shall be selected. 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

I. component selection

our commonly used electronic devices mainly include active devices and passive devices. Active devices mainly refer to IC and module circuits, and passive devices mainly refer to resistance, capacitance, inductance and other components. The selection of these two types of components and the problems to be considered in electromagnetic compatibility are introduced below

EMC selection of active devices

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

selection of passive components

passive components usually include resistance, capacitance, inductance, etc. for the selection of passive components, we should pay attention to the frequency characteristics and distribution parameters of these components

passive components will show different characteristics at some 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 have good high-frequency characteristics, but usually have small capacities. Considering the influence of temperature on components, components with various temperature characteristics are selected according to the design principle

II. PCB design

in PCB design, the impact of interference on the system should be taken into account. The analog part and digital part of the circuit should be strictly separated. As the world's leading R & D manufacturer of aluminum processing products, the core power should be protected. The system ground wire should be looped and the wiring should be as thick as possible. The filter circuit should be added to the domestic power source, and the DC-DC isolation should be adopted. The signal should be photoelectric isolated, and the isolated power supply should be designed, Analyze the parts prone to interference (such as clock circuit, communication circuit, etc.) and the parts prone to interference (such as analog sampling circuit, etc.), and take measures for these two types of circuits respectively. Suppression measures shall be taken for interference elements, isolation and protection measures shall be taken for sensitive elements, and they shall be separated from each other in space and electricity. In the board level design, it should be noted that the components should be placed away from the edge of the printed board, which is beneficial to the protection against air discharge

see Figure 1 for schematic design of sampling circuit:

Figure 1: sampling circuit design

reasonable layout of circuit can reduce interference and improve EMC performance. 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 wiring the printed board, the following aspects should be paid attention to:

1. Keep the loop area to a 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 the loop. The power line should 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-interference performance of the system. The parallel wires are closely put together, and a thick wire is used for connection. The signal wire is laid close to the ground plane to reduce interference. High frequency filter capacitor is added between power supply and ground

2. Shorten the length of the conductor as much as possible, reduce the area of the printed board and reduce the interference on the conductor

3. Adopt complete ground plane design, adopt multi-layer plate design, and 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 panel, handle, screw, etc., keep the chassis in good contact with the ground, and provide a good discharge channel for interference. Process sensitive signal packets to reduce interference

5. Try to use SMD components. The EMC performance of SMD components is much better than that of in-line components

6. Analog ground and digital ground are grounded at one point at the connection between PCB and the outside world

7. The high-speed logic circuit shall be close to the edge of the connector, the low-speed logic circuit and memory shall be arranged away from the connector, and the medium speed logic circuit shall 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 shall not change abruptly, and the corners shall be arc-shaped without right angles or sharp corners

9. The clock line and signal line shall also be as close to the ground wire as possible, and the routing shall not be too long to reduce the loop area of the circuit

III. system wiring design

after the printed board is designed, trial production, welding commissioning and system installation shall be carried out. Considering the electromagnetic compatibility design factors, the following aspects shall be paid attention to in the cabinet structure and cable design:

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

2. The main power incoming line shall be a shielded power line with a magnetic ring. The shielding layer shall be grounded 360 degrees at the place where it enters the cabinet

3. The shielded wire shall be selected for the external signal wire of the system, and the inlet of the shielding layer cabinet shall be well grounded

4. The equipment shell shall be 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. The power line and signal line shall be strictly separated. All surfaces of the equipment shell and all boards and panels shall be in good contact, and the contact resistance shall be less than 0.4 Ω,

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