Everyone EMC colleagues, Xiaobian long-term mixed in the field of SI/PI, entered EMC, please enlighten! This series follows Xiaobian's EMC growth trajectory and recommends several entry-level articles for everyone, covering EMC basic knowledge, testing, design, etc., all of which are widely circulated in the industry. Please try! The theme of this issue is EMC design in the R&D process.
At present, the increasingly deteriorating electromagnetic environment has caused us to gradually pay attention to the working environment of the equipment and to pay increasing attention to the influence of the electromagnetic environment on the electronic equipment. From the beginning of design, we have incorporated electromagnetic compatibility design to make electronic equipment more reliable. For the R&D and design process of a new project, electromagnetic compatibility design needs to go through the entire process. In consideration of electromagnetic compatibility design in the design, it will not be reworked, avoid repeated research and development, shorten the time to market for the entire product, and increase the efficiency of the enterprise.
A project from research and development to the market needs to undergo needs analysis, project approval, project outline design, detailed project design, sample trial production, functional testing, electromagnetic compatibility testing, project production, and investment in the market. In the demand analysis stage, product market analysis, on-site research, excavation of useful information for the project, integration of project development prospects, detailed arrangement of project product working environment, site inspection of installation location, whether there is room 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 bad electromagnetic environment, and whether it is limited to other equipment, whether the product development function greatly improves the production efficiency, or whether it can give people a living or working environment. Bring great convenience, operation and use methods can easily be accepted by people, which requires the project product to meet the site's functional needs, easy to operate, etc., and finally to organize a detailed needs analysis report for demand assessment.
After the review by the relevant person in charge within the company, the requirements analysis report is improved, and then the project is established. The project approval requires the formation of a project team. The software, hardware, structure, and test personnel are assigned to the project team and assigned their respective responsibilities. The next stage of project development is the design of the project summary. The project is decomposed into multiple functional modules. The functional decomposition of the project is performed using the WBS decomposition structure. The time is allocated according to the workload and the specific personnel are arranged. Organize the project summary design report, evaluate the project as a whole, determine the type of power supply used, power distribution, power isolation filtering, system grounding, product shielding, product structure using shielded design, shielded chassis, analysis of signal Lightning, static electricity, group pulse and other interference take protective measures.
After the product summary design report comes out, it needs to be reviewed by related personnel, whether the implementation method is reasonable or not, and whether the implementation plan is feasible. The assessment report is given by the assessment staff. After the project team revise the summary design in conjunction with the assessment report, it enters the product detailed design stage. The contents of the phase include the schematic design, PCB design, PCB procurement and welding, software programming, function debugging and other processes. The schematic design should take into account the impact of electromagnetic compatibility, increase the filter capacitance of the board-level power supply, increase the interface part of the signal Filter circuit, according to the type of signal, select the appropriate filter circuit, if the signal is low-frequency models, should choose low-pass filter circuit, calculate the appropriate cut-off frequency, select the corresponding resistance, capacitance and so on. Another part of the interface design large current drain circuit, set lightning protection devices, so that the third level of lightning protection.
The following describes the EMC design for schematic design, PCB design, component selection, system wiring, and system grounding.
First, component selection
Commonly used electronic devices mainly include active devices and passive devices. Active devices mainly refer to devices such as ICs and module circuits. Passive devices mainly refer to components such as resistors, capacitors, and inductors. In the following, some introductions are made on the selection of these two types of components and the issues to be considered in electromagnetic compatibility.
a. EMC selection of active devices
Good EMC characteristics with wide operating voltage, good EMC characteristics with low operating voltage, good delay (commonly referred to as slow speed) characteristics within the allowable range of design, and good characteristics of small quiescent current and small power consumption The EMC performance of chip-packaged devices is better than that of the device.
b. Passive device selection
Passive devices usually include resistors, capacitors, and inductors 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 wirewound resistors, good low frequency characteristics of electrolytic capacitors, poor high frequency characteristics, and thin film capacitors and ceramic capacitors. High-frequency characteristics are better, but usually the capacity is smaller. Considering the influence of temperature on the components, according to the design principle, choose a variety of temperature characteristics of the device.
Second, the printed board design
When designing a printed circuit board, the effects of interference on the system must be taken into account. The circuit of the analog and digital parts of the circuit must be strictly separated, the core circuit must be protected, the system ground wire should be surrounded, and the wiring should be as thick as possible. The power supply must be added to the filter circuit. , DC-DC isolation, signal isolation using optoelectronics, design isolated power supply, analysis of parts that are prone to interference (such as clock circuits, communication circuits, etc.) and easily disturbed parts (such as analog sampling circuits, etc.), for these two types The circuits take measures respectively. For the interference components to take measures to take isolation and protection measures on the sensitive components, and they will be pulled away in space and electrical distance. When designing at the board level, it is also necessary to pay attention to the placement of components away from the edge of the printed board, which is advantageous for protecting against air discharge.
The rational layout of the circuit can reduce interference and improve electromagnetic compatibility. According to the function of the circuit, several functional modules are divided to analyze the interference sources and sensitive signals of each module for special processing. When wiring the PCB, 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, reduce the induced current of electromagnetic interference on the loop, and place the power cable as close to the ground as possible 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 to connect. The signal wires are routed close to the ground plane to reduce interference. Increase the high-frequency filter capacitor between the power supply and ground.
2. Make the length of the wire as short as possible, reduce the area of ​​the printed circuit board, and reduce the interference on the wire.
3, the use of a complete ground plane design, the use of multi-plate design, laying the ground, to facilitate the release of interference signals.
4. Keep electronic components away from planes where discharge may occur, such as chassis panels, handles, screws, etc. Keep the chassis in good contact with the ground to provide a good drain path for interference. Sensitive signal processing to reduce interference.
5. SMD components are used as much as possible. The electromagnetic compatibility performance of patch devices is much better than that of in-line devices.
6, analog ground and digital ground in the PCB and the outside world to connect a point.
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. The middle-speed logic circuit should be placed between the high-speed logic circuit and the low-speed logic circuit.
8, the width of the printed circuit board should not be abrupt, the corner should be rounded, not right angle or sharp corners.
9. The clock line and signal line should be as close to the ground as possible, and the trace should not be too long to reduce the loop area of ​​the loop.
Third, system wiring design
After the printed circuit board is designed, trial production, welding and debugging, and system installation are considered. Considering electromagnetic compatibility design factors, the cabinet structure and cable design need to pay attention to the following aspects:
1. The cabinet is made of electromagnetic shielding cabinet, which has good shielding performance. It shields the system well and reduces the influence of external electromagnetic interference on the system.
2. Select the shielded power cable for the main power input line and add the magnetic ring. The shielding layer is grounded 360 degrees into the cabinet.
3. Use shielded cables for the external signal lines of the system. Ground the shield at the entrance of the cabinet.
4, the device shell near the cabinet to avoid crossover.
5. The system sets up isolation transformers and ups to ensure that the system supplies pure power.
6, strictly separate the power line and signal line, the device shell between the various surfaces and between the various sub-panel
For good contact, the contact resistance should be less than 0.4 ohms. The smaller the better, the better the housing of the device is connected to the ground. This will not affect the normal operation of the system when there is static discharge.
Fourth, system grounding design
Grounding is the most effective method of suppressing disturbance sources and can solve 50% of EMC problems. The system base is connected to the earth and suppresses electromagnetic harassment. The metal parts of the shell are directly connected to the earth and can also provide leakage paths for electrostatic charges to prevent the accumulation of static electricity. Ground lines are divided into the following types:
1. The safety grounding includes the protective earthing and the lightning proof grounding. It is to provide a discharge circuit for large currents and high voltages when there are some electrical abnormalities, and is mainly a protection measure for the circuit.
a. The protective ground provides a low impedance path for the fault current of the product to enter the earth.
b. Lightning grounding provides access to the amplified current.
2. The reference ground provides a reference level for stable and reliable operation of the product, providing a reference potential for power supplies and signals. The reference ground is mainly the signal ground and the power ground, which is the basis for ensuring the circuit's function.
Common grounding methods are as follows:
1) Floating grounding. There is generally no problem for an independent system with no external interface, but if there are interfaces such as communication ports and sampling lines between the system and other systems, the floating ground is easily affected by static electricity and lightning strikes. Therefore, most electronic products do not use floating ground.
2) Single point grounding. When f is greater than 1MHz, single-point grounding can be selected, which can be divided into parallel single-point grounding and multi-stage circuit series single-point grounding.
a. 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.
b. Series single-point grounding of multi-level circuits: Connect the grounds of circuits with similar characteristics together to form a common point and then connect each common point to a single point ground.
3) Multi-point grounding. Multipoint grounding is used when f is less than 10 MHz. The circuits in the equipment are all referenced to the grounding bus.
The single point grounding circuits are connected at the same point and 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. The multi-point ground is grounded nearby, and each ground line can be very short, providing a low ground impedance. 1MHz~10MHz can be selected according to the actual needs of what kind of grounding method.
4) Hybrid grounding is the advantage of integrating 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.
5) The signal line shielding grounding is divided into high frequency and low frequency. The high frequency cable adopts multi-point grounding, and the low frequency cable adopts single point grounding. Low-frequency electric field shielding requires a single point of grounding at the receiving end, and low-frequency magnetic field shielding requires grounding at both ends. Ground at multiple points, except at both ends, grounded at intervals of 3/20 or 1/10 of the operating wavelength.
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Shenzhen Unitronic Power System Co., Ltd , https://www.unitronicpower.com