PCB anti-interference technology design
Electromagnetic compatibility EMC refers to the ability of an electronic system to work normally in accordance with the design requirements in a specified electromagnetic environment. The electromagnetic interference suffered by electronic systems not only comes from the radiation of electric and magnetic fields, but also the common impedance of lines, the interference between wires and the circuit structure. When developing and designing circuits, we also hope that the printed circuit board designed is not as susceptible to external interference as possible, and that it itself will interfere with other electronic systems as little as possible. There are many factors that affect the anti-interference performance of printed boards, including the thickness of copper foil, the width and length of printed wires and crosstalk between adjacent wires, the rationality of the layout of components inside the board, and the common impedance of wires Electromagnetic fields generated by wires and components in space.
The first task of designing a printed circuit board is to analyze the circuit and determine the key circuits. This is to identify which circuits are interference sources and which circuits are sensitive circuits, and find out what paths the interference sources may interfere with sensitive circuits. In analog circuits, low-level analog circuits are often sensitive circuits, and power amplifiers are often sources of interference. When the working frequency is low, the interference source mainly interferes with sensitive circuits through barrel-to-line coupling; when the working frequency is high, the interference source mainly interferes with sensitive circuits through electromagnetic radiation. In digital circuits, high-speed repetitive signals, such as clock signals and bus signals, are rich in frequency components and are the largest source of interference, often posing a threat to sensitive circuits. Reset circuits, interrupt circuits, etc. are sensitive circuits that are susceptible to interference from spikes, so digital circuits cannot work properly. The input / output circuit (1/0) is connected to the outside world and special attention should be paid. If the UO circuit is close to an interference source such as a clock line, unwanted high-frequency energy will be connected to the input output line, and the noise on the line will interfere with sensitive circuits near the cable through radiation or conduction.
Based on a thorough analysis of the circuit and the determination of key circuits, the circuit must also be properly arranged on the printed board. For digital circuits, high-speed circuits (such as clock circuits, high-speed logic circuits, etc.), low- and medium-speed logic circuits, and UO circuits should be arranged in different areas, and try to separate interference sources from sensitive circuits as much as possible. This can make interference sources Radiated interference to sensitive circuits is greatly reduced.
Anti-interference design of printed board
The purpose of PCB anti-interference design is to reduce the crosstalk between the electromagnetic radiation of the PCB and the circuit on the PCB. In addition, the PCB ground wire design also directly affects the common mode voltage radiation of the 1/0 cable. Therefore, the anti-interference design of PCB is of great significance to reduce the electromagnetic information radiation of the system.
2.1 Layout Design of PCB
The density of printed circuit boards (PCBs) is getting higher and higher, and the quality of the PCB design has a great impact on the ability to resist interference, so the layout of the PCB is very important in the design.
Layout requirements for special components:
1. The connection between high-frequency components should be as short as possible to reduce electromagnetic interference between them; components that are easily affected by interference should not be too close; input and output components should be kept as far away as possible;
2. Some components have a high potential difference. The distance between them should be increased to reduce common mode radiation. The layout of components with high voltage should pay special attention to the rationality of the layout;
3. The thermal element should be far away from the heating element;
4. The capacitor should be close to the power pin of the chip;
5. The layout of adjustable components such as potentiometers, adjustable inductor coils, variable capacitors, and micro-switches should be placed in a convenient position for adjustment according to requirements;
6. The positions occupied by the positioning holes of the printed board and the fixing brackets should be reserved.
Layout requirements for common components:
1. Place the components of each functional circuit unit according to the flow of the circuit so that the direction of signal flow is as consistent as possible;
2. With the core components of each functional circuit as the center and layout around them, the components should be evenly and neatly arranged on the PCB to minimize and shorten the leads and connections between the components;
3. For circuits operating at high frequencies, the interference between components must be considered. Generally, the components should be arranged in parallel as much as possible to facilitate wiring.
4. The PCB's outplace line is generally not less than 80mil from the edge of the circuit board. The optimal shape of the circuit board is rectangular. The aspect ratio is 3: 2 or 4:30.
2.2 Wiring Design of PCB
The wiring density of PCBs is getting higher and higher, so the PCB wiring design is particularly important.
1. The power supply layer of the four-layer board should be as close as possible to the ground layer to obtain the minimum power supply impedance. From top to bottom are: signal line, ground line, power line, signal line. Considering electromagnetic compatibility, the six-layer board is best from top to bottom: signal line, ground line, signal line, power line, ground line, signal line;
2. The clock line should be adjacent to the ground layer, and the line width should be as large as possible, and the line width of each clock line should be the same;
3. The signal layer adjacent to the ground line is provided with high-speed digital signal lines and low-level analog signal lines, and the farthest layer is provided with low-speed signal lines and high-level analog signal lines;
4. The wiring of the input and output terminals should be as close as possible to avoid parallelism and feedback.
5. Generally, 135-degree obtuse angles are taken at the bends of printed wires
6. The width of the power and ground wires should be increased as much as possible, and the width of the wiring for devices with a pitch of 0.5mm is not less than 12mil;
7. The width of general digital circuit signal lines is 8.il-10nul, with a pitch of 6mi1-8mil;
8. The lead of the de-radiation capacitor must not be too long, especially the high-frequency bypass capacitor cannot have leads;
9. The digital ground and analog ground on the mixed-signal circuit board are separated. If the wiring crosses the separation gap, the electromagnetic radiation and signal interference will increase sharply, causing electromagnetic compatibility problems. Therefore, PCB design generally adopts a unified layout layout of digital circuits and analog circuits;
10. For some high-speed signals, differential pair wiring can be used to reduce electromagnetic radiation.
A few practical examples are listed below to explain the interference caused by different causes and their practical solutions.
3.1 Interference from power and ground wires
The above picture is taken from a part of the circuit of an external high-voltage control and protection PCB. (a) is the original design circuit. Because the width of the printed wires of the power and ground wires is too thin, the circuit is subject to external interference during operation; and (b) is an improved circuit whose power and ground wires are thickened to 5mm, which solves the circuit interference problem.
3.2 Interference caused by irrational component layout
The above picture is taken from some circuits of a radar transmitter magnetic field control PCB. The improved PCB circuit (b) has a much improved anti-interference performance than the PCB circuit (a) before the improvement.
3.3 Interference caused by improper wiring
The above picture is taken from a part of the circuit of a radar CFA power control PCB. (a) is the original design circuit. Because the high-voltage sampling signal line is arranged in the closed-loop sampling circuit during wiring, the closed-loop sampling circuit is susceptible to external interference during operation, which often causes false overvoltage faults; and (b) is an improved PCB circuit. The interference caused by the high-voltage sampling signal line is opened, and the improved PCB circuit works reliably and stably.
Multi-layer printed boards have unique anti-interference characteristics. With the continuous development of large-scale integrated circuits and ultra-large-scale integrated circuits, people will increasingly use multi-layer printed boards. In modern electronic systems, with the increase of clock frequency and the increase of chip integration, the rationality and reliability of PCB design become more and more important. Specific issues and specific analysis are needed in the design to obtain high-quality PCB designs.