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Summary of printed circuit board design experience

Release Time:2019-01-25 Edit the author:金致卓 Reading:346

As an electronic engineer, printed circuit boards are essential for electronic engineers to do electronic design. I believe that you have encountered some confusions and problems in electronic design. I have summarized the printed circuit boards here. Some design methods in the process, I hope to give you answers.



First, the size of the printed circuit board and the layout of the device



The size of the printed circuit board should be moderate. When the size is too large, the printed lines are long and the impedance is increased. The noise resistance is reduced and the cost is high. If the size is too small, the heat dissipation is not good, and the adjacent lines are easily interfered. In terms of device layout, as with other logic circuits, the related devices should be placed as close as possible to achieve better noise immunity. Clock generators, crystal oscillators, and CPU clock inputs are prone to noise and are close to each other. Devices that are prone to noise, small current circuits, high-current circuits, etc. should be as far away as possible from the logic circuit. If possible, it is important to make a separate circuit board.



Second, decoupling capacitor configuration



In a DC power supply loop, changes in load can cause power supply noise. For example, in a digital circuit, when a circuit transitions from one state to another, a large spike current is generated on the power line to form a transient noise voltage. Configuring decoupling capacitors to suppress noise due to load changes is a common practice for reliable design of printed circuit boards.



The configuration principles are as follows:



The power input terminal is connected to a 10~100uF electrolytic capacitor. If the position of the printed circuit board is allowed, the anti-interference effect of the electrolytic capacitor above 100uF will be good.



A 0.01 uF ceramic capacitor is provided for each integrated circuit chip. If you encounter a small printed circuit board space and can not fit, you can configure a 1 ~ 10uF tantalum electrolytic capacitor every 4 ~ 10 chips, the high-frequency impedance of this device is particularly small, the impedance is less than 1Ω in the range of 500kHz ~ 20MHz, Moreover, the leakage current is small (0.5 uA or less).



For devices with weak noise capability, large current changes during shutdown, and memory devices such as ROM and RAM, decoupling capacitors should be directly connected between the power supply line (Vcc) and ground (GND) of the chip.



The leads of the decoupling capacitors must not be too long, especially if the high frequency bypass capacitors are not leaded.



Third, the heat dissipation design



From the perspective of facilitating heat dissipation, the printed plate is preferably installed upright, and the distance between the plates and the plate is generally not less than 2 cm, and the arrangement of the devices on the printed plate should follow certain rules:



1. For devices that use free convection air cooling, it is best to arrange the integrated circuits (or other devices) in a vertically long manner; for devices that use forced air cooling, it is best to use integrated circuits (or other devices) in a horizontally long format. Way row.



2. The devices on the same printed board should be arranged as far as possible according to their heat generation and heat dissipation. Devices with low heat generation or poor heat resistance (such as small signal transistors, small scale integrated circuits, electrolytic capacitors, etc.) should be placed. The uppermost flow (at the inlet) of the cooling airflow, the device that generates a large amount of heat or heat (such as a power transistor, a large-scale integrated circuit, etc.) is placed at the most downstream of the cooling airflow.



3. In the horizontal direction, the high-power devices are placed as close as possible to the edge of the printed board to shorten the heat transfer path; in the vertical direction, the high-power devices are placed as close as possible to the top of the printed board, so as to reduce the temperature of other devices when these devices are operated. Impact.



4. The temperature-sensitive device should be placed in the lowest temperature region (such as the bottom of the device). Do not place it directly above the heat-generating device. Multiple devices are preferably staggered on a horizontal surface.



5. The heat dissipation of the printed circuit board in the equipment mainly depends on the air flow, so the air flow path should be studied during the design, and the device or printed circuit board should be properly configured. When the air flows, it tends to flow in a place with low resistance. Therefore, when configuring the device on the printed circuit board, avoid leaving a large air space in a certain area.



Fourth, electromagnetic compatibility design



Electromagnetic compatibility refers to the ability of an electronic device to work in a coordinated and efficient ma


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