C. Decoupling capacitor configuration
In DC power circuits, changes in load can cause power noise. For example, in digital circuits, when a circuit transitions from one state to another, a large peak current is generated on the power line, forming a transient noise voltage. Configuring decoupling capacitors can suppress noise caused by load changes. It is a common practice for the reliability design of printed circuit boards. The configuration principles are as follows:
The input end of the power supply is connected across an electrolytic capacitor of 10 ~ 100uF. If the position of the printed circuit board allows, the anti-interference effect of the electrolytic capacitor of more than 100uF will be better.
Configure a 0.01uF ceramic capacitor for each integrated circuit chip. If the printed circuit board has a small space and cannot be installed, a 1 to 10 uF tantalum electrolytic capacitor can be configured for every 4 to 10 chips. The high-frequency impedance of this device is particularly small, and the impedance is less than 1 Ω in the range of 500 kHz to 20 MHz. And the leakage current is very small (below 0.5uA). Electric iron
For devices with low noise capability and large current changes during shutdown, and storage devices such as ROM and RAM, a decoupling capacitor should be directly connected between the chip's power line (Vcc) and ground line (GND).
The lead of the decoupling capacitor should not be too long, especially the high-frequency bypass capacitor should not have lead.
D. Size of printed circuit board and arrangement of devices
The size of the printed circuit board should be moderate. When it is too large, the printed lines are long and the impedance is increased, which not only reduces the anti-noise ability, but also the cost is too high. In terms of device layout, like other logic circuits, the related devices should be placed as close as possible, so as to obtain better anti-noise effect. Clock generators, crystal oscillators, and clock inputs of the CPU are prone to noise and should be closer to each other. It is very important to keep noise-prone devices, small-current circuits, and high-current circuits as far away as possible from logic circuits. If possible, separate circuit boards should be used.
E. Thermal design
From the perspective of facilitating heat dissipation, the printed board is preferably installed upright. The distance between the board and the board should generally not be less than 50px, and the arrangement of the device on the printed board should follow certain rules:
—— For equipment using free convection air cooling, it is best to arrange integrated circuits (or other devices) in a lengthwise manner; for equipment using forced air cooling, it is best to use integrated circuits (or other devices) in a horizontal length Way row. Electric iron
—— The devices on the same printed board should be arranged as much as possible according to the amount of heat generated and the degree of 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. At the top of the cooling airflow (at the entrance), devices (such as power transistors, large-scale integrated circuits, etc.) with high heat generation or good heat resistance are placed downstream of the cooling airflow.
——In the horizontal direction, high-power devices should be arranged as close to the edge of the printed board as possible to shorten the heat transfer path; in the vertical direction, high-power devices should be arranged as close to the top of the printed board as possible to reduce the temperature of these devices when they are working. Impact.
——It is better to place the temperature-sensitive device in the lowest temperature area (such as the bottom of the device). Do not place it directly above the heat-generating device. It is better to arrange multiple devices staggered on the horizontal plane.
——The heat dissipation of the printed board in the device 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 reasonably configured. When air flows, it tends to flow in a place with low resistance, so when configuring the device on a printed circuit board, avoid leaving a large airspace in a certain area.
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