Summarizing the experience of printed circuit board design
As an electronic engineer, printed circuit boards are a must-have for electronic engineers to do electronic design. I believe that everyone has encountered some confusion and problems in electronic design at work. Here I summarize some design methods in the process of printed circuit boards, hoping to give you answers.
1.The size of printed circuit boards and the layout of devices
The size of printed circuit boards should be moderate. When they are too large, the printed lines are long and the impedance increases, which not only reduces the anti-noise ability but also increases the cost; if they are too small, the heat dissipation is not good and they are easily interfered by adjacent lines. In terms of device layout, like other logic circuits, related devices should be placed as close as possible to obtain better anti-noise effects. The clock input terminals of clock generators, crystal oscillators and CPUs are all prone to noise and should be closer to each other. Devices that are prone to noise, small current circuits, large current circuits, etc. should be kept as far away from logic circuits as possible. If possible, a separate circuit board should be made. This is very important.
2.Decoupling capacitor configuration
In the DC power supply circuit, changes in load will cause power supply noise. For example, in a digital circuit, when the circuit switches from one state to another, a large spike current will be generated on the power line, forming a transient noise voltage. Configuring decoupling capacitors can suppress the noise caused by load changes, which is a common practice for the reliability design of printed circuit boards.
The configuration principles are as follows:
A 10~100uF electrolytic capacitor is connected across the power input terminal. If the position of the printed circuit board allows, the anti-interference effect of using an electrolytic capacitor of more than 100uF will be good.
Configure a 0.01uF ceramic capacitor for each integrated circuit chip. If the printed circuit board space is small and cannot be installed, a 1~10uF tantalum electrolytic capacitor can be configured for every 4~10 chips. The high-frequency impedance of this device is particularly small, and the impedance is less than 1Ω in the range of 500kHz~20MHz, and the leakage current is very small (less than 0.5uA).
For devices with weak noise capability, large current changes when turned off, and storage devices such as ROM and RAM, a decoupling capacitor should be directly connected between the power line (Vcc) and the ground line (GND) of the chip.
The leads of the decoupling capacitors should not be too long, especially the high-frequency bypass capacitors should not have leads.
3.Heat dissipation design
From the perspective of heat dissipation, the printed board is best installed upright, the distance between the boards should generally not be less than 2cm, and the arrangement of the devices on the printed board should follow certain rules:
(1)For equipment using free convection air cooling, it is best to arrange the integrated circuits (or other devices) in a longitudinal manner; for equipment using forced air cooling, it is best to arrange the integrated circuits (or other devices) in a horizontal manner.
(2)The devices on the same printed board should be arranged according to their heat generation and heat dissipation as much as possible. 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 uppermost stream (entrance) of the cooling airflow, and devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) should be placed at the lowest downstream of the cooling airflow.
(3)In the horizontal direction, high-power devices should be arranged as close to the edge of the printed circuit 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 circuit board as possible to reduce the impact of these devices on the temperature of other devices when they are working.
(4).Devices that are sensitive to temperature are best placed in the area with the lowest temperature (such as the bottom of the device). Never place it directly above the heating device. Multiple devices are best arranged in a staggered manner on the horizontal plane.
(5)The heat dissipation of the printed circuit board in the equipment mainly depends on air flow, so when designing, the air flow path should be studied and the devices or printed circuit boards should be reasonably configured. When air flows, it always tends to flow to places with low resistance, so when configuring devices on the printed circuit board, avoid leaving a large airspace in a certain area.
4.Electromagnetic compatibility design
Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and effective manner in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress various external interferences, so that electronic equipment can work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of electronic equipment itself to other electronic equipment.
(1)Choose a reasonable wire width
Since the impact interference generated by transient current on the printed line is mainly caused by the inductance component of the printed wire, the inductance of the printed wire should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and fine wires are beneficial to suppress interference. The signal lines of clock leads, row drivers or bus drivers often carry large transient currents, and the printed wires should be as short as possible. For discrete component circuits, the printed wire width of about 1.5mm can fully meet the requirements; for integrated circuits, the printed wire width can be selected between 0.2 and 1.0mm.
(2).Adopt the correct wiring strategy
Using equal routing can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is best to use a tic-tac-toe mesh wiring structure. The specific method is to wire horizontally on one side of the printed board and vertically on the other side, and then connect them with metallized holes at the cross holes. In order to suppress crosstalk between the wires of the printed board, long-distance equal routing should be avoided as much as possible when designing the wiring.
5.Ground wire design
In electronic equipment, grounding is an important method to control interference. If grounding and shielding can be used correctly in combination, most interference problems can be solved. The ground wire structure in electronic equipment generally includes system ground, chassis ground (shielded ground), digital ground (logic ground) and analog ground. The following points should be noted in ground wire design:
(1)Correctly select single-point grounding and multi-point grounding
In low-frequency circuits, the operating frequency of the signal is less than 1MHz, and the inductance between its wiring and devices has little effect, while the loop formed by the grounding circuit has a greater impact on interference, so one-point grounding should be used. When the signal operating frequency is greater than 10MHz, the ground wire impedance becomes very large. At this time, the ground wire impedance should be reduced as much as possible, and multi-point grounding should be used nearby. When the operating frequency is between 1 and 10MHz, if one-point grounding is used, the ground wire length should not exceed 1/20 of the wavelength, otherwise a multi-point grounding method should be used.
(2)Separate digital circuits from analog circuits
There are both high-speed logic circuits and linear circuits on the circuit board. They should be separated as much as possible, and the ground wires of the two should not be mixed, and they should be connected to the ground wires at the power supply end respectively. Try to increase the grounding area of the linear circuit as much as possible.
(3)Try to thicken the grounding wire
If the grounding wire is very thin, the grounding potential will change with the change of current, causing the timing signal level of the electronic equipment to be unstable and the anti-noise performance to deteriorate. Therefore, the grounding wire should be as thick as possible so that it can pass the allowable current of the three-position printed circuit board. If possible, the width of the grounding wire should be greater than 3mm.
(4)Make the grounding wire into a closed loop
When designing a grounding system for a printed circuit board composed only of digital circuits, making the grounding wire into a closed loop can significantly improve the anti-noise ability. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are components that consume a lot of power. Due to the limitation of the thickness of the grounding wire, a large potential difference will be generated on the ground junction, causing the anti-noise ability to decrease. If the grounding structure is made into a loop, the potential difference will be reduced, and the anti-noise ability of the electronic equipment will be improved.
