PCB Tech - EMC design skills inside PCB circuit board products

At present, electronic equipment is still used in various electronic equipment and systems with printed circuit boards as the main assembly method. Practice has proved that even if the circuit schematic design is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of electronic equipment. For example, if two thin parallel lines on the PCB board are close together, it will cause a delay in the signal waveform, and reflection noise will be formed at the end of the transmission line. Therefore, when designing a printed circuit board, care should be taken to adopt the correct method.

1. PCB design Ground wire design In electronic equipment, grounding is an important method to control interference

If the grounding and shielding can be properly combined and used, most of the interference problems can be solved. The ground structure of electronic equipment roughly includes system ground, chassis ground (shield ground), digital ground (logical ground), and analog ground. The following points should be paid attention to in the ground wire design:

1. Correctly choose single-point grounding and multi-point grounding

In the low-frequency circuit, the working frequency of the signal is less than 1MHz, its wiring and the inductance between the devices have little influence, and the circulating current formed by the grounding circuit has a greater influence on the interference, so one point grounding should be adopted. 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 the nearest multiple points should be used for grounding. When the working frequency is 1～10MHz, if one-point grounding is adopted, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise the multi-point grounding method should be adopted.

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 of the power supply terminal. Try to increase the grounding area of the linear circuit as much as possible.

3. Make the ground wire as thick as possible

If the ground wire is very thin, the ground potential will change with the current change, causing the timing signal level of the electronic device 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 on the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.

4. Form the ground wire into a closed loop

When designing the ground wire system of the printed circuit board composed of only digital circuits, making the ground 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 ground wire, a large potential difference will be generated on the ground junction, resulting in a decrease in the anti-noise ability, If the grounding structure is formed into a loop, the potential difference will be reduced and the anti-noise ability of electronic equipment will be improved.

Two, PCB design 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 all kinds of external interference, so that the electronic equipment can work normally in a specific electromagnetic environment, and at the same time to reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.

1. Choose a reasonable wire width

Since the impact interference generated by the transient current on the printed lines is mainly caused by the inductance of the printed wires, the inductance of the printed wires should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and precise 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, when the printed wire width is about 1.5mm, it can fully meet the requirements; for integrated circuits, the printed wire width can be selected between 0.2 to 1.0mm.

2. Adopt the correct wiring strategy

The use of equal routing can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout permits, it is best to use a grid-shaped wiring structure. The specific method is to wire one side of the PCB horizontally and the other side vertically, and then Connect with metallized holes at the cross holes. In order to suppress the crosstalk between the PCB board wires, long-distance equal wiring should be avoided when designing the wiring.

Three, circuit board design decoupling capacitor configuration

In the DC power supply loop, the change of the load will cause the power supply noise. For example, in digital circuits, when the circuit changes from one state to another, a large spike current will be generated on the power line, forming a transient noise voltage. The configuration of decoupling capacitors can suppress the noise generated by load changes, which is a common practice in 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 location of the printed circuit board allows, the anti-interference effect of using an electrolytic capacitor above 100uF will be better.

▪ Configure a 0.01uF ceramic capacitor for each integrated circuit chip. If the printed circuit board space is small and cannot be installed, one 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 and large current changes during turn-off, and storage devices such as ROM, RAM, etc., a decoupling capacitor should be directly connected between the power line (Vcc) and ground (GND) of the chip.

▪ The leads of decoupling capacitors cannot be too long, especially high-frequency bypass capacitors.

Four, PCB design printed circuit board size and device layout

The PCB size should be moderate. When the size of the PCB is too large, the printed lines will be long and the impedance will increase, not only the anti-noise ability will be reduced, but the cost will also be high; In terms of device layout, like other logic circuits, the devices related to each other should be placed as close as possible so that a better anti-noise effect can be obtained. Clock generators, crystal oscillators, and CPU clock input terminals are all prone to noise, so they should be closer to each other. It is very important that noise-prone devices, low-current circuits, and high-current circuits should be kept away from logic circuits as far as possible. If possible, separate circuit boards should be made.

5. Circuit board design and heat dissipation design

From the perspective of conducive to heat dissipation, the PCB board is best installed upright, the distance between the board and the board should not be less than 2cm, and the arrangement of the components on the PCB board should follow certain rules:

▪ For equipment that uses free convection air cooling, it is best to arrange integrated circuits (or other devices) in a vertical manner; for equipment that uses forced air cooling, it is best to arrange integrated circuits (or other devices) in a horizontal manner Row.

▪ The devices on the same PCB board should be arranged as far as possible according to their calorific value and degree of heat dissipation. Devices with low calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed in the cooling airflow. The uppermost flow (at the entrance), the devices with large heat or heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the most downstream of the cooling airflow.

▪ In the horizontal direction, high-power devices are placed as close as possible to the edge of the PCB board to shorten the heat transfer path; in the vertical direction, high-power devices are placed as close as possible to the top of the PCB board to reduce the impact of these devices on the temperature of other devices.

▪ Temperature-sensitive devices are best placed in the lowest temperature area (such as the bottom of the device). Do not place it directly above the heating device. It is best to stagger multiple devices on the horizontal plane.

▪ The heat dissipation of the PCB board in the equipment mainly relies on 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 always tends to flow in places with low resistance, so when configuring devices on a printed circuit board, avoid leaving a large airspace in a certain area.