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Internal EMC design skills - engineers must

Posted:05:56 PM June 11, 2018 writer: G

1. Ground design

In electronic devices, grounding is an important method of controlling interference. If you can use the correct combination of grounding and shielding, you can solve most of the interference problems. The ground structure of the electronic device is roughly systematic, chassis ground (shielded ground), digital ground (logical ground), and analog ground. The following points should be noted in the grounding design:

1.1 Correct choice of single-point grounding and multi-point grounding

In the low-frequency circuit, the operating frequency of the signal is less than 1MHz, and its influence on the inductance between the wiring and the device is small, and the circulation current formed by the grounding circuit has a great influence on the interference, so a point should be grounded. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large. At this time, the impedance of the ground wire should be reduced as much as possible, and the nearby multi-point grounding should be adopted. When the operating frequency is from 1 to 10 MHz, if a point is grounded, the length of the ground wire should not exceed 1/20 of the wavelength. Otherwise, the multi-point grounding method should be adopted.

1.2. Separate digital and 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 lines of the two should not be mixed. They are connected to the power supply ground. Try to increase the grounding area of the linear circuit.

1.3 as much as possible to increase the ground wire

If the grounding wire is very fine, the grounding potential changes with the change of the current, causing the timing signal level of the electronic equipment to be unstable and the anti-noise performance to deteriorate. Therefore, the ground wire should be as thick as possible so that it can pass three allowable currents on the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.

1.4. Make the ground wire a closed loop

When designing a printed circuit board ground system consisting of only digital circuits, making the ground wire a closed loop can significantly improve the noise immunity. The reason lies in the fact that there are many integrated circuit components on the printed circuit board, especially when there are many components that consume more power, due to the limitation of the thickness of the grounding wire, a large potential difference will be generated in the ground junction, resulting in a decrease in the anti-noise ability. If the grounding structure is looped, the potential difference will be reduced and the anti-noise capability of the electronic equipment will be improved.

2. Electromagnetic Compatibility Design

Electromagnetic compatibility refers to the ability of electronic devices to work in coordination and effectively in various electromagnetic environments. The purpose of the electromagnetic compatibility design is to enable electronic devices to both suppress various external interferences and enable the electronic devices to work properly in a specific electromagnetic environment while reducing the electromagnetic interference of the electronic devices themselves to other electronic devices.

2.1. Choose a reasonable wire width

Because the impact interference produced by the transient current on the printed lines is mainly caused by the inductance components of the printed conductors, the inductance of the printed conductors should be minimized. The inductance of a printed conductor is proportional to its length and inversely proportional to its width, so a short, fine conductor is good for suppressing interference. Clock lines, row drivers, or bus driver signal lines often carry large transient currents. Printed conductors must be as short as possible. For discrete component circuits, when the width of the printed conductor is about 1.5mm, it can fully meet the requirements; for integrated circuits, the width of the printed conductor can be chosen between 0.2~1.0mm.

2.2. Adopt the correct cabling strategy

Equivalent traces can reduce the inductance of the wire, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is better to adopt the grid structure of the grid structure. The specific approach is to laterally route one side of the printed circuit board, and the other is to make vertical wiring. Then connect them with metallized holes at the cross holes. In order to suppress the crosstalk between printed circuit board conductors, it is necessary to avoid long-distance equal alignment when designing the wiring.

3. Decoupling capacitor configuration

In a DC power circuit, changes in the load can cause power supply noise. For example, in a digital circuit, when the circuit is switched from one state to another, a large spike current is generated on the power line, forming a transient noise voltage. Configuring decoupling capacitors can suppress the noise caused by load changes. It is a common practice for the reliability design of printed circuit boards. The configuration principles are as follows:

10-100uF electrolytic capacitor is connected across the input of the power supply. If the position of the printed circuit board is allowed, the anti-interference effect of using an electrolytic capacitor of 100uF or more will be good.

 0.01uF ceramic capacitor is provided 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 to 10 chips. The high frequency impedance of this device is particularly small, and the impedance is less than 1Ω in the range of 500kHz to 20MHz. And the leakage current is very small (less than 0.5uA).

For devices with low noise power, large current changes at turn-off, and storage devices such as ROM and RAM, decoupling capacitors should be directly connected between the power supply line (Vcc) and the ground (GND) of the chip.

The lead of the decoupling capacitor cannot be too long, especially the high-frequency bypass capacitor cannot lead.

4. Printed circuit board size and device layout

The printed circuit board should be of a moderate size. When the printed circuit board is too large, the printed lines are long and the impedance increases. This not only reduces the ability to resist noise, but also has a high cost. When the printed circuit board is too small, the heat dissipation is not good, and it is easily disturbed by adjacent lines. In the device layout, as with other logic circuits, the related devices should be placed as close as possible, so that a good anti-noise effect can be obtained. Clock generators, crystal oscillators, and the CPU's clock inputs are all prone to noise and should be close to each other. Devices that are prone to noise, low-current circuits, high-current circuits, etc. should be as far away as possible from logic circuits. If possible, it is important to make another circuit board. This is very important.

5. 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 not be less than 2cm in general, and the arrangement of the device on the printed board should follow certain rules:

• For devices using free convection air cooling, it is best to arrange the integrated circuits (or other devices) in a lengthwise manner; for devices using forced air cooling, it is best to use integrated circuits (or other devices) in a horizontally long manner. row.

• 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 or poor heat resistance (such as small-signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be cooled. At the top of the airflow (at the entrance), devices with large heat or good 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 edges of the printed board to shorten the heat transfer path; in the vertical direction, high-power devices are placed as close to the printed board as possible, so as to reduce the temperature of other devices when these devices work. influences.

• Devices that are sensitive to temperature are best placed in the lowest temperature area (such as the bottom of the device). Do not place it directly above the heat generating device. Multiple devices are best laid out in a staggered horizontal plane.

• The heat dissipation of printed boards in equipment is mainly dependent on air flow. Therefore, air flow paths should be studied during design, and devices or printed circuit boards should be properly configured. When air flows, it tends to flow where the resistance is low. Therefore, when configuring the device on the printed circuit board, avoid leaving a large airspace in a certain area.

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