Summary of EMC design tips for printed circuit boards
At present, printed circuit boards are still the main assembly method for electronic equipment used in various electronic devices and systems. Practice has proved that even if the circuit schematic design is correct and the printed circuit board is not designed properly, it will adversely affect the reliability of electronic equipment. For example, if two thin parallel lines of a printed circuit board are close to each other, a delay of the signal waveform will be formed, and a reflection noise will be formed at the end of the transmission line. Therefore, when designing a printed circuit board, pay attention to the correct method.
A. Ground wire design
In electronics, grounding is an important method for controlling interference. If the correct combination of grounding and shielding can be used, most interference problems can be solved. The ground structure in electronic equipment is roughly system ground, chassis ground (shielded ground), digital ground (logical ground), and analog ground. Pay attention to the following points in the ground wire design:
1. Correctly select single-point grounding and multi-point grounding
In low-frequency circuits, the signal's operating frequency is less than 1MHz, its wiring and the inductance between the components have a small effect, and the circulating current formed by the ground circuit has a large effect on interference, so a little ground should be used. When the signal operating frequency is greater than 10MHz, the impedance of the ground wire becomes very large. At this time, the impedance of the ground wire should be reduced as much as possible, and multiple nearby grounds should be used. When the operating frequency is between 1 and 10 MHz, if one point of grounding is used, the length of the ground wire should not exceed 1/20 of the length of the wave [url =] B [/ url], otherwise a multi-point grounding method should be used.
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 connected to the power-side ground wire respectively. Try to increase the ground 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 change of the current, causing the timing signal level of the electronic equipment to be unstable and the anti-noise performance to be deteriorated. Therefore, the ground wire should be made as thick as possible, so that it can pass three permissible currents on the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.
4. Make the ground wire into a closed loop
When designing a ground wire system for a printed circuit board consisting of only digital circuits, making the ground wire into a closed loop can significantly improve the ability to resist noise. 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, which will reduce the anti-noise ability. If the ground structure is looped, the potential difference will be reduced and the anti-noise capability of electronic equipment will be improved.
B. Electromagnetic compatibility design
Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and effective manner in a variety of electromagnetic environments. The purpose of electromagnetic compatibility design is to make electronic equipment not only suppress various external interferences, enable electronic equipment to work normally in a specific electromagnetic environment, but also reduce the electromagnetic interference of electronic equipment itself to other electronic equipment.
1.Select a reasonable wire width
Since the impact interference of the transient current on the printed lines is mainly caused by the inductance component of the printed wire, the inductance of the printed wire should be minimized. The inductance of a printed wire is directly proportional to its length and inversely proportional to its width, so a short and fine wire is beneficial to suppress interference. Clock leads, signal lines for row drivers or bus drivers often carry large transient currents, and printed conductors should be as short as possible. For discrete component circuits, when the width of the printed wire is about 1.5mm, the requirements can be fully met; for integrated circuits, the width of the printed wire can be selected between 0.2 and 1.0mm.
2.Use the right wiring strategy
The use of equal wiring can reduce the inductance of the wire, but the mutual inductance and the distributed capacitance between the wires increase. If the layout allows, it is best to use a square grid wiring structure. The specific method is to horizontally route one side of the printed board and the other side to longitudinally. Metallized holes are then connected at the cross holes. In order to suppress the crosstalk between the printed circuit wires, long distance equal routing should be avoided when designing the wiring.