Research on the design of PCB board based on electromagnetic compatibility technology

1Electromagnetic compatibility

Electromagnetic compatibility (EMC) is a comprehensive subject, which mainly studies the problems of electromagnetic interference and anti-interference. Electromagnetic compatibility means that the performance of electronic equipment or system will not be reduced due to electromagnetic interference under the specified electromagnetic environment level, and the electromagnetic radiation generated by them is not greater than the limit level of verification, and will not affect the normal operation of other electronic equipment or systems, and meet the requirements of non-interference and reliable operation between equipment and equipment, and between systems. Electromagnetic compatibility (EMC) includes two aspects: electromagnetic radiation and anti-electromagnetic interference of products. A good electronic product must consider electromagnetic compatibility issues. It should not have electromagnetic radiation to interfere with other electronic equipment, and it should have low electromagnetic sensitivity and be able to resist the specified electromagnetic interference. Electromagnetic interference is the consequence of electromagnetic disturbance, which will reduce the performance of electronic equipment, transmission channels, systems and printed circuit board assemblies. As the basic component of electronic equipment, printed circuit boards also have electric fields and magnetic fields in use. There are electromagnetic compatibility issues when there are electric and magnetic fields. In particular, modern electronic equipment uses a large number of digital circuits and high-speed logic circuits, and the signal transmission speed is greatly improved, which also increases the factors causing electromagnetic radiation and electromagnetic interference. Therefore, considering electromagnetic compatibility issues is an important part of printed circuit board design.

2. Design of electromagnetic interference suppression on printed circuit boards

As the density of electronic devices and circuits on PCBs continues to increase, and the frequency of signals continues to increase, the problems of E-correlation (electromagnetic compatibility) and I-correlation (electromagnetic interference) will inevitably be introduced.

External conducted interference and radiated interference have little effect on the circuits on the PCB. In fact, taking the right measures in the design can often play a role in both anti-interference and emission suppression. When designing a printed circuit board, first of all, according to actual needs, choose the appropriate type of printed board (board and board layer), then determine the location of components on the board, and then lay out and design ground and signal lines.

2.1 Selection of printed circuit boards

Printed circuit boards can be divided into single-sided, double-sided and multi-layer boards. Single-sided and double-sided boards are generally used for circuits with low and medium density wiring and circuits with low integration. Multi-layer boards are suitable for high-speed digital circuits with high-density wiring and high-integration chips. From the perspective of electromagnetic compatibility, multi-layer boards can reduce the electromagnetic radiation of circuit boards and improve the anti-interference ability of circuit boards. Because in multi-layer boards, special power layers and ground layers can be set up, so that the distance between the signal line and the ground line is only the distance between the layers of the printed circuit board. In this way, the loop area of ​​all signals on the board can be minimized, thereby effectively reducing differential mode radiation.

2.2 Component Layout

Designing a printed circuit board is not just about connecting the components with printed wires. More importantly, the characteristics and requirements of the circuit should be considered and the components should be placed correctly.

l) Circuit units and interconnected components should be laid out close to each other to reduce the length of the wiring and connections between components and reduce radiation and interference.

2) The layout should be divided into zones according to the relative high and low operating frequencies of the circuits or the switching speeds of the devices. The operating frequencies of the circuits should decrease from the I/O end to the far end of the board. In the higher frequency region, high-speed oscillating devices should not be placed close to the I/O end.

3) Keep components that easily generate electromagnetic radiation (such as clocks, oscillators, etc.) away from electromagnetic sensitive devices or wiring, and take electromagnetic shielding measures when necessary.

2.3 Layout of signal lines

The signal lines should be arranged according to the signal flow order to make the signal flow smoothly on the circuit board. Incompatible signal lines should be kept away from each other and not run in parallel. The signal lines on the same layer should be kept at a certain distance, and it is best to isolate them with corresponding ground loops to reduce signal crosstalk between lines. The signal lines distributed on different layers should be perpendicular to each other, so as to reduce the interference of electric and magnetic fields between lines. The loop area of ​​high-speed signals should be as small as possible to avoid radiation interference.

1) High-speed signal lines should be placed on the same layer as much as possible, and do not change layers; 2) Make the signal line transition smoothly to avoid signal reflection caused by sudden changes in line width; 3) Do not have part of the printed line close to the ground line and part of it not close to cause impedance mutation; 4) The signal line should not be too close to the edge of the printed board, otherwise it will cause characteristic impedance changes, and it is easy to generate fringe fields, increasing outward radiation: 5) For signal lines with strict requirements on the characteristic impedance of the wire, stripline or microstrip wiring should be used, which is conducive to adjusting the characteristic impedance by wire width, thickness and insulation layer thickness; 6) When the operating frequency of the circuit exceeds SMHz or the edge rate of the device exceeds sns, multi-layer boards should be used first to reduce electromagnetic interference; 7) There should be as few vias as possible in the high-speed signal transmission line, and the aperture should be small, which is conducive to reducing the parasitic capacitance of the hole. Small aperture vias, buried vias or blind vias are usually used.

2.4 Ground Layout

1) 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. The ground wires of the two should not be mixed and should be connected to the ground wire of the power supply end respectively. The ground area of ​​the linear circuit should be increased as much as possible.

2) Correctly choose 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 current formed by the grounding circuit has a greater impact on interference, so single-point grounding should be used. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large. At this time, the ground impedance should be reduced as much as possible, and multi-point grounding should be used nearby.

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 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 times the allowable current of the printed circuit board.

4) The isolation holes on the ground wire should not be too close, and the insulating ring of the isolation hole should not be too large to avoid forming unintentional grooves that affect the impedance of the upper layer signal line.

5) The ground wire forms a closed loop. When designing a ground wire system for a printed circuit board consisting only of digital circuits, making the ground wire into a closed loop can significantly improve the noise resistance.

1Electromagnetic compatibility

Electromagnetic compatibility (EMC) is a comprehensive subject, which mainly studies the problems of electromagnetic interference and anti-interference. Electromagnetic compatibility means that the performance of electronic equipment or system will not be reduced due to electromagnetic interference under the specified electromagnetic environment level, and the electromagnetic radiation generated by them is not greater than the limit level of verification, and will not affect the normal operation of other electronic equipment or systems, and meet the requirements of non-interference and reliable operation between equipment and equipment, and between systems. Electromagnetic compatibility (EMC) includes two aspects: electromagnetic radiation and anti-electromagnetic interference of products. A good electronic product must consider electromagnetic compatibility issues. It should not have electromagnetic radiation to interfere with other electronic equipment, and it should have low electromagnetic sensitivity and be able to resist the specified electromagnetic interference. Electromagnetic interference is the consequence of electromagnetic disturbance, which will reduce the performance of electronic equipment, transmission channels, systems and printed circuit board assemblies. As the basic component of electronic equipment, printed circuit boards also have electric fields and magnetic fields in use. There are electromagnetic compatibility issues when there are electric and magnetic fields. In particular, modern electronic equipment uses a large number of digital circuits and high-speed logic circuits, and the signal transmission speed is greatly improved, which also increases the factors causing electromagnetic radiation and electromagnetic interference. Therefore, considering electromagnetic compatibility issues is an important part of printed circuit board design.

2. Design of electromagnetic interference suppression on printed circuit boards

As the density of electronic devices and circuits on PCBs continues to increase, and the frequency of signals continues to increase, the problems of E-correlation (electromagnetic compatibility) and I-correlation (electromagnetic interference) will inevitably be introduced.

External conducted interference and radiated interference have little effect on the circuits on the PCB. In fact, taking the right measures in the design can often play a role in both anti-interference and emission suppression. When designing a printed circuit board, first of all, according to actual needs, choose the appropriate type of printed board (board and board layer), then determine the location of components on the board, and then lay out and design ground and signal lines.

2.1 Selection of printed circuit boards

Printed circuit boards can be divided into single-sided, double-sided and multi-layer boards. Single-sided and double-sided boards are generally used for circuits with low and medium density wiring and circuits with low integration. Multi-layer boards are suitable for high-speed digital circuits with high-density wiring and high-integration chips. From the perspective of electromagnetic compatibility, multi-layer boards can reduce the electromagnetic radiation of circuit boards and improve the anti-interference ability of circuit boards. Because in multi-layer boards, special power layers and ground layers can be set up, so that the distance between the signal line and the ground line is only the distance between the layers of the printed circuit board. In this way, the loop area of ​​all signals on the board can be minimized, thereby effectively reducing differential mode radiation.

2.2 Component Layout

Designing a printed circuit board is not just about connecting the components with printed wires. More importantly, the characteristics and requirements of the circuit should be considered and the components should be placed correctly.

l) Circuit units and interconnected components should be laid out close to each other to reduce the length of the wiring and connections between components and reduce radiation and interference.

2) The layout should be divided into zones according to the relative high and low operating frequencies of the circuits or the switching speeds of the devices. The operating frequencies of the circuits should decrease from the I/O end to the far end of the board. In the higher frequency region, high-speed oscillating devices should not be placed close to the I/O end.

3) Keep components that easily generate electromagnetic radiation (such as clocks, oscillators, etc.) away from electromagnetic sensitive devices or wiring, and take electromagnetic shielding measures when necessary.

2.3 Layout of signal lines

The signal lines should be arranged according to the signal flow order to make the signal flow smoothly on the circuit board. Incompatible signal lines should be kept away from each other and not run in parallel. The signal lines on the same layer should be kept at a certain distance, and it is best to isolate them with corresponding ground loops to reduce signal crosstalk between lines. The signal lines distributed on different layers should be perpendicular to each other, so as to reduce the interference of electric and magnetic fields between lines. The loop area of ​​high-speed signals should be as small as possible to avoid radiation interference.

1) High-speed signal lines should be placed on the same layer as much as possible, and do not change layers; 2) Make the signal line transition smoothly to avoid signal reflection caused by sudden changes in line width; 3) Do not have part of the printed line close to the ground line and part of it not close to cause impedance mutation; 4) The signal line should not be too close to the edge of the printed board, otherwise it will cause characteristic impedance changes, and it is easy to generate fringe fields, increasing outward radiation: 5) For signal lines with strict requirements on the characteristic impedance of the wire, stripline or microstrip wiring should be used, which is conducive to adjusting the characteristic impedance by wire width, thickness and insulation layer thickness; 6) When the operating frequency of the circuit exceeds SMHz or the edge rate of the device exceeds sns, multi-layer boards should be used first to reduce electromagnetic interference; 7) There should be as few vias as possible in the high-speed signal transmission line, and the aperture should be small, which is conducive to reducing the parasitic capacitance of the hole. Small aperture vias, buried vias or blind vias are usually used.

2.4 Ground Layout

1) 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. The ground wires of the two should not be mixed and should be connected to the ground wire of the power supply end respectively. The ground area of ​​the linear circuit should be increased as much as possible.

2) Correctly choose 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 current formed by the grounding circuit has a greater impact on interference, so single-point grounding should be used. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large. At this time, the ground impedance should be reduced as much as possible, and multi-point grounding should be used nearby.

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 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 times the allowable current of the printed circuit board.

4) The isolation holes on the ground wire should not be too close, and the insulating ring of the isolation hole should not be too large to avoid forming unintentional grooves that affect the impedance of the upper layer signal line.

5) The ground wire forms a closed loop. When designing a ground wire system for a printed circuit board consisting only of digital circuits, making the ground wire into a closed loop can significantly improve the noise resistance.

2.5 Width of printed conductors

The conductor width should be suitable for meeting electrical performance and facilitating production. Its minimum value is determined by the current it bears, but the minimum should not be less than 0.2mm. In high-density, high-precision printed circuits, the conductor width and spacing can generally be 0.3mm. The conductor width should also take into account its temperature rise under high current conditions. Single-panel tests show that when the copper foil thickness is 80um, the conductor width is 1-1.2mm, and the current passing through is 2A, the temperature rise is very small.

In the routing of the DIP package, when two wires pass between the two pins, the pad diameter can be set to 50 mil, and the line width and line spacing are both 10 mil; when only one wire passes between the two pins, the pad diameter can be set to 64011, and the line width and diameter are both 12 mil.

3. Software Anti-interference Measures

A watchdog circuit is used to prevent the system from shutting down or the program from entering an infinite loop due to external interference, hardware failure, and program errors; a software filtering method is used to eliminate high-frequency pulse interference.

4 Summary

The electromagnetic compatibility problem in printed circuit boards is very complicated, and corresponding measures should be taken for specific problems. In the design, we should be good at adopting new design methods, absorbing advanced design experience, and using mature installation technology to effectively reduce electromagnetic interference. With the continuous improvement of PCB technology and the in-depth development of electromagnetic compatibility, the electromagnetic compatibility performance of electronic products will also be significantly improved.