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This article introduces you how to analyze EMC PCB design technology, the content is very detailed, interested friends can refer to it, I hope it can help you.

In addition to component selection and circuit design, good printed circuit board (PCB) design is also a very important factor in electromagnetic compatibility. The key to PCB EMC design is to minimize the return area and let the return path flow in the direction of design. The most common return current problems arise from cracks in the reference plane, shifting reference plane layers, and signals flowing through connectors. Jumper capacitors or decoupling capacitors may solve some problems, but the overall impedance of capacitors, vias, pads, and wiring must be considered. This lecture will introduce EMC PCB design technology from three aspects: PCB layering strategy, layout skills and routing rules.

PCB layering strategy

The thickness, via process and number of layers in circuit board design are not the key to solve the problem. Good layered stacking is the key to ensure bypass and decoupling of power bus, minimize transient voltage on power plane or ground plane, and shield signal and power electromagnetic fields. From the signal trace point of view, a good layering strategy should be to put all the signal traces on one or several layers, which are next to the power plane or ground plane. For power, a good layering strategy should be that the power plane is adjacent to the ground plane and the distance between the power plane and the ground plane is as small as possible. This is what we call a "layering" strategy. Below we will talk specifically about good PCB layering strategies.

1. The projection plane of the routing layer should be within its reflow plane layer area. If the wiring layer is not in the projection area of its reflow plane layer, there will be signal lines outside the projection area during wiring, resulting in "edge radiation" problems, and also lead to an increase in the signal loop area, resulting in increased differential mode radiation.

2. Avoid adjacent wiring layers as much as possible. Because parallel signal traces on adjacent wiring layers will cause signal crosstalk, if it is impossible to avoid adjacent wiring layers, the layer spacing between two wiring layers should be appropriately increased and the layer spacing between wiring layers and their signal loops should be reduced.

3. Adjacent planar layers should avoid overlapping their projection planes. Because the coupling capacitance between layers causes noise coupling between layers when projections overlap.

multilayer board design

When the clock frequency exceeds 5MHz, or the signal rise time is less than 5ns, in order to make the signal loop area can be well controlled, it is generally necessary to use a multilayer board design. The following principles should be noted when designing multilayer boards:

1. Critical routing layers (clock lines, bus lines, interface signal lines, RF lines, reset signal lines, chip select signal lines, and various control signal lines) should be adjacent to, and preferably between, the complete ground plane, as shown in Figure 1. Key signal lines are generally strong radiation or extremely sensitive signal lines, close to the ground plane wiring can reduce the signal loop area, reduce its radiation intensity or improve anti-interference ability.

Figure 1 Critical routing layer between two planes

2. The power plane should be recessed relative to its adjacent ground plane (recommended value 5H~20H). The power plane is retracted relative to its return ground plane, which can effectively suppress the "edge radiation" problem.

In addition, the main operating power plane of the board (the most widely used power plane) should be located in close proximity to its ground plane to effectively reduce the loop area of the power supply current, as shown in Figure 3.

Figure 3 Power plane should be in close proximity to its ground plane

3. Whether there is no signal line ≥50MHz on TOP and BOTTOM layers of single board. If so, it is best to put the high-frequency signal between two planar layers to suppress its radiation to space.

Single and double plate designs:

For the design of single-layer board and double-layer board, the main attention should be paid to the design of key signal lines and power lines. There must be a ground wire adjacent to and parallel to the power line to reduce the power current loop area.

"Guide Ground Line" shall be laid on both sides of key signal line of single-layer board. The key signal line ground projection plane of double-layer board shall be paved with large area, or the ground treatment method of single-layer board shall be the same, and "Guide Ground Line" shall be designed, as shown in Figure 5. On the one hand, the "guard ground" on both sides of the key signal line can reduce the signal loop area, and on the other hand, it can prevent crosstalk between the signal line and other signal lines.

Fig. 5 Large area paving on the projection plane of key signal line of double-layer board

In general, PCB layering can be designed according to the following table.

PCB Layout Tips

PCB layout design, should fully comply with the signal flow along the line placed in the design principle, try to avoid back and forth around. This can avoid direct signal coupling and affect signal quality. In addition, in order to prevent the layout of electronic components and components between circuits, the following principles should be observed:

Figure 6 Circuit modules placed in a straight line along the signal flow direction

1. If the interface "clean ground" is designed on the board, the filter and isolation devices shall be placed on the isolation band between the "clean ground" and the working ground. This prevents the filter or isolation devices from coupling to each other through the planar layer, weakening the effect. In addition, on the "clean ground", no other devices can be placed except filtering and protective devices.

2. When multiple module circuits are placed on the same PCB, digital circuits and analog circuits, high speed and low speed circuits should be laid out separately to avoid mutual interference between digital circuits, analog circuits, high speed circuits and low speed circuits. In addition, when there are high, medium and low speed circuits on the circuit board at the same time, in order to avoid high frequency circuit noise radiating through the interface, the layout principle should be followed.

3. The filter circuit of the power input port of the circuit board should be placed close to the interface to avoid the filtered circuit being coupled again.

Figure 8 The filter circuit of the power input port should be placed close to the interface.

4. The filtering, protection and isolation devices of the interface circuit are placed close to the interface, as shown in Figure 9, which can effectively achieve the effects of protection, filtering and isolation. If there are both filter and protection circuits at the interface, the principle of first protection and then filter should be followed. Because the protection circuit is used for external overvoltage and overcurrent suppression, if the protection circuit is placed after the filter circuit, the filter circuit will be damaged by overvoltage and overcurrent. In addition, since the input and output lines of the circuit are coupled to each other, the filtering, isolation or protection effect will be weakened. The layout should ensure that the input and output lines of the filter circuit (filter), isolation and protection circuit are not coupled to each other.

Figure 9. Filtering, protection and isolation of interface circuits placed close to the interface

5. Sensitive circuits or devices (such as reset circuits) shall be at least 1000 mils away from each edge of the board, especially the edge of the board interface side.

6. Energy storage and high-frequency filter capacitors should be placed near unit circuits or devices with large current variations (such as input and output terminals of power modules, fans and relays) to reduce the loop area of large current loops.

7. Filter components should be placed side by side to prevent interference with the filtered circuit.

8. Crystal, crystal oscillator, relay, switching power supply and other strong radiation devices away from the board interface connector at least 1000 mils. This allows interference to be radiated either directly or by coupling out current on outgoing cables.

PCB routing rules

In addition to component selection and circuit design, good printed circuit board (PCB) routing is also a very important factor in electromagnetic compatibility. Since PCBs are an inherent component of the system, enhancing EMC in PCB routing does not impose additional costs on the final completion of the product.

Anyone should remember that a poor PCB routing can cause more EMC problems than eliminate them, and in many cases, even adding filters and components will not solve them. In the end, the entire board had to be rewired. Therefore, developing good PCB routing habits at the beginning is the most cost-effective way. The following will introduce some common rules of PCB wiring and design strategies for power lines, ground lines and signal lines. Finally, according to these rules, improvement measures are proposed for typical printed circuit board circuits of air conditioners.

1. wiring separation

The purpose of routing separation is to minimize crosstalk and noise coupling between adjacent lines within the same PCB layer. The 3W specification states that all signals (clock, video, audio, reset, etc.) must be isolated line-to-line and edge-to-edge as shown in Figure 10. To further reduce magnetic coupling, a ground reference is placed near the critical signal to isolate coupling noise from other signal lines.

Figure 10 Trace Isolation

2. protection and shunt circuit

Shunt and protection circuits are very effective ways to isolate and protect critical signals, such as system clock signals, in a noisy environment. In Figure 21, the parallel or guard lines in the PCB are routed along the critical signal lines. Guard lines not only isolate coupling flux generated by other signal lines, but also isolate critical signals from coupling to other signal lines. The difference between a shunt line and a protection line is that the shunt line does not have to be terminated (connected to ground), but both ends of the protection line must be connected to ground. To further reduce coupling, the protection circuit in the multilayer PCB can be added with a path to ground at every other segment.

3. Power cord design

According to the size of the printed circuit board current, try to thicken the width of the power line and reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability. In single-sided or double-sided panels, if the power line is long, a decoupling capacitor should be added to ground every 3000 mils, and the capacitance value is 10uF+1000 pF.

4. Ground design

The principles of ground design are:

(1)Digital ground is separated from analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. Low-frequency circuit ground should be single-point parallel grounding as far as possible, the actual wiring difficulties can be part of the series and then parallel grounding. High-frequency circuit should adopt multi-point series grounding, ground wire should be short and rent, high-frequency components around as much as possible with grid-like large area of ground foil.

(2)Ground wire should be as thick as possible. If the grounding wire is made of a very thin line, the grounding potential changes with the change of current, so that the noise resistance is reduced. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the grounding wire should be more than 2~3 mm.

(3)The ground wire forms a closed loop. For printed circuit boards composed of digital circuits only, the grounding circuits are mostly arranged into a group loop to improve the anti-noise ability.

5. signal line design

For key signal lines, if the single board has an internal signal routing layer, the key signal lines such as clocks are arranged in the inner layer, and the preferred wiring layer is given priority. In addition, critical signal lines must not cross the partition area, including reference plane gaps caused by vias and pads, otherwise it will lead to an increase in signal loop area. Moreover, the critical signal line should be ≥3H from the edge of the reference plane (H is the height of the line from the reference plane) to suppress the edge radiation effect.

For clock lines, bus lines, RF lines and other strong radiation signal lines and reset signal lines, chip selection signal lines, system control signals and other sensitive signal lines, should be far away from the interface outgoing signal lines. Therefore, the interference on the strong radiation signal line is prevented from being coupled to the outgoing signal line and radiating outward, and the external interference brought in by the outgoing signal line of the interface is also prevented from being coupled to the sensitive signal line, resulting in system misoperation.

For differential signal lines, they shall be routed on the same layer, of equal length and in parallel, with consistent impedance, and no other routing between differential lines. Because the common-mode impedance of differential pairs is equal, the anti-interference ability can be improved.

According to the above wiring rules, the typical printed circuit board circuit of air conditioner is improved and optimized, as shown in Figure 12.

Figure 12 Typical printed circuit board circuit for an improved air conditioner

In general, PCB design for EMC improvement is: before wiring, first study the design of the return path, there is the best chance of success, can achieve the goal of reducing EMI radiation. Moreover, before the actual wiring has been started, it does not cost any money to change the wiring layer, which is the cheapest way to improve EMC.

About how to analyze EMC PCB design technology to share here, I hope the above content can be of some help to everyone, you can learn more knowledge. If you think the article is good, you can share it so that more people can see it.

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