Analysis on PCB Design Technology Based on EMC

In addition to the selection of components 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 reduce the reflow area as much as possible and let the reflow path flow in the direction of the design. The most common return current problems come from cracks in the reference plane, changing the reference plane layer, and the signal flowing through the connector. 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’s PCB design technology from three aspects: PCB layering strategy, layout skills and wiring rules.


PCB layering strategy

The thickness, via process and the number of layers in the circuit board design are not the key to solving the problem. Good layered stacking is to ensure the bypass and decoupling of the power bus and minimize the transient voltage on the power layer or ground layer. The key to shielding the electromagnetic field of the signal and power supply. From the perspective of signal traces, a good layering strategy should be to put all signal traces on one or several layers, and these layers are next to the power layer or ground layer. For the power supply, a good layering strategy should be that the power layer is adjacent to the ground layer, and the distance between the power layer and the ground layer is as small as possible. This is what we call the “layering” strategy. Below we will specifically talk about the excellent PCB layering strategy. 1. The projection plane of the wiring layer should be in its reflow plane layer area. If the wiring layer is not in the projection area of ​​the reflow plane layer, there will be signal lines outside the projection area during wiring, which will cause the “edge radiation” problem, and will also cause the signal loop area to increase, resulting in increased differential mode radiation . 2. Try to avoid setting up adjacent wiring layers. Because parallel signal traces on adjacent wiring layers can cause signal crosstalk, if it is impossible to avoid the adjacent wiring layers, the layer spacing between the two wiring layers should be increased appropriately, and the layer spacing between the wiring layer and its signal circuit should be reduced. 3. Adjacent plane layers should avoid overlapping of their projection planes. Because when the projections overlap, the coupling capacitance between the layers will cause the noise between the layers to couple with each other.

Multilayer board design

When the clock frequency exceeds 5MHz, or the signal rise time is less than 5ns, in order to control the signal loop area well, a multilayer board design is generally required. The following principles should be paid attention to when designing multilayer boards: 1. The key wiring layer (the layer where the clock line, bus line, interface signal line, radio frequency line, reset signal line, chip select signal line and various control signal lines are located) should be adjacent to the complete ground plane, preferably between the two ground planes, such as Shown in Figure 1. The key signal lines are generally strong radiation or extremely sensitive signal lines. Wiring close to the ground plane can reduce the area of ​​the signal loop, reduce the radiation intensity or improve the anti-interference ability.

Figure 1 The key wiring layer is between the two ground planes

2. The power plane should be retracted relative to its adjacent ground plane (recommended value 5H~20H). The retraction of the power plane relative to its return ground plane can effectively suppress the “edge radiation” problem.

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

Figure 3 The power plane should be close to its ground plane

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

Single-layer board and double-layer board design

For the design of single-layer and double-layer boards, the design of key signal lines and power lines should be paid attention to. There must be a ground wire next to and parallel to the power trace to reduce the area of ​​the power current loop. “Guide Ground Line” should be laid on both sides of the key signal line of the single-layer board, as shown in Figure 4. The key signal line projection plane of the double-layer board should have a large area of ​​ground, or the same method as the single-layer board, design “Guide Ground Line”, as shown in Figure 5. The “guard ground wire” on both sides of the key signal line can reduce the signal loop area on the one hand, and also prevent crosstalk between the signal line and other signal lines.

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

PCB layout skills

When designing the PCB layout, fully comply with the design principle of placing in a straight line along the signal flow direction, and try to avoid looping back and forth, as shown in Figure 6. This can avoid direct signal coupling and affect signal quality. In addition, in order to prevent mutual interference and coupling between circuits and electronic components, the placement of circuits and the layout of components should follow the following principles:

1. If a “clean ground” interface is designed on the board, the filtering and isolation components should be placed on the isolation band between the “clean ground” and the working ground. This can prevent the filtering or isolation devices from coupling to each other through the planar layer, which weakens the effect. In addition, on the “clean ground”, apart from filtering and protection devices, no other devices can be placed. 2. When multiple module circuits are placed on the same PCB, digital circuits and analog circuits, and 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 high, medium, and low-speed circuits exist on the circuit board at the same time, in order to prevent high-frequency circuit noise from radiating outward through the interface.

3. The filter circuit of the power input port of the circuit board should be placed close to the interface to prevent the circuit that has been filtered from 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 components 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 is both a filter and a protection circuit at the interface, the principle of first protection and then filtering 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 will weaken the filtering, isolation or protection effect when they are coupled with each other, ensure that the input and output lines of the filter circuit (filter), isolation and protection circuit do not couple with each other during layout.

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

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

7. The filter components must be placed side by side to prevent the filtered circuit from being interfered again.

8. Keep strong radiation devices such as crystals, crystal oscillators, relays, and switching power supplies at least 1000 mils away from the board interface connectors. In this way, the interference can be radiated directly or the current can be coupled to the outgoing cable to radiate outward.

PCB wiring rules

In addition to the selection of components and circuit design, good printed circuit board (PCB) wiring is also a very important factor in electromagnetic compatibility. Since PCB is an inherent component of the system, enhancing electromagnetic compatibility in PCB wiring will not bring additional costs to the final completion of the product. Anyone should remember that a poor PCB layout can cause more electromagnetic compatibility problems, rather than eliminate them. In many cases, even the addition of filters and components cannot solve these problems. In the end, the entire board had to be rewired. Therefore, it is the most cost-effective way to develop good PCB wiring habits at the beginning. The following will introduce some general rules of PCB wiring and the design strategies of power lines, ground lines and signal lines. Finally, according to these rules, improvement measures are proposed for the typical printed circuit board circuit of the air conditioner. 1. Wiring separation The function of wiring separation is to minimize crosstalk and noise coupling between adjacent circuits in the same layer of the PCB. The 3W specification states that all signals (clock, video, audio, reset, etc.) must be isolated from line to line, edge to edge, as shown in Figure 10. In order to further reduce the magnetic coupling, the reference ground is placed near the key signal to isolate the coupling noise generated by other signal lines.

2. Protection and shunt line setting Shunt and protection line is a very effective method to isolate and protect key signals, such as system clock signals in a noisy environment. In Figure 21, the parallel or protection circuit in the PCB is laid along the circuit of the key signal. The protection circuit not only isolates the coupling magnetic flux generated by other signal lines, but also isolates key signals from coupling with other signal lines. The difference between the shunt line and the 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 the ground. In order to further reduce the coupling, the protection circuit in the multilayer PCB can be added with a path to the ground every other segment.

3. The power line design is based on the size of the printed circuit board current, and the width of the power line is as thick as possible to 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 a single or double panel, if the power line is very long, a decoupling capacitor should be added to the ground every 3000 mil, and the value of the capacitor is 10uF+1000pF.

Ground wire design

The principles of ground wire design are:

(1) The digital ground is separated from the analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be grounded in parallel at a single point as much as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. The high-frequency circuit should be grounded at multiple points in series, the ground wire should be short and leased, and the grid-like large-area ground foil should be used around the high-frequency component as much as possible.

(2) The grounding wire should be as thick as possible. If the ground wire uses a very tight line, the ground potential changes with the change of the current, which reduces the anti-noise performance. 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 2~3mm or more.

(3) The ground wire forms a closed loop. For printed boards composed only of digital circuits, most of their grounding circuits are arranged in loops to improve noise resistance.

Signal line design

For key signal lines, if the board has an internal signal wiring layer, the key signal lines such as clocks should be laid on the inner layer, and priority is given to the preferred wiring layer. In addition, key signal lines must not be routed across the partition area, including reference plane gaps caused by vias and pads, otherwise it will lead to an increase in the area of ​​the signal loop. And the key signal line should be more than 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, radio frequency lines and other strong radiation signal lines and reset signal lines, chip select signal lines, system control signals and other sensitive signal lines, keep them away from the interface and outgoing signal lines. This prevents the interference on the strong radiating signal line from coupling to the outgoing signal line and radiating outward; and also avoids the external interference brought in by the interface outgoing signal line from coupling to the sensitive signal line, causing system misoperation. Differential signal lines should be on the same layer, equal length, and run in parallel, keeping the impedance consistent, and there should be no other wiring between the differential lines. Because the common mode impedance of the differential line pair is ensured to be equal, its anti-interference ability can be improved. According to the above wiring rules, the typical printed circuit board circuit of the air conditioner is improved and optimized.