Besides the selection of components and circuit design, good printed circuit board (PCB) design is also a very important factor in electromagnetic compatibility. The key of PCB design is to minimize the reflow area and make the reflow path flow in the designed direction. The most common return current problems come from cracks in the reference plane, changing the reference plane layer, and signals flowing through the connector. Crossover 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
Thickness, via process and the number of layers in circuit board design are not the key to solve the problem. Excellent layered stacking is the key to ensure the bypass and decoupling of the power bus, minimize the transient voltage on the power layer or ground layer and shield the electromagnetic field of the signal and power supply. From the point of view of signal routing, a good layering strategy should be to place all signal routing on one or several layers, which are next to the power layer or the ground layer. For power supply, a good layering strategy should be that the power supply layer is adjacent to the ground layer, and the distance between the power supply layer and the ground layer is as small as possible, which is what we call “layering” strategy. Next, we will specifically talk about the excellent PCB layering strategy.
1. The projection plane of the wiring 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, which will lead to the problem of “edge radiation” and the increase of signal loop area and differential mode radiation.
2. Try to avoid the arrangement of adjacent wiring layers. Because parallel signal traces on adjacent wiring layers will lead to signal crosstalk, if adjacent wiring layers cannot be avoided, the layer spacing between two wiring layers should be appropriately widened and the layer spacing between wiring layers and their signal loops should be narrowed.
3. Adjacent plane layers should avoid overlapping their projection planes. Because when the projections overlap, the coupling capacitance between layers will lead to the noise coupling between layers.
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, it is generally necessary to use multilayer board design. 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, RF 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. The key signal lines are generally strong radiation or extremely sensitive signal lines. Wiring close to the ground plane can reduce the area of its signal loop, reduce its radiation intensity or improve its anti-interference ability.
2. The power plane should be retracted relative to its adjacent ground plane (recommended value is 5h ~ 20h). The problem of “edge radiation” can be effectively suppressed by shrinking the power plane relative to its return ground plane.
In addition, the main working power plane of a single board (the most widely used power plane) should be close to its ground plane to effectively reduce the loop area of power current.
3. Whether there is no signal line ≥50MHz on the TOP and BOTTOM layers of the single board. If so, it’s best to walk the high-frequency signal between two plane layers to suppress its radiation to space.
Single-layer and double-layer plate design:
For the design of single-layer board and double-layer board, we should pay attention to the design of key signal lines and power lines. There must be a ground wire near the power supply wiring, which is adjacent to and parallel to it, so as to reduce the area of the power supply current loop.
Guide Ground Line should be laid on both sides of key signal line of single layer board. The projection plane of the key signal line of the double-layer board should be paved in a large area, or “Guide Ground Line” should be designed in the same way as the single-layer board. On the one hand, the “protective ground wire” on both sides of the key signal line can reduce the area of the signal loop, and on the other hand, it can prevent the crosstalk between the signal line and other signal lines.
Generally speaking, the layering of PCB can be designed according to the following table.
PCB layout skills
During PCB layout design, the design principle of placing signals in a straight line along the signal flow direction should be fully observed, and it should be avoided to go back and forth as far as possible. This can avoid the direct coupling of signals, which will affect the 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 the interface “clean ground” is designed on the single board, the filter and isolation devices should be placed on the isolation belt between the “clean ground” and the working place. In this way, the filter or isolation devices can be prevented from being coupled with each other through the plane layer, which weakens the effect. In addition, on the “clean ground”, no other devices can be placed except filtering and protection 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 among 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 the high-frequency circuit noise 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 filtered circuit from being coupled again.
4. The filtering, protection and isolation devices of the interface circuit are placed close to the interface, which can effectively achieve the effect of protection, filtering and isolation. If there are both filtering and protection circuits at the interface, the principle of protection before filtering should be followed. Because the protection circuit is used to suppress external overvoltage and overcurrent, if the protection circuit is placed behind the filter circuit, the filter circuit will be damaged by overvoltage and overcurrent. In addition, as the input and output traces of the circuit are coupled with each other, the filtering, isolation or protection effect will be weakened, so the input and output traces of the filter circuit (filter), isolation and protection circuit should not be coupled with each other during layout.
5. Sensitive circuits or devices (such as reset circuits, etc.) are at least 1000mil away from the edges of the single board, especially the interface side edges of the single board.
6. Energy storage and high-frequency filter capacitors should be placed near unit circuits or devices with large current changes (such as input and output terminals of power modules, fans and relays) to reduce the loop area of high-current loops.
7. Filters should be placed side by side to prevent the filtered circuit from being interfered again.
8. Crystal, crystal oscillator, relay, switching power supply and other strong radiation devices shall be at least 1000mil away from the single board interface connector. In this way, the interference can be radiated directly or the current can be coupled out on the outgoing cable.
PCB wiring rules
Besides the selection of components and circuit design, good wiring of printed circuit board (PCB) 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 cost to the final completion of the product. Anyone should remember that a poor PCB wiring can lead to more EMC problems, rather than eliminate them. In many cases, even adding filters and components can’t solve these problems. In the end, the whole board had to be rewired. Therefore, it is the cheapest way to develop good PCB wiring habits at the beginning. In the following, some general rules of PCB wiring and the design strategies of power lines, ground lines and signal lines are introduced. Finally, according to these rules, some improvement measures for typical printed circuit boards of air conditioners are put forward.
1. Wiring separation
The function of wiring separation is to minimize the crosstalk and noise coupling between adjacent lines in the same layer of PCB. The 3W specification indicates that all signals (clock, video, audio, reset, etc.) are isolated from line to line and edge to edge. In order to further reduce the magnetic coupling, the reference ground is placed near the key signal to isolate the coupling noise generated on other signal lines.
2. Protection and shunt lines
Setting shunt and protection lines is a very effective way to isolate and protect key signals, such as the system clock signal in a noisy environment. Parallel or protection lines in PCB are laid along the lines of key signals. The protection circuit not only isolates the coupling flux generated by other signal lines, but also isolates the key signals from the 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 the ground), but both ends of the protection line must be connected to the ground. In order to further reduce the coupling, the protective circuit in the multilayer PCB can be added with a path to the ground every other section.
3. Power cord design
According to the current of printed circuit board, try to widen the width of power line and reduce the loop resistance. At the same time, make the direction of power line and ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability. In single panel or double panel, if the power line is long, the coupling capacitor should be removed to the ground every 3000mil, and the value of the capacitor should be 10uF＋1000pF.
4. Ground wire design
The design principle of ground wire is:
(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 far as possible. The grounding of low-frequency circuits should be grounded by single-point parallel connection as far as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. Multi-point series grounding should be adopted for high-frequency circuits, and the ground wire should be short and rented. Grid-shaped large-area ground foil should be used as far as possible around high-frequency components.
(2) The grounding wire should be as bold as possible. If the grounding wire is made of a heavy thread, the grounding potential will change with the change of current, which will reduce the anti-noise performance. Therefore, the grounding wire should be thickened so that it can pass three times the allowable current on the printed circuit board. If possible, the grounding wire should be more than 2~3mm.
(3) The grounding wire forms a closed loop. For printed boards composed of digital circuits only, most of the grounding circuits can improve their anti-noise ability by arranging them into a cluster loop.
5. Signal line design
For the key signal lines, if the single board has an internal signal routing layer, the key signal lines such as clocks are laid in the inner layer, and the preferred wiring layer is given priority. In addition, the key signal lines must not be routed across the division area, including the reference plane gap caused by vias and pads, otherwise the signal loop area will be increased. Moreover, the key 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 strong radiation signal lines such as clock line, bus line and RF line, and sensitive signal lines such as reset signal line, chip select signal line and system control signal, keep away from the interface outgoing signal line. Thereby preventing the interference on the strong radiation signal line from coupling to the outgoing signal line and radiating outwards; It also prevents the external interference brought by the outgoing signal line of the interface from being coupled to the sensitive signal line, resulting in the wrong operation of the system.
For differential signal lines, they should be on the same floor, of equal length, and run in parallel, keeping the impedance consistent, and there are no other lines 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.
Generally speaking, the improvement of PCB design to EMC is: before wiring, the best chance of success is to study the design scheme of reflow path, which can achieve the goal of reducing EMI radiation. Moreover, it doesn’t cost any money to change the wiring layer before the actual wiring, which is the cheapest way to improve EMC.
