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 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. 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 techniques, and wiring rules.

PCB layering strategy

In circuit board design, the thickness, via process and number of circuit board layers are not the key to solving the problem. Good layering stacking is the key to ensure bypass and decoupling of the power bus, minimize transient voltage on the power layer or ground layer, and shield the electromagnetic field of the signal and power supply.

From the perspective of signal routing, a good layering strategy should be to put all signal routing in one or several layers, which are close to the power layer or ground layer. For 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. Let's talk about the good PCB layering strategy in detail.

1. The projection plane of the wiring layer should be within the area of ​​its return plane layer. If the wiring layer is not within the projection area of ​​its return plane layer, there will be signal lines outside the projection area during wiring, resulting in "edge radiation" problems, and will also increase the area of ​​the signal loop, resulting in increased differential mode radiation.
2. Try to avoid adjacent wiring layers. 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 the two wiring layers should be appropriately increased, and the layer spacing between the wiring layer and its signal loop should be reduced.
3. Adjacent plane layers should avoid overlapping projection planes. Because when the projections overlap, the coupling capacitance between 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 noted when designing a multilayer board:

1. The key wiring layer (clock line, bus, interface signal line, RF line, reset signal line, chip select signal line and various control signal lines) should be adjacent to the complete ground plane, preferably between two ground planes, as shown in the figure below. Key signal lines are generally strong radiation or extremely sensitive signal lines. Routing close to the ground plane can reduce the signal loop area, 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 5H to 20H). Retracting the power plane relative to its return ground plane can effectively suppress the "edge radiation" problem, as shown in the following figure.

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 current, as shown in the following figure.

3. Are there any signal lines ≥50MHz on the TOP and BOTTOM layers of the single-layer board? If so, it is best to route the high-frequency signal between the two plane layers to suppress its radiation to the space.

Single-layer and double-layer board design

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

"Guide Ground Line" should be laid on both sides of the key signal line of the single-layer board. The projection plane of the key signal line of the double-layer board should have a large area of ​​ground, or the same treatment method as the single-layer board should be used to design the "Guide Ground Line", as shown in the figure below. The "Guide Ground Line" on both sides of the key signal line can reduce the signal loop area on the one hand, and prevent crosstalk between the signal line and other signal lines on the other hand.

PCB layout tips

When designing PCB layout, you should fully comply with the design principle of placing the signal in a straight line and try to avoid back and forth, as shown in the figure below. This can avoid direct coupling of signals 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 comply with the following principles:

1. If the interface "clean ground" is designed on the single board, the filtering and isolation devices should be placed on the isolation belt between the "clean ground" and the working ground. This can prevent the filtering or isolation devices from coupling with each other through the plane layer and weakening the effect. In addition, no other devices can be placed on the "clean ground" except filtering and protection devices.

2. When multiple module circuits are placed on the same PCB, digital circuits and analog circuits, high-speed circuits 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 the high-frequency circuit noise from radiating outward through the interface, the layout principles in the figure below should be followed.

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

4. The filtering, protection and isolation components of the interface circuit are placed close to the interface, as shown in the figure below, which can effectively achieve the protection, filtering and isolation effects. If there are both filtering and protection circuits at the interface, the principle of protection first and filtering later should be followed. Because the protection circuit is used to suppress external overvoltage and overcurrent, if the protection circuit is placed after the filtering circuit, the filtering 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 effects when coupled with each other, it is necessary to ensure that the input and output lines of the filtering circuit (filter), isolation and protection circuit are not coupled with each other during layout.

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

6. Energy storage and high-frequency filter capacitors should be placed near 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. Filter components need to be placed side by side to prevent the filtered circuit from being interfered with again.

8. Strong radiation devices such as crystals, crystal oscillators, relays, switching power supplies, etc. should be at least 1000mil away from the board interface connector. This can radiate interference directly outward or couple current on the outgoing cable to radiate outward.

PCB layout rules

In addition to component selection and circuit design, good printed circuit board (PCB) layout is also a very important factor in electromagnetic compatibility. Since the PCB is an inherent component of the system, enhancing electromagnetic compatibility in PCB layout will not bring additional costs to the final product. Everyone should remember that a poor PCB layout can cause more electromagnetic compatibility problems rather than eliminate them. In many cases, even adding filters and components cannot solve these problems. In the end, the entire board has to be re-wired.

Therefore, it is the most cost-effective way to develop good PCB layout habits at the beginning. The following will introduce some general rules for PCB layout and design strategies for power lines, ground lines and signal lines. Finally, based on these rules, improvement measures are proposed for a typical printed circuit board circuit of an air conditioner.

1. Wiring separation

The role of wiring separation is to minimize crosstalk and noise coupling between adjacent lines on the same layer of the PCB. The 3W specification indicates that all signals (clock, video, audio, reset, etc.) must be isolated from line to line and edge to edge as shown in Figure 10. To further reduce magnetic coupling, place the reference ground near key signals to isolate the coupling noise generated by other signal lines.

2. Protection and Shunt Lines

Setting up shunt and protection lines is a very effective way to isolate and protect critical signals, such as system clock signals in a noisy environment. In the figure below, the parallel or protection lines in the PCB are laid along the lines of critical signals. The protection line not only isolates the coupling flux generated by other signal lines, but also isolates the critical signal 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. To further reduce coupling, the protection line in the multi-layer PCB can be added with a path to the ground at every other section.

3. Power line design

According to the current of the printed circuit board, the width of the power line should be increased as much as possible to reduce the loop resistance. At the same time, the direction of the power line and the ground line should be consistent with the direction of data transmission, which will help enhance the anti-noise ability. In a single-sided or double-sided board, if the power line is very long, a decoupling capacitor should be added to the ground every 3000mil, and the capacitance value should be 10uF + 1000pF.

4. Ground line design

The principles of ground line design are:

(1) Digital ground and analog ground are separated. 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 connected to the ground at a single point as much as possible. If the actual wiring is difficult, it can be connected in series and then connected in parallel. High-frequency circuits should adopt multi-point series grounding. The ground line should be short and loose, and a large area of ​​grid-shaped ground foil should be used around high-frequency components as much as possible.

(2) The ground line should be as thick as possible. If the ground line is a very thin line, the ground potential changes with the change of 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 circuit board. If possible, the ground wire should be more than 2~3mm.
(3) The ground wire forms a closed loop. For printed circuit boards composed only of digital circuits, the ground circuit is mostly arranged in a group loop to improve the noise resistance.

5. Signal line design

For key signal lines, if the single board has an internal signal routing layer, the key signal lines such as the clock are arranged in the inner layer, and the preferred routing layer is given priority. In addition, the key signal lines must not be routed across the partition area, including the reference plane gap caused by vias and pads, otherwise it will lead to an increase in the signal loop area. In addition, the key signal line should be ≥3H away 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 lines, buses, and RF lines, and sensitive signal lines such as reset signal lines, chip select signal lines, and system control signals, they should be kept away from the interface outbound signal lines. This prevents interference on strong radiation signal lines from coupling to outgoing signal lines and radiating outward; it also prevents external interference brought in by interface outgoing signal lines from coupling to sensitive signal lines, causing system misoperation.

Differential signal lines should be routed in the same layer, of equal length, and in parallel, with consistent impedance, and no other routing between differential lines. Because ensuring that the common-mode impedance of the differential line pair is equal can improve its anti-interference ability.

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

In general, the improvement of EMC by PCB design is: before wiring, the design of the return path is studied first, which will give you the best chance of success and achieve the goal of reducing EMI radiation. Moreover, before the actual wiring is done, changing the wiring layer does not cost any money, which is the cheapest way to improve EMC.