What is PCB EMC wiring design skills and rules?

 Abstract: EMC is about how to solve the problem of how electronic devices interfere with other devices or prevent external devices from generating electromagnetic field interference to their own electronic devices. In PCB EMC wiring design, especially high-speed circuit design, you must consider the electromagnetic compatibility of the circuit, otherwise, your product may not pass the 3C standard. Today we will talk about the specific design details and issues that need to be paid attention to in the PCB layout design of electromagnetic compatibility (EMC).

Table of content

1. PCB EMC wiring design skills

2. PCB EMC wiring design rules

    2.1 Wiring separation

    2.2 Protection and shunt line

    2.3 Power cord design

    2.4 Ground wire design

    2.5 Signal line design

 

1. PCB EMC wiring design skills

When designing the PCB layout design, engineers should fully comply with the design principles of placing in a straight line along the signal flow direction and try to avoid back and forth. Such a layout can avoid direct signal coupling and affect signal quality, as shown in Figure 1:

 

Figure 1 Circuit modules are placed in a straight line along with the signal flow

In addition, in order to prevent the PCB layout design of electronic components and components between circuits, the following principles should be followed:

A. If a “clean ground” interface is designed on a single board, the filtering and isolation devices 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.

B. 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 out through the interface, the PCB EMC wiring design principle in Figure 2 should be followed.

 

Figure 2 Principles of high, medium and low-speed PCB layout design 

C. 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 3 The filter circuit of the power input port should be placed close to the interface

 

D. The filtering, protection and isolation components of the interface circuit are placed close to the interface, which can effectively achieve the effects of protection, filtering and isolation, as shown in Figure 4. 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.

 

                            Figure 4 The filtering, protection and isolation components of the interface circuit are placed close to the interface

 

E. 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.

F. 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.

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

H. 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.

 

2. PCB EMC wiring design rules

In addition to the selection of components and circuit design, well printed circuit board wiring is also a very important factor in electromagnetic compatibility. The following will discuss some general rules of PCB EMC wiring design and the design of power lines, ground lines and signal lines. Strategy is introduced.

2.1 Wiring separation

The function of wiring separation is to minimize the crosstalk and noise coupling between adjacent lines in the same layer of the PCB. Isolate lines and lines, 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 by other signal lines.

2.2 Protection and shunt line

Setting the shunt and protection circuit is a very effective way to isolate and protect key signals, such as the system clock signal in a noisy environment. For example, 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.

2.3 Power cord design

According to the size of the printed circuit board current, try to increase the width of the power line as much 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.

2.4 Ground wire design

The principle of ground PCB EMC wiring design is:

(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 ground 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.

2.5 Signal line design

For key signal lines, if the single 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 strong radiation signal lines such as clock lines, bus lines, and radiofrequency lines, and sensitive signal lines such as reset signal lines, chip select signal lines, system control signals, etc., keep the signal lines away from the interface. 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 sensor 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.　

 

Generally speaking, the improvement of PCB EMC wiring design is: before wiring, first study the design of the return path, there is the best chance of success, and the goal of reducing EMI radiation can be achieved.