PCB design technology of EMC

In addition to component selection and circuit design, good printed circuit board (PCB) design is also a very important factor in EMC. The key to PCB EMC design is to minimize the backflow area and allow the backflow path to flow in the designed direction. The most common return current problems come from cracks in the reference plane, transforming the reference plane layer, and signals flowing through the connector. Bonding capacitors or de-coupling capacitors may solve some problems, but the overall impedance of the capacitor, through hole, pad, and wiring must be taken into account. This lecture will introduce the PCB design technology of EMC from three aspects of PCB layering strategy, layout skills and wiring rules.

PCB layering strategy

In circuit board design, thickness, hole process and the number of layers of circuit board are not the key to solve the problem. Good layered stacking is the key to ensure the bypass and decoupling of the power busbar, to minimize the transient voltage in the power supply layer or the ground and to shield the signal from the electromagnetic field of the power supply. From the point of view of signal routing, a good layering strategy would be to place all signal routing in one or more layers, which are adjacent to the power supply layer or connection layer. For power supply, a good stratification 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, this is what we talk about "stratification" strategy. We'll talk about a good PCB layering strategy in detail.

1. The projected plane of the wiring layer shall be within its reflux plane layer region. If the wiring layer is not in the projection area of the backflow plane layer, there will be signal lines outside the projection area during wiring, leading to the "edge radiation" problem, and the increase of the signal loop area and the increase of the differential mode radiation.

2. Try to avoid setting cabling layers adjacent to each other. Because parallel signal routing on adjacent wiring layers will lead to signal crosstalk, so if the adjacent wiring layers cannot be avoided, the spacing between the two wiring layers should be appropriately extended to reduce the spacing between the wiring layers and the signal loop.

3. Adjacent plane layers should avoid overlapping their projection planes. Because when the projection overlaps, the coupling capacitance between layers causes the noise between layers to be coupled to each other.

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 required to use multilayer board design. In the design of multilayer boards should pay attention to the following principles:

The key wiring layer (the layer where clock line, bus, interface signal line, radio frequency line, reset signal line, chip selection signal line and various control signal lines are located) should be adjacent to the complete ground plane, preferably between the two planes, as shown in Figure 1. Key signal lines are usually strong radiation or extremely sensitive signal lines, and wiring close to the ground plane can reduce the area of the signal loop, reduce the radiation intensity or improve the anti-interference ability.

Fig. 1 The key wiring layer is between the two planes

2. The power plane should shrink relative to its adjacent ground plane (suggested value 5H ~ 20H). The "edge radiation" problem can be effectively suppressed by the power plane shrinking relative to its backflow ground plane, as shown in Fig. 2.

Fig. 2 The power plane should shrink with respect to its adjacent ground plane

In addition, the single-board main operating power plane (the most widely used power plane) should be adjacent to its ground plane to effectively reduce the loop area of the supply current, as shown in Fig. 3.

Fig. 3 The power supply plane shall be adjacent to the ground plane

For the design of single-layer and double-layer panels, the main attention should be paid to the design of key signal lines and power lines. In order to reduce the area of the power supply current loop, there must be a ground wire close to and parallel to the power supply circuit.

"Guide Ground Line" should be placed on both sides of key signal lines of single-layer board, as shown in Fig. 4. There should be a large area of Ground paving on the Ground projection plane of the key signal lines of the double-layer slabs, or the same single-layer slabs should be used to deal with the design of "Guide Ground Line", as shown in Figure 5. On the one hand, the "ground guard" on both sides of the key signal line can reduce the area of the signal loop, and on the other hand, it can also prevent the crosstalk between the signal line and other signal lines.

Fig. 4 "Guide Ground Line" on both sides of key signal lines of single-layer board

Fig. 5 A large area of ground is laid on the ground projection plane of the key signal lines of the double-deck slab

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

PCB layout tips

When designing PCB layout, the design principle of placing in a straight line along the signal flow direction should be fully complied with, and circling back and forth should be avoided as far as possible, as shown in Figure 6. In this way, direct signal coupling can be avoided and the signal quality can be affected. In addition, in order to prevent interference and coupling between circuits and electronic components, the placement of circuits and the layout of components shall follow the following principles:

Fig. 6 The circuit modules are placed in a straight line along the signal flow direction

1. If the interface on the veneer is designed to be "clean ground", the filter and isolation devices shall be placed on the isolation belt between the "clean ground" and the working place. This prevents the filtering or isolating devices from coupling with each other through a planar layer, weakening the effect. In addition, no other devices can be placed on the "clean ground" other than filtering and protection devices.

2. When multiple module circuits are placed on the same PCB, digital circuit and analog circuit, high speed circuit and low speed circuit should be arranged separately to avoid the mutual interference between digital circuit, analog circuit, high speed circuit and low speed circuit. In addition, when there are high, medium and low speed circuits on the circuit board at the same time, the layout principle in Figure 7 should be followed in order to avoid the radiating of high frequency circuit noise through the interface.

Fig. 7 layout principles of high, medium and low speed circuits

3. The filter circuit of the power input port of the circuit board should be placed near the interface to avoid the circuit that has been filtered being coupled again.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 realize the effect of protection, filtering and isolation. If the interface has both a filter and a guard circuit, the principle of guard first and filter later should be followed. Because the guard circuit is used for external overvoltage and overcurrent suppression, if the guard circuit is placed behind the filter circuit, the filter circuit will be damaged by overvoltage and overcurrent. In addition, because the input and output lines of the circuit can be coupled to each other to weaken the filtering, isolation, or guard effect, the layout should ensure that the input and output lines of the filtering circuit (filter), isolation, and guard circuit are not coupled to each other.

Fig. 8 Filtering, protection, and isolation devices of the interface circuit are placed near the interface

5. Sensitive circuit or device (such as reset circuit, etc.) should be at least 1000mil away from each edge of the veneer, especially the edge of the veneer interface side.

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 supply modules, fans and relays) to reduce the loop area of large current loops.

7. The filter elements 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 away from the single board interface connector at least 1000mil. This can radiate the interference directly outward or the outgoing current can be coupled to the outgoing cable to radiate outward.

PCB wiring rules

In addition to component selection and circuit design, good printed circuit board (PCB) wiring is a very important factor in EMC. Since the PCB is an inherent part of the system, the enhancement of EMC in the PCB wiring does not incur additional costs for the final completion of the product. One should remember that a poor PCB wiring can cause more EMC problems rather than eliminate them, and in many cases, even adding filters and components will not solve them. Eventually, the entire board had to be rewired. Therefore, it is the most economical way to develop good PCB wiring habits at the beginning. The following will introduce some general rules of PCB wiring and the design strategy of power cord, ground wire and signal line. Finally, according to these rules, the improvement measures for typical PCB circuit of air conditioner are proposed.

1. Wiring separation

The purpose of wiring separation is to minimize crosstalk and noise coupling between adjacent lines 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. 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.

Fig. 9 trace isolation

2. Protection and shunt lines

Setting up shunt and guard circuits is a very effective way 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 lines laid along the critical signals. The guard circuit not only isolates the coupled flux generated on the other signal lines, but also isolates the critical signal from the coupling with the other signal lines. The difference between a shunt line and a guard line is that the shunt line does not have to be terminated (connected to ground), but both ends of the guard line must be connected to ground. To further reduce coupling, guard circuits in multilayer PCB can be added to ground paths at regular intervals.

3. Power cord design

According to the size of the printed circuit board current, try to increase the width of the power line, reduce the resistance of the loop. At the same time, the direction of the power line, the ground line and the direction of data transmission are consistent, which helps to enhance the anti-noise ability. In single or double panels, if the power cord is long, add a decoupling capacitor to ground every 3000mil, and the capacitance is 10uF+1000pF.

4. Ground wire design

The principles of ground wire design are:

(1) Digital and analog separation. If there are both logical and linear circuits on the circuit board, they should be separated as far as possible. The ground of low frequency circuit should be used as far as possible in single point parallel grounding. When the actual wiring is difficult, the parallel grounding can be partially connected in series. High-frequency circuit should use multi-point series grounding, the ground wire should be short and rent, high-frequency components around as far as possible with a large area of grid foil.

(2) the grounding wire should be thickened as far as possible. If the grounding wire is very thin, the grounding potential changes with the current, so that the anti-noise performance is reduced. Therefore, the grounding 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 above 2~3mm.

(3) the ground wire constitutes a closed loop. The noise resistance of printed boards consisting only of digital circuits can be improved in most cases when the ground circuitry is distributed in a group loop.

5. Signal line design

For key signal lines, if the veneer has an internal signal routing layer, then key signal lines such as clocks are distributed in the inner layer, and the preferred wiring layer is given priority. In addition, the key signal line must not cross the segmentation area, including the reference plane clearance caused by holes and pads, otherwise it will lead to the increase of the signal loop area. In addition, the key signal line should be more than 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 clock line, bus, radio frequency line and other strong radiation signal line and reset signal line, chip selection signal line, system control signal and other sensitive signal line, should be away from the interface signal line. So as to avoid the interference on the strong radiation signal line coupling to the outgoing signal line, outward radiation; In addition, the interference brought in by the outgoing signal line of the interface is also avoided from coupling to the sensitive signal line, resulting in system misoperation.

For differential signal lines, they should be of the same layer, equal length, and walk along the lines. The impedance should be consistent, and there is no other route between the difference lines. Because the common mode impedance of the difference line pair is equal, its anti-interference ability can be improved.

Fig. 12 A typical PCB circuit for an improved air conditioner

In general, the improvement of PCB design to EMC is that the design of backflow paths before wiring has the best chance of achieving the goal of reducing EMI radiation. The cheapest way to improve EMC is to change the wiring layer without spending any money before actually wiring it.