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PCB board design principles to reduce electromagnetic interference
PCB Blog
PCB board design principles to reduce electromagnetic interference

PCB board design principles to reduce electromagnetic interference

2022-01-12
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Author:pcb

PCB board effective anti-interference design is a key link in the design of electronic products, which affects the reliability and stability of the circuit work. The article analyzes the main reasons for the existence of electromagnetic interference on circuit boards, and summarizes the effective suppression and prevention of electromagnetic interference in PCB board design from the selection of circuit boards, the layout of circuit board components, the wiring of power and ground, and the wiring of signal lines. measures and principles. The printed circuit board is the carrier of circuit components in electronic products, providing electrical connections between circuit components, and is the basic component of various electronic equipment. Its performance is directly related to the quality of electronic equipment. With the development of the information society and the development of electronic technology, the integration of circuits is getting higher and higher, the size of circuit boards is getting smaller and smaller, the density of components on circuit boards is getting higher and higher, and the running speed of electronic products is getting higher and higher. Therefore, the problem of electromagnetic interference and compatibility caused by itself is more prominent. Therefore, how to reduce the electromagnetic interference of PCB boards has become a hot topic in today's electronic technology. The electromagnetic compatibility problem of a circuit board is the key to whether an electronic system can work normally, which affects the reliability and stability of the circuit or system. Therefore, the electromagnetic interference problem should be effectively solved when designing the PCB board.

PCB board

In terms of the reasons for electromagnetic interference, the measures and principles to reduce electromagnetic interference that should be considered in PCB board design are summarized.
1. The reason for the existence of electromagnetic interference on the circuit board
In the high-speed electronic system composed of switching power supply and microprocessor, the electromagnetic interference of the circuit board mainly comes from the existing radio frequency interference source, components, basic loop and differential mode and common mode noise.

1.1 The source of radio frequency interference present on the circuit board
In an intelligent high-speed electronic system, the source of radio frequency interference on the circuit board mainly comes from the microprocessor system, the power supply system and the oscillator circuit.
1) Microprocessor system
The radio frequency (RF) noise of a microprocessor is generated inside the chip and coupled to the outside in many different possible ways. It exists at all inputs, outputs, power and ground at the same time. It is potential noise that makes every lead of the microprocessor. There may be problems with the feet. The problem is noise from the input and output pins (I/O) of the microprocessor. These noises are mainly generated by the clock switching inside the chip, connected to the internal and external cables through the input and output pins and radiated out, mainly manifested as short-time pulse waveform interference.
2) Power supply system
The power supply system includes the power regulator and its bypass capacitors on the regulator and microcontroller side. These circuits are the source of all RF energy in the system and provide the required switching currents for the on-chip sequential circuits.
3) Oscillator circuit
The oscillator circuit provides a fast clock signal to the system, in a digital system, since the output buffer of the oscillator is digital, harmonics are generated on the output side when converting a sine wave to a square wave. Any noise generated by internal operations, such as clock buffers, will show up at the output and propagate through component coupling.

1.2 Other causes of electromagnetic interference
1) SMD components and through-hole components
SMD devices (SMDs) are better at handling RF energy than leaded chips because of their lower inductive reactance and closer placement of components. Typically, the lead capacitance of through-hole components will self-oscillate (change from capacitive to inductive) at about 80MHz. Therefore, the noise above 80MHz must be controlled, and many serious problems must be considered if through-hole components are used in the design.
2) Basic circuit
Each edge transition transmitted from the microprocessor to another chip is a current pulse that flows to the receiving chip, out of the ground pin of the receiver chip, and then back to the ground pin of the microprocessor through the ground wire, so form a basic circuit. Such loops exist everywhere in the circuit, and any noise voltage and its accompanying currents travel through the impedance path back to where it originated, causing an effect. A return can be a signal line and its return path, a bypass between power and ground, a crystal oscillator and a driver within a microprocessor, or a return from the voltage regulator of the power supply to the bypass capacitor. The larger the geometric area of the loop, the stronger the radiation, so we can mitigate noise propagation by controlling the shape and impedance of the return path.
3) Differential mode and common mode noise
Differential mode noise is the noise that occurs when a signal travels through the line to the receiving chip and then returns along the return line. There is a differential voltage between the two lines, which is the noise that each signal must generate to perform its function. The electric field strength generated by this noise is proportional to the square of the frequency, the magnitude of the current, and the area of the current loop, and inversely proportional to the distance from the observation point to the noise source. Therefore, the method to reduce the differential mode radiation is to reduce the operating frequency of the circuit, reduce the area of the signal loop or reduce the strength of the signal current. An effective method in practice is to control the area of the signal loop. Common mode noise is the noise caused by the impedance shared by the signal and return lines while the voltage is traveling along the signal and return lines at the same time, with no differential voltage between them. Common-mode impedance noise is a common source of noise in most microprocessor-based systems. The electric field strength produced by this noise is proportional to the magnitude of the frequency, the magnitude of the current and the length of the cable, and inversely proportional to the distance from the observation point to the noise source. The methods to reduce the common mode radiation are: reduce the impedance of the ground wire, shorten the length of the line, and use a common mode choke coil.

2. PCB board design principles
Since the integration degree and signal frequency of the circuit board are getting higher and higher with the development of electronic technology, electromagnetic interference will inevitably be brought about. Therefore, the following principles should be followed when designing the PCB board to control the electromagnetic interference of the circuit board within a certain range. It can meet the design requirements and standards and improve the overall performance of the circuit.

2.1 Selection of circuit boards
The primary task of PCB board design is to properly select the size of the circuit board. If the size is too large, the impedance value of the line will increase and the anti-interference ability will decrease because the connection between components is too long. The dense arrangement of devices is not conducive to heat dissipation, and the wiring is too thin and dense, which is easy to cause crosstalk. Therefore, the circuit board of the appropriate size should be selected according to the required components of the system. Circuit boards are divided into single-sided, double-sided and multi-layer boards. The selection of the number of layers of the circuit board depends on the function to be implemented by the circuit, the noise index, the number of signals and network cables, etc. A reasonable number of layers can reduce the electromagnetic compatibility problem of the circuit itself. The usual selection principle is: when the signal frequency is medium and low frequency, there are few components, and the wiring density is low or medium, choose single-sided or double-sided; for high wiring density, high integration and many components, use multi-layer ③ For high signal frequency, high-speed integrated circuits, and dense components, choose 4 or more layers of circuit boards. In the design of multi-layer boards, a single layer can be used as a power layer, a signal layer and a ground layer. The signal loop area is reduced and the differential mode radiation is reduced. For this reason, the multi-layer board can reduce the radiation of the circuit board and improve the anti-interference ability.


2.2 Layout of circuit board components
After determining the size of the PCB board, the positions of special components should be determined first, and all components of the circuit should be laid out in blocks according to the functional units of the circuit. The digital circuit unit, the analog circuit unit and the power supply circuit unit should be separated, and the high frequency circuit unit and the low frequency circuit unit should also be separated. Common circuit board layout principles are as follows.

1) The principle of determining the location of special components:
①The heating element should be placed in a position conducive to heat dissipation, such as the edge of the PCB board, and away from the microprocessor chip;
② Special high-frequency components should be placed next to each other to shorten the connection between them;
③Sensitive components should be kept away from noise sources such as clock generators and oscillators;
④ The layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, and key switches should conform to the structural requirements of the whole machine and facilitate adjustment;
⑤ Components with heavier mass should be fixed with brackets;
⑥EMI filter should be placed close to the EMI source.

2) The principle of laying out the umbrella components of the circuit according to the circuit functional unit:
① Each functional circuit should determine the corresponding position according to the signal flow between them to facilitate wiring;
② Each functional circuit should first determine the position of the components, and place other components around the components to shorten the connection between the components as much as possible;
③ For high-frequency circuits, the distribution parameters between components should be considered;
④ Components placed on the edge of the circuit board should be no less than 2mm away from the edge of the circuit board.
⑤ The DC/DC converter, switch tube and rectifier should be placed as close to the transformer as possible to reduce external radiation;
⑥ Voltage regulating components and filter capacitors should be placed close to the rectifier diode.
    
2.3 The wiring principle of power supply and ground
Whether the wiring of the power supply and the ground of the PCB board is reasonable is the key to reducing the electromagnetic interference of the entire circuit board. The design of power lines and ground lines is a problem that cannot be ignored in the PCB board, and is often a difficult design. The design should follow the following principles.

1) Wiring skills for power and ground
The wiring on the PCB is characterized by distributed parameters such as impedance, capacitive reactance and inductive reactance. In order to reduce the influence of the distribution parameters of the PCB board wiring on the high-speed electronic system, the wiring principles for the power supply and the ground are as follows:
①Increase the spacing of the traces to reduce the crosstalk of capacitive coupling;
② The power line and the ground line should be routed in parallel to make the distributed capacitance reach;
③ According to the size of the carrying current, try to increase the width of the power line and the ground line as much as possible, reduce the loop resistance, and at the same time make the direction of the power line and the ground line in each functional circuit consistent with the transmission direction of the signal, which will help improve the performance. Anti-interference ability;
④ The power supply and the ground should be routed directly above each other, thereby reducing the inductive reactance and making the loop area, and try to make the ground wire go under the power line as much as possible;
⑤ The thicker the ground wire, the better, generally the width of the ground wire is not less than 3mm;
⑥ The ground wire is formed into a closed loop to reduce the potential difference on the ground wire and improve the anti-interference ability;
⑦ In the multi-layer board wiring design, one of the layers can be used as the "full ground plane", which can reduce the ground impedance and at the same time play a shielding role.

2) Grounding skills of each functional circuit
The grounding methods of each functional circuit of the PCB board are divided into single-point grounding and multi-point grounding. Single-point grounding is divided into single-point series grounding and single-point parallel grounding according to the connection form. Single-point series grounding is often used for protective grounding because the length of each grounding wire is different, the grounding impedance of each circuit is different, and the electromagnetic compatibility performance is reduced. Each circuit of single-point parallel grounding has its own grounding wire, so the mutual interference is small, but it may extend the grounding wire and increase the grounding impedance. It is often used for signal grounding, analog grounding, and power grounding. Multi-point grounding means that each circuit has a grounding point, as shown in Figure 5. Multi-point grounding is often used in high-frequency circuits, with short grounding lines and small grounding impedance values, reducing the interference of high-frequency signals. In order to reduce the interference caused by grounding, the grounding must also meet certain requirements:
①The ground wire should be as short as possible, and the ground plane should be large;
② Avoid unnecessary ground loops and reduce the interference voltage of the common ground;
③The grounding principle is to adopt different grounding methods for different signals, and all groundings cannot be taken to the same grounding point;
④ When designing a multi-layer PCB board, the power supply layer and the grounding layer should be placed in the adjacent layers as much as possible, so that the capacitance between the layers can be formed in the circuit and the electromagnetic interference can be reduced;
⑤ Try to avoid strong and weak current signals, digital and analog signals in common ground.

3) Place the gridded plane
Gridding is an important design technique for two-layer boards. Gridding is to extend the ground wire on the PCB board and use the ground fill pattern to construct a grid network connected to the ground, forming an effective ground plane, which can reduce noise like a four-layer board. It has two purposes: ① imitating the ground plane of a four-layer board, providing a return path below for each signal line; ② reducing the impedance between the microprocessor and the voltage regulator. The principles that should be paid attention to when designing are:
①Each ground wire extends to fill the space of the printed circuit board as much as possible;
② Place as many grilles as possible on the two-layer board;
③ Use as many through holes as possible to connect the top and bottom grids when the size is appropriate;
④ The lines do not have to be at right angles or the same width.

4) Use of high frequency decoupling capacitors and ferrite beads
In digital circuits, when the state of the logic gate changes, a large spike will be generated on the power supply, forming an instantaneous noise voltage. In this case, decoupling capacitors or ferrite beads are generally used to limit the sudden change of current. change to reduce radiation. Usually, a high-frequency decoupling capacitor with a capacity of about 0.01μF ~ 0.1μF is added between the power supply and the ground of each chip, and ferrite beads are placed on the power line close to the chip to block the radio frequency from the power line. current source. When designing, try to:
① Use tantalum capacitors instead of aluminum electrolytic capacitors, which have large internal inductance;
②The closer the capacitor is to the chip, the better, and the lead of the decoupling capacitor should not be too long;
③The ferrite beads are only used on the +V power supply line, not on the ground line;
④ Place the ferrite beads as close to the noise source as possible.

2.4 Wiring principles of signal lines

1) Reduce the capacitive and inductive crosstalk of the line
When wiring, there is capacitive and inductive crosstalk between lines that are run in parallel even over short distances. When capacitively coupled, a rising edge on the source causes a rising edge on the victim. With inductive coupling, the voltage change on the victim is the opposite of the change on the source. Most crosstalk is capacitive, and the magnitude of the noise is proportional to the parallel distance, frequency, source voltage amplitude, and victim impedance, and inversely proportional to the distance the two lines are separated. Therefore, the measures to reduce crosstalk are:
① Keep the lines connected to the microprocessor that carry radio frequency noise away from other signals;
②The return ground wire of the signal that may be the victim of noise should be routed below it;
③ Do not take noise lines on the outer edge of the circuit board;
④ If possible, route some noisy lines together and then surround them with a ground wire;
⑤ Keep non-noisy lines away from areas on the board that are prone to receiving noise, such as connectors, oscillator circuits, relays, and relay drivers.

2) Reasonably arrange the number of return ground wires
In the computer industry, it is a common practice to have at least 1 ground wire for every 9 signal wires in a cable or wire. At high speeds, this ratio changes to 5:1. Principles that can be considered when designing signal lines and return lines:
① is that each signal line in the cable has a return ground wire to form a twisted pair;
②Do not exceed one return ground for every 9 signal lines;
③If the cable is more than one foot long, there should be a return ground wire for every 4 signal wires;
④If possible, a solid metal bracket should be used as a mechanical bracket, soldered between the two circuit boards, both as a mounting bracket and as a reliable RF return ground.

3) Other wiring principles
①The copper foil used as the wire will make the impedance of the wire discontinuous at a 90-degree turn, which may cause reflection interference, so the 90-degree wire should be changed to a 135-degree trace, which will help reduce reflection interference;
② For the PCB board with double-sided wiring, the wiring of the upper and lower layers should cross vertically to reduce coupling and help suppress interference;
③Isolated wiring is used. In many circuits that have to be routed in parallel, a grounded isolated wiring can be considered to be added to the two signal lines;
④ All lines should be laid along the DC ground as far as possible, and try to avoid laying along the AC ground;
⑤ Use short wiring. In the case that the lines cannot be arranged or can be connected only by winding in large circles, simply connect with insulated "flying wires" instead of printed wires, or directly bridge with the leads of resistance-capacitance components;
⑥ The DC circuit should be wired away from the AC circuit, and the input signal line and the output signal line should be separated;
⑦ Signal traces should not have branches, and should be connected from one component to the next to avoid reflection interference or harmonic interference;
⑧The high-frequency signal lines such as clocks should be routed close to the ground line to make the loop area to reduce differential mode radiation.

3. Conclusion
It is impossible to completely eliminate electromagnetic interference in electronic products. We can only take necessary measures to reduce electromagnetic interference and control electromagnetic interference within a certain range. A good printed circuit board design is to reduce electromagnetic interference. important part. When designing a printed circuit board, you can refer to the design principles mentioned above, but these principles are not static. Various anti-interference methods should be flexibly applied according to the specific circuit conditions in order to meet the requirements of electromagnetic compatibility. This requires PCB board design The usual experience accumulation and summary.