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Aug 12 2019

PCB Design Principle for Reducing Electromagnetic Interference

Warm hints: This article contains about 3393 words and reading time is about 10 mins.

Introduction

Printed circuit board (PCB) is the carrier of circuit components in electronic products, which provides the electrical connection between circuit components and is the most basic part of all kinds of electronic equipment. Its performance is directly related to the quality of electronic equipment. With the development of information society and the development of electronic technology, the integration of circuit is higher and higher, the size of circuit board is smaller and smaller, the density of components on circuit board is higher and higher, and the running speed of electronic products is faster and faster. Therefore, the electromagnetic interference and compatibility problems caused by themselves are more prominent. Therefore, how to reduce the electromagnetic interference of PCB board has become the heat of electronic technology. The topic of the door. The electromagnetic compatibility of a circuit board is the key to the normal operation of an electronic system, which affects the reliability and stability of the circuit or the system. Therefore, the electromagnetic interference problem should be effectively solved in the design of PCB.

Printed circuit board 

Here, we will analyze the causes of electromagnetic interference in modern intelligent high-speed electronic systems, and summarize the measures and principles to reduce electromagnetic interference that should be considered in PCB design.

Article Core

PCB

Purpose

PCB design principle for reducing electromagnetic interference

Application

Semiconductor industry

Keywords

PCB


Catalog

Introduction

Ⅰ The reason why the circuit board

 has electromagnetic interference

1.1 RF interference sources present on the circuit board.

1.2Other causes of electromagnetic interference

Ⅱ Design principles of PCB

2.1 Selection of circuit board

2.2 Layout of circuit board components.

2.3 Wiring principle of power supply and ground wire

2.4 Wiring principle of signal Line

III Conclusion


The reason why the circuit board has electromagnetic interference

In the high speed electronic system composed of switching power supply and microprocessor, the electromagnetic interference of circuit board mainly comes from its own RF interference sources, components, basic circuits and differential mode and common mode noise.

1.1 RF interference sources on circuit boards

In the intelligent high-speed electronic system, the RF interference source on the circuit board mainly comes from the microprocessor system, the power supply system and the oscillator circuit.

1.1.1 Microprocessor system

The Radio Frequency RF noise of the microprocessor is generated from the inside of the chip and coupled to the outside in many different possible ways. It exists at the same time in all inputs, outputs, power supplies and ground. It is a potential noise, which makes every pin of the microprocessor possible. The biggest problem is the noise from the microprocessor input and output pins (I/ O). The noise is mainly generated in the clock switch inside the chip, and is connected to the internal and external cables through the input and output pins and radiated out, mainly for short-time pulse.

1.1.2 Power supply system

The power supply system includes the power regulator and its voltage regulator and the bypass capacitance of the micro controller. These circuits are the source of all RF energy in the system and provide the required switching current for the sequential circuits in the chip.

1.1.3 Oscillator circuit

The oscillator circuit provides a fast clock signal for the system. In a digital system, because the output buffer of the oscillator is digital, harmonics will be generated on the output side when the sine wave is converted into a square wave. Any noise generated by internal operation, such as clock buffering, is displayed at the output and propagated through component coupling.

1.2 Other causes of electromagnetic interference

1.2.1 Patch devices and through-hole components

The patch device (SMD) is better than the lead chip in processing RF energy because of its small inductance and close placement of components. Generally, the lead capacitance of the through-hole component will oscillate itself (from capacitive to inductive) when it is about 80MHz. Therefore, the noise higher than 80MHz should be controlled, and many serious problems should be considered if the through-hole components are used in the design.

1.2.2 Fundamental circuit

Each edge jump transmitted from the microprocessor to another chip is a current pulse that flows to the receiving chip, flows out of the ground pin of the receiving chip, and then returns to the ground pin of the microprocessor through the ground wire, forming a basic loop. Such a loop exists everywhere in the circuit, and any noise voltage and its auxiliary current return to where it produces through the lowest impedance path, resulting in an impact. A circuit can be a signal line and its return path, a bypass between the power supply and the ground, a driver in the crystal oscillator and the microprocessor, or a circuit from the voltage regulator supplied by the power supply to the bypass capacitance. The larger the geometric area of the loop, the stronger the radiation, so we can reduce the noise propagation by controlling the shape and impedance of the return path.

Fundamental circuit

1.2.3 Difference mode and common mode noise

Differential mode noise is the noise generated when the signal is transmitted through the line to the receiving chip and then returned along the return line. There is a differential voltage between the two lines, which is the noise that each signal must produce in order to complete its function. The electric field intensity produced by this noise is proportional to the square of frequency, the size of current and the area of current loop, which is 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 working frequency of the circuit, reduce the area of the signal loop or reduce the intensity of the signal current. And The most effective method in practical work is to control the area of the signal loop.

Common mode noise is voltage transmitted along both signal and return lines. There is no difference voltage between the two. The noise caused by the common impedance of the signal line and the return line is caused by the common impedance of the signal line and the return line. Common mode impedance noise is the most common noise source in most microprocessor-based systems. The electric field intensity produced by this noise is proportional to the frequency, the current and the length of the cable, which is inversely proportional to the distance from the observation point to the noise source. So the method of reducing the common-mode radiation includes reducing the ground impedance, shortening the length of the line, and using a common-mode choke.


Ⅱ Design principles of PCB

 Because the integration and signal frequency of circuit board are getting higher and higher with the development of electronic technology, it is inevitable to bring electromagnetic interference, so the following principles should be followed in the design of PCB, so that the electromagnetic interference of circuit board can be controlled in a certain range, meet the design requirements and standards, and improve the overall performance of the circuit.

  2.1 Selection of Circuit Board

  The primary task of PCB design is to select the size of the circuit board properly, If the size is too large, the connection between the components is too long, resulting in an increase in the impedance of the line and a decrease in the anti-interference ability. However, too small size will lead to dense arrangement of components, which is not conducive to heat dissipation, and the connection is too thin and too dense, which is easy to cause crosstalk. Therefore, the appropriate size of the board should be selected according to the components required by the system.

   The circuit board is divided into single panel, double panel and multi-layer board. The selection of the number of layers of the circuit board depends on the function to be realized by the circuit, the noise index, the number of signals and network lines, and so on. Reasonable layer setting can reduce the electromagnetic compatibility problem of the circuit itself. The usual selection principles are: ①For the medium and low frequency, fewer components, and lower or medium wiring density, single panel or double panel is used; ②Multi-layer board for high wiring density, high integration and many components; ③For high signal frequency, high speed integrated circuits, components dense selection of 4 layers or more layers of circuit boards. The multi-layer board can be used as the power supply layer, the signal layer and the connecting layer at the time of design. The signal loop area is reduced,thus reducing the differential mode radiation. Therefore, the multi-layer board can reduce the radiation of the circuit board and improve the anti-interference ability.

Printed Circuit Board 

 2.2 Layout of circuit board components

 After determining the size of the PCB, the position of the special component should be determined first, and finally, according to the functional unit of the circuit, all the components of the circuit should be laid out in blocks. 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. Generally, when arranging high-speed, medium-speed, and low-speed circuits, components should be arranged with reference to Figure 1. When arranging components with clocks, CPU, memory, controllers, and input and output circuits, the components should be arranged with reference to figure 2. The layout principles of commonly used circuit boards are as follows.

circuit layout


Figure 1 High, medium and low speed circuit layout

Figure 2 High speed logic circuit layout

(1) Principles for determining the position of special components: ①The heating element shall be placed in a position convenient for heat dissipation, such as the edge of the PCB and away from the microprocessor chip; ②Special high frequency elements should be placed next to each other in order to shorten the connection between them; ③The sensitive element should be far from the noise source such as clock generator, oscillator and so on; ④The layout of adjustable components, such as potentiometer, adjustable inductor, variable capacitor, key switch, etc., should meet the structural requirements of the whole machine and be convenient to adjust; ⑤The heavy quality components should be fixed by bracket; ⑥EMI filter should be placed close to EMI source.

component layout 

(2) The principle of laying out the umbrella components of the circuit according to the functional unit of the circuit: ①Each functional circuit should determine the corresponding position according to the signal flow direction between them, so as to facilitate the wiring; ②Each functional circuit should first determine the position of the core components and place other components around these to shorten the connection between the components as much as possible; ③For high frequency circuits, the distribution parameters between components should be considered; ④Components placed on the edge of the circuit board should be not less than 2mm from the edge of the circuit board; ⑤DC/DC converter, switch tube and rectifier should be placed as close as possible to the transformer in order to reduce the external radiation; ⑥Voltage regulating elements and filter capacitors should be placed close to rectifier diode.

2.3Wiring principle of Power supply and ground Wire

Whether the wiring of PCB power supply and ground wire is reasonable is the key to reduce electromagnetic interference of the whole circuit board. The design of power cord and ground wire is a problem that can not be ignored in PCB, and it is often the most difficult design. The following principles should be followed in design.

2.3.1Wiring skills for power and ground Wire

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 PCB layout parameters on high-speed electronic system, the wiring principles for power supply and ground are as follows: ①To reduce capacitance coupling crosstalk by increasing the spacing of traces; ②The power and ground wires should be routed in parallel to optimize the distributed capacitance; ③According to the magnitude of the carrying current, the width of the power line and the ground line should be increased as much as possible to reduce the loop resistance, and the direction of the power line and the ground line in each functional circuit and the direction of signal transmission are the same, which helps to improve the anti-interference ability; ④The power supply and ground wire shall be routed directly above each other to reduce inductive reactance and minimize the area of the loop, and make the ground wire below the power line as far as possible; ⑤The thicker the ground wire, the better, the width of the general ground wire is not less than 3mm; ⑥The ground wire is formed into a closed loop to reduce the potential difference on the ground wire and improve the anti-interference ability; ⑦In the design of multi-layer board wiring, one of the layers can be regarded as the "all-ground plane", which can reduce the grounding impedance and act as a shield at the same time.

2.3.2 Grounding skills of each functional Circuit

The grounding mode of each functional circuit of PCB is divided into single point grounding and multi-point grounding. According to the connection form, single point grounding can be divided into single point series grounding and single point parallel grounding, as shown in figures 3 and 4. Single point series grounding is often used to protect grounding wire because of the different length of grounding wires and the different grounding impedance of each circuit, and the electromagnetic compatibility is reduced. Each circuit of single point and parallel connection has a separate grounding wire, so the interference between each other is small, but it may prolong the grounding wire and increase the grounding impedance, which is often used for signal grounding, analog grounding and power grounding. Multipoint grounding refers to the fact that each circuit has a connection point, as shown in figure 5.  Multi-point grounding is often used in high frequency circuits, which has short grounding wire and small grounding impedance, so as to reduce the interference of high frequency signal.

Three grounding 

Figure 3 single point series grounding

Figure 4 single point parallel grounding

Figure 5 Multipoint grounding

In order to reduce the interference caused by grounding, grounding should also meet certain requirements: ①The grounding line should be as short as possible and the ground surface should be large; ②To avoid the unnecessary grounding loop and to reduce the interference voltage of the common ground;

③The grounding principle is to adopt different grounding modes for different signals, and all grounding can not be taken at the same place; ④In the design of multi-layer PCB, the power layer and the connecting layer should be placed in the adjacent layer as much as possible, so that the capacitance of the layer can be formed in the circuit and the electromagnetic interference should be reduced; ⑤Try to avoid strong and weak electrical signals, digital and analog signals

2.3.3Placinf the gridded plane

Grid is the most important design technique for two-layer panel. Rasterization is to extend the ground wire on the PCB and use the ground filling mode to construct the grid network connected to the ground to form an effective ground plane, which can reduce the noise as well as the four-layer board. It has two purposes: ①Simulating the layer of the four-layer plate and provide the return path at the bottom for each signal line; ②To Reduce the impedance between the microprocessor and the voltage stabilizer.

The principles to be noted in the design are: ①Each ground wire extends to fill the space of the printed circuit board as much as possible; ②Place as many grille as possible on the two-layer board; ③Using as many holes as possible to connect the top and bottom grille at the right time; ④The line does not have to have a right angle or the same width.

2.3.4 Application of High Frequency decoupling Capacitor and Ferrite beads

In digital circuits, when the state of the logic gate changes, a large peak pulse will be generated on the power supply, forming an instantaneous noise voltage. In this case, decoupling capacitors or ferrite beads are commonly used to limit the sudden change in the current and to reduce the radiation. Typically, a high-frequency decoupling capacitor with a capacity of about 0.01. m u.F to 0.1. m u.F is applied between the power supply of each chip and the ground, and a ferrite bead is placed on the power line close to the chip to block the RF current source from the power supply line. We should try our best to do so at the time of design: ①Tantalum capacitors are used instead of aluminum electrolytic capacitors, which have large internal inductors; ②The closer the capacitance to the chip, the better the lead of the decoupling capacitor is not too long; ③The ferrite magnetic beads are used only on the power supply line of the + V and do not need to be on the ground wire; ④The Ferrite beads are placed as close as possible to the noise source.

Application of Ferrite Beads in Decoupling Circuits 

Application of Ferrite Beads in Decoupling Circuits

2.4 Wiring principle of signal Line

2.4.1 Reducing Line Capacitive and Inductive Crosstalk

When doing wire, the Wiring principle of signal Line has capacitive and inductive crosstalk even between lines within a short distance and on the line. When capacitive coupling, the rising edge of the source end causes a rising edge on the victim. During inductive coupling, the voltage change on the victim is opposite to the change at the source end. most of the crosstalk is capacitive and the magnitude of the noise is proportional to the parallel distance, the frequency, the amplitude of the source voltage and the impedance of the victim, inversely proportional to the distance from the two lines. Therefore, measures to reduce crosstalk are: ①Keep the RF noise-carrying line connected to the microprocessor away from other signals; ②The return ground wire of the signal that may be the victim of the noise should be routed below it; ③Do not leave the noise line at the outer edge of the circuit board; ④If possible, route some noise lines together and surround them with ground wires; ⑤Keep non-noisy lines away from areas on the circuit board that are easy to receive noise, such as connectors, oscillator circuits, relays and relay drivers.

2.4.2 Reasonably arrange the number of return ground wires

In the computer industry, there are at least 1 ground wire per 9 signal lines in a cable or wire, which is a very common experience. At high speed, the ratio changes to 5%. Principles that can be considered when designing signal lines and return lines: ①It is best to have a return ground wire for each signal line in the cable, forming a pair of twisted-pair wires; ②Do not exceed one return ground wire for every 9 signal lines; ③If the cable is more than one foot long, a return ground wire shall be provided for every 4 signal lines; ④If possible, a solid metal bracket should be used as a mechanical bracket, welded between two circuit boards, both as an installation bracket and as a reliable radio frequency return ground wire.


Conclusion

It is impossible to completely eliminate the electromagnetic interference in electronic products. We can only take the necessary measures to reduce the electromagnetic interference so that the electromagnetic interference can be controlled within a certain range. A good design of printed circuit board is an important link to reduce electromagnetic interference. In the design of printed circuit board, the above-mentioned design principles can be referred to, but these principles are not unchanged. According to specific circuit conditions, various anti-interference methods shall be applied flexibly to meet the requirements of electromagnetic compatibility to the maximum extent, which requires the accumulation and summary of designers' usual experience.


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