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Common EMC system integration problems and solutions

Release time:

2022-06-15 00:00

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　　Electromagnetic compatibility system refers to the ability of equipment or system to work normally in its electromagnetic environment without causing unbearable electromagnetic harassment to any equipment or anything in the environment. Then, let's understand the common electromagnetic compatibility system integration problems and solutions.

　　How to improve the anti-interference ability and compatibility of electromagnetic compatibility system:

　　1, the following part of the system should pay special attention to electromagnetic interference.

　　1) Microcontroller clock frequency is particularly high, bus cycle is particularly fast system.

　　2) The system includes high-power and high-current drive circuits, such as relays that generate sparks and high-current switches.

　　3) System, including weak analog signal circuit and high precision A/D conversion circuit.

　　2. In order to improve the anti-interference ability of electromagnetic compatibility system, the following measures shall be taken:

　　1) Select low frequency microcontrollers

　　Choosing a microcontroller with a low external clock frequency can effectively reduce noise and improve the anti-interference ability of the system. Even for square and sinusoidal waves of the same frequency, the high frequency component of square wave is much more than that of sinusoidal wave, and the high frequency component of square wave is smaller than that of fundamental wave, but the higher the frequency, the easier it is to become a noise source. The high-impact high-frequency noise produced by microcontrollers is about three times the clock frequency.

　　2) Reduce distortion in signal transmission

　　The microcontroller is mainly manufactured by high-speed CMOS technology. The static input current of the signal input terminal is about 1mA, the input capacitance is about 10PF, and the input impedance is quite high. The output terminals of high-speed CMOS circuits have quite large load capacity, that is, quite large output value. The delay time of the signal on the printed substrate is related to the characteristic impedance of the lead, that is, the characteristic impedance of the printed circuit board material. The transmission speed of the signal on the lead of the printed substrate is about 1/1 of the speed of light. The Tr (standard delay time) of the frequently used logic telephone component in the system composed of microcontrollers is between 18ns.

　　3) Reduce crosstalk between signal lines

　　CMOS microcontroller has high input impedance, high noise and high noise tolerance. Digital circuit even superimposed 100~200mv noise does not affect the action. If the AB line in the figure is an analog signal, such interference cannot be allowed. If the printed circuit board is 4 layers of board, one layer is a large area of the ground, or two sides of the board, the other side of the signal line is a large area of the ground, then the crosstalk between the signals becomes smaller. This is because the characteristic impedance of the signal line is greatly reduced, the reflection of the signal at the d end is greatly reduced, the characteristic impedance is inversely proportional to the square of the dielectric constant between the signal line and the ground, and is proportional to the natural logarithm of the thickness of the medium. When the AB line is an analog signal, in order to avoid the interference of the digital circuit signal line CD to AB, a large area of ground is required under the AB line. The distance between line AB and line CD is two to three times greater than the distance between line AB and the ground. Use a partially shielded ground to lay ground cables on the left and right sides of the knoted wire.

　　4) Reduce power supply noise

　　A power supply for the noise is added while providing energy for the EMC system integration. The reset line, interrupt line and other control lines of the microcontroller in the circuit are susceptible to interference from external noise. Strong noise on the grid enters the circuit from the power supply, and even in battery-powered systems, the batteries themselves have high-frequency noise. The analog signal in the analog circuit cannot withstand the noise from the power supply.

　　5) Pay attention to the high frequency characteristics of printed circuit boards and components

　　At high frequency, the wires, holes, resistors, capacitors, connectors of distributed inductors and capacitors on the printed circuit board can not be ignored. The distributed inductance of the inductor cannot be ignored, and the distributed capacitance of the inductor cannot be ignored. By reflecting the high frequency signal from the resistance, the distributed capacitance of the lead acts. When the length is greater than 1/20 of the wavelength corresponding to the noise frequency, an antenna effect occurs and noise is emitted outward through the lead.

　　6) Component configuration must be reasonably divided

　　Electromagnetic interference countermeasures should be fully considered when the components are arranged on the printed circuit board. One of the principles is to shorten the wires between the components as much as possible. The layout is reasonably divided into analog signal part, high-speed digital circuit part, noise source part, such as relay, large current switch and other three parts, so that the signal coupling between each other is small.

　　(7) Treatment of grounding wire

　　Electromagnetic compatibility system is integrated on printed circuit board, power cord and ground wire are important, and the important means to overcome electromagnetic interference is ground wire. On the double-sided substrate, the configuration of ground wire is particularly exquisite. By using a single point grounding method, the power supply and ground wire are connected from both ends of the power supply to the printed circuit board. One point, one point, printed circuit board of the power supply, there are multiple return ground wires, they gather at the contact of the return power supply, so called single point ground, analog, digital, high power equipment separation means that the wiring is separated, gathered at this ground point. When connecting signals other than printed circuit boards, shielded cables are usually used. For high frequency signals and digital signals, both ends of the shielded cable are grounded. For low frequency analog signals, one end of the shielded cable is grounded.

　　The above is the common electromagnetic compatibility system integration problems and solutions, if you need to know more, please feel free to contact us!

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