Essential Hardware EMC Design Specifications for Engineers
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The three elements of electromagnetic interference are interference source, interference transmission path and interference receiver. EMC focuses on these issues. The most basic interference suppression technology is shielding, filtering and grounding. They are mainly used to cut off the transmission path of interference. Broadly speaking, electromagnetic compatibility control technology includes suppressing the emission of interference sources and improving the sensitivity of interference receivers, but it has extended to other disciplines. This specification focuses on the EMC design of single boards, with some necessary EMC knowledge and rules. Considering electromagnetic compatibility in the design stage of printed circuit boards will reduce the occurrence of electromagnetic interference in circuit prototypes. Types of problems include common impedance coupling, crosstalk, radiation from high-frequency current-carrying conductors, and noise pickup through loops formed by interconnect wiring and printed wiring. In high-speed logic circuits, this type of problem is particularly vulnerable for many reasons: 1. The impedance of the power and ground lines increases with frequency, and common impedance coupling occurs more frequently; 2. The impedance of the power and ground lines increases with frequency, and common impedance coupling occurs more frequently. 2. The signal frequency is high, and the coupling to the wiring through parasitic capacitance is more effective, making crosstalk more likely to occur; 3. The signal loop size is comparable to the wavelength of the clock frequency and its harmonics, and the radiation is more significant. 4. Impedance mismatch causes signal line reflection. 1. The 515 rule, that is, if the clock frequency reaches 5MHz or the pulse rise time is less than 5ns, the PCB board must use a multilayer board. 2. Different power planes cannot overlap. 3. Common impedance coupling problem. Model: 359487VN1=I2ZG is the noise voltage induced in circuit No. 1 by the power supply I2 flowing through the ground plane impedance ZG. Because ground plane currents can be generated by multiple sources, the induced noise may be higher than the sensitivity of the analog circuit or the immunity of the digital circuit. Solution:①The analog and digital circuits should have their own loops and finally be grounded at a single point;②The wider the power cord and return line, the better; 255)]③Shorten the length of the printed line;④Decouple the power distribution system. 4. Reduce the loop area and the cross-link area of the two loops. 5. An important idea is that the EMC on the PCB mainly depends on the Z of the DC power line. 359488[/attach] 2. Layout [attach]51)]Here are the circuit board layout guidelines: 1. Place the crystal oscillator as close to the processor as possible 2. Analog circuits and digital circuits occupy different areas , 255, 255, 255)]3. Place the high frequency at the edge of the PCB and arrange them layer by layer. 4. Fill the empty area with ground. 5. Place the high frequency at the edge of the PCB and arrange them layer by layer. 6. Fill the empty area with ground. 7. Fill the empty area with ground. 8. Fill the empty area with ground. 9. Fill the empty area with ground. 10. Fill the empty area with ground. 11. Fill the empty area with ground. 12. Fill the empty area with ground. 13. Fill the empty area with ground. 14. Fill the empty area with ground. 15. Fill the empty area with ground. 255)]1. Keep the power line and return line as close as possible. The best method is to run them on each side. 2. Provide a zero volt return line for the analog circuit. The ratio of signal line to return line should be less than 5:1. 3. For crosstalk of long parallel lines, increase the spacing between them or add a zero volt line between the lines. 4. Manual clock routing should be kept away from I/O circuits. Consider adding a dedicated signal return line. 5. Key lines such as reset lines should be close to ground return lines. 6. To minimize crosstalk, use double-sided #-shaped routing. 7. Avoid running high-speed lines at right angles. 8. Separate strong and weak signal lines. 4. Shield 1. Shield> Model: [backcolor=rgb(255, 255, Shielding effectiveness SE (dB) = reflection loss R (dB) + absorption loss A (dB) Shielding effectiveness SE (dB) = reflection loss R (dB) + absorption loss A (dB) The key to high-frequency RF shielding is reflection, and absorption is the key mechanism of low-frequency magnetic field shielding. 2. When the operating frequency is lower than 1MHz, the noise is generally caused by electric or magnetic fields (interference caused by magnetic fields is generally within a few hundred Hz). Above 1MHz, electromagnetic interference should be considered. The shielding entities on the board include transformers, sensors, amplifiers, DC/DC modules, etc. The larger ones involve shielding between boards, subracks, and racks. 3. Electrostatic shielding does not require the shielding body to be closed, but only requires high conductivity materials and grounding. Electromagnetic shielding does not require grounding, but requires that the induced current has a path on it, so it must be closed. Magnetic shielding requires high magnetic permeability materials to make a closed shielding body. In order to allow the magnetic flux generated by eddy current and the magnetic flux generated by interference to cancel each other out and achieve the purpose of absorption, there are requirements on the thickness of the material. In the case of high frequency, the three can be unified, that is, closed and grounded with high conductivity materials (such as copper). 4. For low frequency, the absorption attenuation of high conductivity materials is reduced, and the magnetic field shielding effect is not good. High magnetic permeability materials (such as galvanized iron) are required. 255)]5. Magnetic field shielding also depends on the thickness, geometry, and maximum linear dimension of the holes. 6. The noise voltage induced by magnetic coupling is UN = jwB.A.coso = jwM.I1, (A is the area of the closed loop of circuit 2; B is the magnetic flux density; M is the mutual inductance; I1 is the current of the interference circuit. There are two ways to reduce the noise voltage. For the receiving circuit, B, A and COS0 must be reduced; for the interference source, M and I1 must be reduced. Twisted pair is a good example. It greatly reduces the loop area of the circuit and simultaneously generates an opposite electromotive force on the other twisted core wire. 7. The empirical formula for preventing electromagnetic leakage: gap size < λmin/20. The coverage rate of a good cable shield should be more than 70%. 5. Grounding 1. Below 300KHz, generally single-point grounding, above multi-point grounding, mixed grounding frequency range 50KHz ~ 10MHz. Another classification is: < 0.05λ single-point grounding; < 0.05λ multi-point grounding. 2. Good grounding method: tree grounding 3. Grounding of the signal circuit shielding cover. The grounding point is selected on the ground wire of the output end of the amplifier. 4. For the cable shield, when L < 0.15λ, it is generally grounded at a single point at the output end. When L<0.15λ, multi-point grounding is adopted, and the shield is generally grounded at intervals of 0.05λ or 0.1λ. In mixed grounding, one end of the shield is grounded and the other end is grounded through a capacitor. 5. For RF circuit grounding, the grounding wire should be as short as possible or grounding should be achieved without wiring. The best grounding wire is a flat copper braid. When the length of the ground wire is an odd multiple of the λ/4 wavelength, the impedance will be very high, and it will be equivalent to a λ/4 antenna, radiating interference signals outward. 6. There are multiple digital and analog grounds in a single board, and only one common ground is allowed. 7. Grounding also includes using wires as power return lines, overlapping, etc. 1. Select EMI signal filter to filter out the high-frequency interference components that are not needed on the wire, and solve the high-frequency electromagnetic radiation and reception interference. It must ensure good grounding. It is divided into circuit board mounted filters, through filters, and connector filters. From the circuit form, there are single capacitor type, single inductor type, L type, and π type. The π type filter has the best transition performance from passband to stopband and can best guarantee the quality of the working signal. 2. Select AC/DC power supply filters to suppress the conducted and radiated interference on the internal and external power lines, which can prevent EMI from entering the power grid and endangering other circuits, and protect the equipment itself. It does not attenuate the power frequency power. DM (differential mode) interference is dominant at frequencies < 1MHz. CM is dominant at > 1MHz.255)]3. Use ferrite beads installed on the leads of the components for decoupling, filtering and parasitic oscillation suppression of high-frequency circuits. 4. Decouple the chip power supply as much as possible (1-100nF), and filter the DC power supply entering the board and the output of the regulator and DC/DC converter (uF). Cmin≈△I△t/△Vmax △Vmax is generally taken as 2% interference level. Pay attention to reducing the inductance of the capacitor lead and increasing the resonant frequency. Even four-core capacitors can be used for high-frequency applications. The selection of capacitors is a very particular issue and is also a means of single-board EMC control. VII. Others The interference suppression of a single board involves a wide range of aspects, from impedance matching of transmission lines to EMC control of components, from production processes to wire binding methods, from coding technology to software anti-interference, etc. The conception and birth of a machine is actually an EMC project. The most important thing is that engineers need to inject EMC awareness into their designs.
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