Introduce the electromagnetic compatibility design and structure design
EMC design content list
1. Scheme design
1) Clarify the electromagnetic compatibility standards to be met by the developed equipment or system. Sometimes, according to user requirements or actual conditions (for example, there are high-sensitivity receivers around, or equipment that produces strong interference), project-specific electromagnetic compatibility requirements need to be proposed.
2) Design interface circuits, try to use balanced interface circuits. When necessary, isolation transformers, optical coupling devices, etc. can be used on the interface circuit to improve the ability to resist common mode interference;
3) Avoid using high-speed pulse signals in the circuit. The rise and fall of the pulse signal should be as smooth as possible, and the bandwidth of the analog circuit should be as narrow as possible (the bandwidth should be limited by selecting devices with appropriate bandwidth and adding appropriate filter circuits);
4) Try to use large-scale integrated circuits, so as to obtain a small loop area and improve anti-interference;
5) Determine the key circuit parts in the system, including strong interference source circuits and highly sensitive circuits, and consider taking special isolation measures (local shielding, filtering) for these circuits;
6) Determine the isolation interface of the circuit to be isolated. The principle of determining the isolation interface is: as few signal lines pass through the interface as possible, and the frequency is as low as possible (in order to take filtering measures);
7) According to the working principle of the system and the principle of ground wire design, draw the ground wire diagram of the system (the ground wire diagram may not mark specific devices and circuits, only different circuit modules), and different circuits of different natures Ground wire, different ground wires are represented by different symbols;
8) Determine what interference suppression measures need to be taken, such as shielding, filtering, etc., the required shielding effectiveness and filtering performance (including frequency range, attenuation, etc.);
9) Cable grouping, group the signal wires according to the characteristics of high frequency, low frequency, digital, analog, power, etc. Do not arrange the signals of different groups in one cable, otherwise it will not only cause mutual interference problems, but also disadvantageous to take filtering measures .
2. Structural design
1) First determine the material of the shielding case, analyze the requirements for shielding effectiveness, and see if there is a low-frequency magnetic field (below 1KHz) shielding requirements. If not, you can choose steel, aluminum, copper and other commonly used materials as shielding materials or use plastic case Electromagnetic shielding paint is sprayed inside. If there is a requirement for low-frequency magnetic field shielding, materials with high permeability such as permalloy should be used;
2) If high permeability materials are used, are there conditions for heat treatment to recover the permeability lost due to processing;
3) Determine the parts on the chassis that need to be connected with low impedance; (such as the seam of the shield, the path of electrostatic discharge current, the grounding of the filter, the common ground of the system, etc.)
4) The realization method of low impedance overlap (to ensure the necessary low impedance), for permanent connection, the most ideal method is welding, for long gaps, continuous welding (note that deformation will occur during continuous welding);
5) The treatment method of non-permanent lap joints is generally to adopt electromagnetic sealing gaskets, what kind of electromagnetic sealing gaskets to choose, and comprehensively consider the shielding effectiveness, sealing gasket installation methods, electrochemical compatibility, price and other issues;
6) How to ensure the conductivity of the surface where the electromagnetic sealing gasket is installed;
7) Fully consider the deformation of the panel caused by the rebound force formed by the electromagnetic sealing gasket, and the rigidity of the panel should be sufficient;
8) When used in harsh environments (humid, salt spray, etc.), or when the gasket material is electrochemically incompatible with the shielding matrix material, use appropriate environmental sealing measures to isolate moisture;
9) Whether measures have been taken for the cables entering and exiting the shielded case, such as shielding or filtering (shielding generally has a better effect on lower frequency interference suppression, and the effect at high frequencies depends on the structure of the shielded cable and the termination method of the shielding layer), Whether the shielding layer of the cable and the chassis at both ends of the cable meet the requirements of the "dumbbell model";
10) For cables that transmit low-frequency signals, or shielded cables with no shield at one end (such as cables connected to sensors), filtering is the best solution at the cable port;
11) Whether the installation method of the filter is correct, whether the problem of good grounding of the filter, coupling between the input and output ends of the filter, and the excessively long wire between the filter and the cable entrance are solved;
12) If a filter connector or filter array board is used, an electromagnetic shielding gasket should be installed between the filter connector or filter array board and the chassis;
13) The shell of the power line filter should be directly connected to the metal case, and the power line should be as short as possible;
14) It is best (especially for digital equipment) to use a filter with a power outlet at the power inlet;
15) The gaps or holes on the chassis should be as far away as possible from strong radiation sources (such as wires, cables, circuit boards, etc.) or sensitive circuits; 16) Any metal objects on the chassis cannot directly pass through the chassis;
17) If honeycomb panels are used on the ventilation holes, electromagnetic sealing gaskets must be used between the honeycomb panels and the chassis;
18) The processing method of the display window. If shielding glass is used, an electromagnetic sealing gasket must be used between the shielding glass and the chassis;
19) For all parts of the equipment that will be subjected to the electrostatic discharge test, according to the principle that the current flows through the path with the least impedance, predict a current discharge path, and then analyze; whether there is a sensitive circuit on or near the electrostatic discharge path;
20) Electromagnetically shield sensitive circuits near the electrostatic discharge path, and connect the shielding layer to the circuit ground;
21) If an unshielded case is used, a larger metal plate should be installed at the cable entrance to provide conditions for filtering at the cable interface and termination of the cable shielding layer;
3. Circuit and circuit board design
1) Determine the number of circuit board layers, considering the electromagnetic compatibility requirements and costs. When the cost permits, try to use circuit boards with more than four layers and set up a ground plane;
2) Fully consider the position and direction of the device;
3) Avoid coincidence of clock harmonics and make a harmonic table for each clock signal;
4) For multilayer circuit boards, if high-speed signals and highly sensitive signals are arranged adjacent to the ground plane;
5) According to the circuit's working frequency, level size, digital circuit/analog circuit division, the circuits of different nature are arranged in different areas of the circuit board to keep the interference circuit away from the sensitive circuit;
6) Circuits in different areas (corresponding to circuits of different nature) use different ground wires and power supplies, and different ground wires and power supplies are connected at one point;
7) For multi-layer circuit boards, the ground planes in different areas must meet the 20H rule at the edges (that is, the edge of the ground plane must extend 20H beyond the edge of the power layer or the signal line layer. H is the ground plane and the signal Height between line layers);
8) For multi-layer circuit boards with a ground plane, avoid long gaps (discontinuous ground wires) on the ground plane (not including the gaps that are intentionally set to separate different ground wires). Long gaps, no signal line can pass through the cracks on the ground plane (cross the gap above or below the gap);
9) The loop area of ​​the clock signal must be as small as possible;
10) Set a return line (signal ground line) near the key signal line (high frequency or extremely sensitive signal);
11) If a double-layer circuit board is used, a ground wire network must be set up (a horizontal line is placed on the A side and a vertical line on the B side, and the two are connected at the intersection of the two through a metalized via to be used as a ground wire);
12) The high-speed clock line should be as short as possible, and the wiring should not be changed, and the corners should not be 90 degrees to avoid sudden changes in impedance and signal reflection;
13) For all traces, if its length (inches) is greater than the signal rise/fall time (ns), termination resistors (typically 33Ω) should be used;
14) Perform simulation analysis on all traces whose length (inches) is greater than the signal rise/fall time (ns);
17) An independent ground wire is used on the I/O interface. This independent ground wire is connected to the other part of the ground wire on the circuit board at only one point. This ground wire is designed to provide a clean ground for the filtering and shielding layer;
18) The I/O interface filter installed on the circuit board should be as close as possible to the cable inlet and outlet, so that the connection line between the filter and the cable connector (between the shielded chassis) is the shortest
19) The drive circuit of the I/O interface cable should be close to the connector on the chassis;
20) Perform common mode filtering on all I/O cables, and concentrate all I/O cables in the set I/O area of ​​the circuit board;
21) The connection between the bypass capacitor used for I/O interface filtering and the chassis must have a low impedance;
22) The heat sink installed on the chip should be connected to the signal ground at multiple points;
23) The local shield on the circuit board must choose the interface with the least wiring, and filter all the wiring passing through the shielding box;
24) The capacity of the power supply decoupling capacitor should be as small as possible;
25) The lead between the power decoupling capacitor and the chip power pin and ground pin should be as short as possible;
26) Use multiple power decoupling capacitors with the same capacity, at least 2 for double-row leaded devices, and at least 4 for square packaged devices;
4. Cable design
1) As far as possible, the flat cable should be equipped with a ground wire next to each signal wire. If conditions do not permit, every two signal wires should be equipped with a ground wire; 2) When the situation permits, use twisted pair wires, but when using twisted pair wires , Pay attention to the grounding of the circuits at both ends, do not form a large ground loop;
3) When using a coaxial cable, pay attention to the termination of the outer layer and the grounding of the circuits at both ends, and do not form a second return path other than the outer layer;
4) Keep the cable away from the gaps and openings on the shield;
5) The shielding layer of the external cable of the equipment and the shielding case shall be overlapped 360 degrees;
6) For the external cables of the equipment, ensure a low impedance overlap between the shielding layer and the shielding chassis;
7) Install a ferrite magnetic ring on the cable, and adjust the number of turns of the wire wound on the ferrite magnetic ring as needed;
8) Try not to arrange different signal lines in a connector or cable.
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