The central topic of electromagnetic compatibility:
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Brief introduction of electromagnetic compatibility technology
Electromagnetic compatibility example application analysis
Electromagnetic compatibility solution:
Grounding, shielding, filtering
Integrate EMC technology into product development processes
Abstract : This paper briefly introduces various technologies related to electromagnetic compatibility. Through the analysis of grounding, shielding, filtering and other technologies, how to achieve good electromagnetic compatibility, how to integrate electromagnetic compatibility technology into product development process. For the case analysis, combined with the electromagnetic compatibility theory, the processing method in the actual test is explained, and the analysis and verification are carried out from the interference source, the coupling path and the sensitive source to improve the reliability of the product.
At present, electromagnetic compatibility technology has developed into a special technology for anti-electromagnetic interference and electromagnetic radiation of electronic products, and has become an important indicator for examining the safety and reliability of electronic products, covering all electronic products.
When each electronic device works in the same space, it will generate a certain intensity of electromagnetic field around it. These electromagnetic fields are coupled to other electronic devices through certain ways (radiation, conduction), affecting the normal operation of other devices, and may cause communication errors or systems. Deadlocks, etc., mutual interference between devices, this effect is not only between devices, but also between components and components, between systems and systems. It even exists inside the integrated chip.
Electromagnetic compatibility technology mainly includes grounding, filtering, shielding technology, etc., which need to be paid attention to in different occasions. In terms of structure, attention should be paid to shielding and grounding. Pay attention to grounding and filtering in terms of cables. In PCB design, attention should be paid. Signal layout, filtering, etc.
First, electromagnetic compatibility technology
Firstly, it starts from the three elements that constitute electromagnetic interference, namely the interference source, the sensitive source and the coupling path. The interference source is the device that generates electromagnetic interference, and the interference sensitive source device is affected by the coupling path such as cable and space radiation. High-frequency voltage/current is the source of interference. There are two ways for electromagnetic energy to propagate between devices: conducted emission and radiated emission. Conducted emission uses wire as the medium and current as the phenomenon. Radiated emission is the space radiation. Taking electromagnetic waves as a phenomenon. Common sources of interference include lightning, wireless communication, pulse circuits, static electricity, inductive load switching, antennas, and cable conductors. Any circuit can become a sensitive source, digital circuits have better anti-interference, but the risk is high. Large pulse spikes may be digital circuit malfunctions, and audio analog circuits are sensitive to RF signals. The coupling path is divided into spatial coupling and conductive coupling. The spatial coupling includes mutual inductance coupling, capacitive coupling, and antenna radiation. The conductive coupling includes conduction on the ground and power lines.
Electromagnetic compatibility design mainly includes knowledge of grounding design, shielding design, and filter design. The ground line is divided into safe ground, AC ground, DC ground, digital ground, analog ground, chassis ground, lightning protection ground, etc. The ground wire from the voltage concept is to provide an equipotential body, from the current concept is to provide a current path. The impedance of the ground line determines the anti-interference of the line. The impedance of the wire determines the potential difference of the ground. The loop impedance determines the actual ground current. The existence of the ground loop is the main cause of the interference of the circuit. The area reduces the impact on the line, and the use of shielded or coaxial cables may reduce the area of ​​the signal loop, thereby reducing the effects of interference. The ground current is always a path with a relatively small impedance of the ground line. The impedance of the line is different at high frequency and low frequency. The signal path can be designed as needed. The multi-layer board is better than the double-layer board because the multi-layer board has a special ground layer and power layer to ensure that each signal loop has the smallest signal loop area. If it is a double-layer board, it is better to lay the ground network. Grid to ensure the smallest loop area.
Single-ended grounding is to reduce the influence of the electric field on the equipment. The grounding at both ends reduces the influence of the magnetic field on the equipment. The two ends are grounded to form a magnetic field loop. The external magnetic field generates an induced current in the loop formed by the original signal and the ground. Inductive current Is is generated in the circuit formed by the shielding layer and the ground line, and Is also induces a magnetic field, but this magnetic field is opposite to the original magnetic field magnetic field and cancels each other, resulting in a decrease in the total magnetic field and a reduction in interference.
Shielding technology is mainly applied to the structure of the system, and also shields the critical circuits of the line, such as clock circuits and CPUs. Inspect the shielding effectiveness of the system by using electrostatic testing. If the shielding of the system is done well, the static electricity will be discharged along the shielding body without affecting the internal wiring. The key factor of good electromagnetic shielding is the continuity of the conductor of the shield. If the guide wire must be opened, the shielded cable should be used. The shield must be grounded by a 360-degree loop to ensure the integrity of the shield. Shielded cables are selected based on the frequency characteristics of the transmission impedance of the different shield layers and the signal operating frequency.
Filtering includes power line filtering and signal filtering. The cable is a good antenna. Sometimes, even if the shielding is done very well, it can't pass the radiation emission and radiation sensitivity test. This is because the radiation generated by the cable is much higher than the leakage of the circuit board itself and the incomplete shielding of the chassis. The radiation produced. One way to solve this problem is to install a filter at the port of the cable to filter out the interference current. The cutoff frequency of the filter is selected according to the frequency of the interference to effectively filter out the interference. A system uses a second-order LC low-pass filter. The radiation test is still too late. The pre-capacitor is removed, and the radiation emission is not exceeded. It indicates that the cut-off frequency needs to be reduced to filter out some of the interference and increase the number of filters. The steepness of the curve improves the filtering performance at the operating frequency and does not filter out the lower frequency interference. The filter capacitor leads should be short, and the "V" connection method can be used to reduce the loop impedance at high frequencies. It is also possible to add mounting beads on the leads, increase the inductance on the leads, and enhance the filtering effect. The resistance of the film capacitor is large, and the ceramic capacitor should be used for filtering. The impedance characteristics of the ceramic capacitor are good.
Electromagnetic compatibility technology should be carried out throughout the product development, including product outline design, detailed design, schematic printing board design, structure, assembly and debugging, etc., should consider electromagnetic compatibility design, the product design environment needs to be investigated in the outline design. Analyze the type of on-site interference and assess the risk of interference. In the detailed design, specific countermeasures need to be taken, and corresponding countermeasures are needed, which requires comprehensive design. Schematic diagram of the printed board diagram design needs to embody the various measures in the schematic diagram, and simulate if necessary. The printed board diagram design needs to follow the modular design, pay attention to the layout and wiring, and the electromagnetic compatibility protection of the sensitive circuit. The structure is also a major part of the electromagnetic compatibility design. The structure of the product has a great relationship with static electricity, group pulse, radiation, etc. The structure requires good shielding and grounding. In the assembly and debugging process, it is necessary to pay attention to signal integrity, ensure the continuity of the grounding, pay attention to the panel contact problem, and take corresponding measures according to the actual situation encountered in the testing process.
Second, the application analysis of electromagnetic compatibility examples
The overall goal of learning electromagnetic compatibility technology is to systematically learn the knowledge of electromagnetic compatibility. By learning electromagnetic compatibility design theory, these methods, rules and measures are integrated into the actual work to ensure that the products are as reliable as possible.
1, grounding problem
Example 1: A system equipment is doing the RF field induction conduction test of the 422 communication serial port. The twisted pair shielded cable is used, and the single-ended grounding is adopted. The bit error rate occurs during the test, and there is almost no correct data. The terminal is reliably grounded and the communication is normal.
Example 2: When a system equipment is used to test the RF field induction conduction of the video mouse line, the display has ripples in the lower frequency band (below 3M), flashing up and down, and then the display side of the video line is reliably grounded, and the interference is significantly reduced. , hardly affects the display.
Analysis: These two phenomena occur in the inductive conduction test of the RF field. The inductive conduction immunity test of the RF field is essentially: the device lead becomes a passive antenna, accepting the induction of the RF field, and becoming a conducted interference intrusion. Inside the device, the near-field electromagnetic field formed by the RF voltage and current affects the operation of the device, mainly based on the low-frequency magnetic field.
Twisted pair can effectively suppress magnetic field interference, not only because there is a small loop area between the two wires of the twisted pair, but also because the current induced on every two adjacent loops of the twisted pair has the opposite The direction is therefore offset by each other. The denser the twisted pair of twisted pairs, the more obvious the effect.
When the shielding layer is grounded at both ends, the external magnetic field generates an induced current in the loop formed by the original signal and the ground line, and also generates an induced current Is in the loop formed by the shield layer and the ground line, and Is also induces a magnetic field, but this The magnetic field is opposite to the original magnetic field and cancels each other, resulting in a reduction in the total magnetic field and a reduction in interference.
2, shielding problem
Example 3: A system is a cabinet and a chassis structure, wherein the control part is a chassis structure, a sub-board bus board structure, and a sub-board is installed with a panel. When performing electrostatic test, when the contact discharge is +5.5kv, when the main board panel and the left and right adjacent panels are subjected to electrostatic test, the control board restarts or freezes, and then the finger reed is installed between the panels near the control board, and the system is in contact. The operation is normal when the discharge is ±6.6kv.
Example 4: A system test, using a common cabinet, the system is very sensitive, the cluster lead test (communication line) for group pulse test, using coupling clip coupling, the interference is added, the system is not normal, increase at both ends of the communication line Magnetic ring, the effect is not obvious, then there is no way, replace the shielded cabinet, carry out the test, there is obvious effect, after a few rounds, the system will appear the idea of ​​the machine, after the communication line into the cabinet to increase the installation of the magnetic ring, the system The work was normal. After several rounds of tests, there was no downtime and the system worked normally.
Analysis: Many systems are now chassis structure, that is, the control board, acquisition board, and driver board are all installed in the same chassis for data exchange and control. After the installation is completed, each circuit board will have a certain gap. The electrostatic pulse passes through the panel gap, and the distributed capacitance is coupled to the main board to make the power supply distorted or control the system to restart and crash. The finger spring is installed between the panels to make the chassis a good shield. Due to the "skin effect" of the electric charge, when there is static electricity, the static electricity will be discharged to the earth along the surface, and the influence on the internal circuit is reduced. Small or disappear.
The shielded cabinet handles the gaps and doors of the cabinet. The conductive reed is installed at the gap. The conductive cloth pad is installed at the contact position between the door and the cabinet to improve the shielding effectiveness of the cabinet, improve the overall anti-interference of the cabinet, and the essence of the group pulse interference. It is the accumulation effect of the distributed capacitance energy of the line. When the energy is accumulated to a certain extent, it may cause the circuit (or even the equipment) to work incorrectly. Usually, if the test equipment is in error, it will continue to make mistakes, even if the pulse voltage is slightly reduced, the error situation is still explained. Pulse grouping occurs, the pulse repetition frequency is higher, the waveform rise time is short, the energy is small, and generally no equipment failure occurs, which makes the equipment malfunction.
3, the role of the magnetic ring
Example 5: Perform a group pulse experiment on a chassis structure system. The chassis contains control boards, acquisition boards, drive boards, etc. The acquisition lines and drive lines are out of the cabinet. Signal line group pulse experiments are required. When interference is applied to the acquisition line. All the indicators on the acquisition board are flashing, the acquisition loop is analyzed, the input and input have opto-isolated devices, and the collection loop is a dynamic 12V output. When the interference is applied, the voltage on the collection return line may be distorted, resulting in an indication. The light flashes, find a closed magnetic ring, install it on the collection return line, carry out the experiment, the indicator light flashes under a certain polarity, indicating that the magnetic ring has a function, and then according to its impedance characteristics, it is wound 2 times, the experimental effect is not Obviously, after testing, it was wound for 3 laps. As a result, the collection indicator showed normal, and the system was normal after repeated tests.
Analysis: The magnetic ring has a good inhibitory effect on the group pulse interference. It is installed at both ends or one end of the communication line according to the actual situation. The magnetic ring has different impedance characteristics, and the frequency analysis of the interference signal is performed. The cutoff frequency of the designed magnetic ring is just right. It falls near the frequency of the interference signal, so that the magnetic ring exhibits a large impedance to suppress interference.
The number of turns of the magnetic ring affects the impedance characteristics of the magnetic ring. The more the number of turns, the impedance characteristic curve moves toward the low frequency direction, that is, the impedance at a lower frequency is larger, and if the frequency is closer to the interference frequency, it can be very Good inhibition of interference.
The integration of electromagnetic compatibility technology into electronic product development and design can improve the safety and reliability of the product. If there is a defect in one aspect in actual testing, it can be tested step by step from the electromagnetic interference mode. There are two forms of electromagnetic interference: Conducted and radiated emissions are searched from their respective coupling paths. If a system indicator exceeds the standard, it can be solved first from the radiation emission. Whether the equipment is well shielded, the hole on the casing is sealed with a conductive cloth, the conductive cloth should be in good contact with the casing, and then the test is carried out. If it exceeds the standard, the interference is mainly When the conducted emission is caused, a signal filter and a power supply filter are installed at the exit of the equipment casing to test. If the standard is exceeded, the interference is transmitted through the cable radiation and conduction. By grounding the shielding layer, the ground ring is reduced. Measures such as roads must be able to find the cause and solve it.
Third, the conclusion
The products need to be gradually updated and perfected in order to achieve certain safety and reliability. Electromagnetic compatibility technology needs to be accumulated continuously to ensure the safety and reliability of the products. The application occasions are different, the electromagnetic interference encountered is different, and the performance of the products is different. The actual application environment, analyze the interference source, find the coupling path, clarify the sensitive source, take isolation measures for the interference source, cut off the coupling path or channel the interference, and adopt shielding and filtering measures for the sensitive source to ensure the safe and reliable operation of the product.
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