EMI is harmful electromagnetic energy. It can interrupt, hinder, reduce or limit the normal operation of electrical and electronic equipment. EMC failures may occur in severe cases. When EMC problems occur, EMI diagnosis and elimination must be carried out. The diagnosis of EMI is inseparable from the accurate judgment of the three elements of EMI (interference source, transmission coupling path and receiving response), as well as the analysis and synthesis of EMI phenomena. Test is the basic method of EMI diagnosis. In the process of diagnosis, it is necessary to apply contradictory laws and related principles to discover the organic connection between the three elements of interference and reveal its causal relationship. In addition, make necessary test plans to avoid messy and time-consuming experiments. All methods and measures that are conducive to controlling EMI signals, and environmental conditions should be given sufficient attention.
First check the structure of the equipment and the connections between the various modules to determine which are the main diagnostic targets. For equipment or systems with antennas, EMI diagnosis should give priority to the interference input or output from the antenna. For equipment or systems composed of sensors, priority should be given to checking whether the sensor is an electromagnetic interference input or output unit. The signal lines, cabinets, ground wires, power lines, vents, high-level, high-current cables of high-sensitivity equipment are the focus of diagnosis. Among the various modules of the equipment and system, the electromagnetic interference of the power supply equipment and power supply system should be given priority. Motors, converters, choppers, rectifiers, etc. may also be sources of interference. Power line interconnection cables, copper pads on PCB boards, etc. are often transmission coupling paths for electromagnetic interference. Interference phenomena are often related to working bandwidth, harmonics, parasitic response and oscillation, non-linear working components, etc. Environmental interference is also an important aspect of EMI diagnosis.EMC整改案例
Secondly, develop specific diagnostic methods according to different objects. If the object is a complex system composed of multiple systems and multiple modules, self-check and mutual check methods can be used for diagnosis. Turn off part of the power supply and make the corresponding part inoperative. This is the easiest way to diagnose EMI. If conditions permit, the method of turning off the two modules or systems or selectively turning off the power is judged one by one to make the scope of suspicion smaller. Substitution and replacement of suspected equipment or components may aid diagnosis. For example, use stable, reliable and mature products to replace the corresponding parts of the new equipment. Measures such as temporary shielding, filtering, and grounding of new equipment or system components and circuits are also helpful for diagnosis. Inserting an opto-isolator in the line is a convenient way to determine the ground loop current interference. The use of frequency management control is an important means of diagnosis. It is also one of the commonly used methods to perform detailed diagnosis with lines and blocks as units. In the process of diagnosis methods and measures taken, no new electromagnetic interference should be generated, and the electromagnetic field structure of the diagnosis object should not be destroyed. Pay attention to personal safety in the process of diagnosis and taking measures. The use of high-sensitivity instruments such as spectrum analyzers or receivers and electric and magnetic field probes make the diagnosis more flexible and practical.
1. If there is an antenna, first check the directivity of the antenna, the side-lobe interference of the antenna, whether the grounding wire and grounding point of the antenna meet the requirements, the radiation and conduction interference generated by the rotation of the antenna axial mechanism (such as the motor), and the poor contact of the antenna , Interference caused by non-linear effects after contact corrosion, antenna installation is not tight, or electrical signal interference due to jitter, whether the antenna connection is shielded wire, or whether the shielding of the connection has discontinuous breakpoints, and the connection is The wiring and the adjacent electromagnetic environment. Check whether the process structure of the equipment rack and shell, front and rear panels, PCB, etc. meets EMC requirements. For example, whether the opening of the enclosure is too large, whether conductive rubber gaskets or gaskets, conductive adhesives, conductive films, etc. are used can diagnose radiation interference caused by poor shielding or poor contact of the enclosure, and signal lines, control lines and control lines that enter and exit the equipment. Whether the power cord is equipped with necessary filtering and decoupling devices at the entrance and exit of the casing to prevent the leakage of internal interference of the equipment and the intrusion of external interference. The leading and trailing edges of the pulse signal’s rapid rise and fall will produce interference with a wide spectrum. For example, the switching signal of a switching power supply, if the pulse signal rise or fall time is shorter, the efficiency of the switching power supply is higher, but if one-sided pursuit of efficiency, the electromagnetic interference generated by it may reach the point where it cannot be suppressed. Therefore, a good balance should be made in terms of efficiency and electromagnetic compatibility. A current and voltage buffer network can be added to the switch tube to suppress electromagnetic interference. Check whether electronic circuits and integrated blocks with high anti-interference ability and low noise are used. Check whether the oscillation in the output waveform of the integrated circuit and the non-linear component is filtered out by taking suppression measures; whether the selected filter capacitor and magnetic beads meet the requirements, and the impedance of the printed board line is too large to cause interference; check the strip line Whether the arrangement of ground wire-signal wire-ground wire-signal wire-ground is adopted.
2. Check whether the filtering and decoupling capacitors on the power supply board meet the EMC requirements, such as whether the pins of the filtering and decoupling capacitors are too long, and whether the resonance frequency of the selected capacitor is much greater than the frequency to be suppressed. The use of public power supply will cross-infect the interference of the equipment. If the power transformer does not meet the requirements, serious conducted interference will occur on the power line.
3. Whether the overlap meets EMC requirements. When radio frequency interference occurs, you should first check whether the laps on the electronic equipment chassis overlap the rack, the chassis in the chassis and the chassis overlap, and the tightly bonded metal parts in the radiation field should be overlapped with the chassis. Check the overlap. Check whether all the contact surfaces are corroded, whether the lap piece meets EMC requirements (length: width=5:1), whether the length of the lap piece is much shorter than the wavelength, check the connector and the casing, the contact piece and the cable Whether the contact is good.
4. Wiring check. Check whether the power cord is arranged close to the ground, and whether the strong and weak signal lines or wires with a level difference of more than 40dB are tied in
Together, or close to each other, check whether the wiring isolates the input and output, whether the ground wire of the single-point grounding shield is far away from the strong interference cable and the strong electromagnetic field area, and check whether the circuit module is properly arranged, the components with strong electromagnetic interference and other sensitive Whether the component axis is arranged in a vertical way.
5. Check the grounding condition. Check whether the ground wire is too long, and check whether there is a common ground wire between the signal wire and the power wire. The main check for low-frequency electronic circuits is whether there is ground loop interference. Several grounding generally follow the following principles: single-point grounding is used for frequencies below 1MHz, multi-point grounding is used for frequencies above 10MHz, 1MHz-10MHz, if the length of the grounding wire is less than 1/20λ, single-point grounding can be used, otherwise multipoint grounding . Check whether the ground wire is thick enough, whether the ground wire is routed smoothly, etc.
6. Check the filter. Poor filtering or no filter is the main cause of interference. Check whether the filter is close to the filtering equipment to reduce the interference, check whether the input and output of the filter are isolated, and whether the switch and relay are filtered.