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EMI Rectification Plan | Several Methods for Electromagnetic Compatibility Rectification
Author.
LCS
Source:
Post time:
2023-09-27
Firstly, it is necessary to diagnose the product based on the actual situation, analyze the sources of interference and the ways and means of mutual interference. Based on the analysis results, targeted rectification will be carried out, and the following points are for reference only.
1. To weaken the interference source, based on identifying the interference source, the allowable range of interference sources should be weakened. The method for weakening the interference source is related to the following aspects:
a. Add a decoupling capacitor between the VCC and GND of the IC, with a capacity of 0.1 μ F~1 μ Pay attention to the lead of the capacitor during installation, making it as short as possible.
b. Whether the filtering, protection, and isolation devices of interface signals are placed close to the interface connector, with protection first and filtering later; Is the power module, filter, and power protection device placed close to the entrance of the power supply, ensuring that the input line of the power supply is as short as possible, the input and output of the power supply are separated, and the wiring is not crossed with each other.
c. Are strong radiation devices or sensitive devices such as crystals, crystal oscillators, relays, and switching power supplies far away from single board handles, connectors, input/output terminals, and PCB edges.
d. Is the filter capacitor placed close to the power pins of the IC, with appropriate position and quantity.
e. Check if the clock circuit is close to the load and placed evenly.
f. Whether there are no other signal lines running offline from the input line of the power filter, and whether the input and output signal lines of the filter and other devices are not parallel or crossed with each other.
2. The classification and organization of wires and cables in electronic equipment, inter line coupling is an important way and also an important cause of interference. Due to frequency factors, it can be roughly divided into high-frequency coupling and low-frequency coupling. Due to different coupling methods, there are also differences in their rectification methods:
(1) Low frequency coupling: When the length of the guide line is equal to or less than 1/16 of the wavelength, low frequency coupling can be divided into electric and magnetic field coupling. The physical model of electric field coupling is capacitive coupling. Therefore, the main purpose of rectification is to reduce the distributed coupling capacitance or the coupling amount. The following methods can be used:
a. Increasing circuit spacing is the most effective way to reduce distributed capacitance.
b. Add a high conductivity shielding cover and make the shielding cover grounded at a single point to effectively suppress low-frequency electric field interference.
c. Adding a filter can reduce the coupling between two circuits.
d. Reduce input impedance, for example, CMOS circuits have high input impedance and are extremely sensitive to electric field interference. It is possible to connect a capacitor or a resistor with a lower resistance value to the input end within the allowable range. The physical model of magnetic field coupling is inductive coupling, which is mainly coupled through the distributed mutual inductance between lines. Therefore, the main method for rectification is to destroy or reduce its coupling amount, and the following methods can be generally adopted:
-When adding a filter, pay attention to the input/output impedance and frequency response of the filter.
-Reduce the loop area between the sensitive circuit and the source circuit, that is, try to make the signal line or current carrying line close to or twisted together with its return line as much as possible.
-Increase the distance between two circuits to reduce mutual inductance between lines and reduce coupling.
-If possible, try to reduce the coupling between the sensitive circuit and the source circuit by making the plane orthogonal or nearly orthogonal.
-Using high permeability magnetic materials to wrap sensitive wires can effectively solve the problem of magnetic field interference. It is worth noting that a closed magnetic circuit should be formed, and reducing the magnetic resistance of the magnetic circuit will be more effective.
(2) High frequency coupling: Refers to the increase in coupling due to the presence of standing waves of voltage and current in a circuit for wiring longer than 1/4 wavelength. The following methods can be used to solve this problem:
a. Try to shorten the grounding wire as much as possible and use surface contact with the casing grounding as much as possible.
b. Reorganize the input and output lines of the filter to prevent coupling between input and output lines, ensuring that the filtering effect of the filter remains unchanged.
c. The shielding layer of the shielded cable adopts multi-point grounding.
d. Connect the suspended pin of the connector to the ground potential to prevent its antenna effect.
3. The ideal ground wire for improving the ground wire system is a physical entity with zero impedance and zero potential. It is not only the reference point of the signal, but also does not generate voltage drop when current flows through it.
In specific electrical and electronic equipment, this ideal ground wire does not exist, and voltage drop will inevitably occur when current flows through the ground wire. Based on this, the formation mechanism of interference in the ground wire can be summarized as the following two points: first, reducing the low impedance and power supply feeder impedance. Secondly, choose the correct grounding method and barrier grounding loop, which can be divided into floating grounding, single point grounding, multi-point grounding, and mixed grounding according to the grounding method. If the interference of sensitive lines mainly comes from external space or system shells, floating ground can be used to solve this problem. However, floating ground equipment is prone to electrostatic accumulation, and when the charge reaches a certain level, electrostatic discharge will occur. Therefore, floating ground is not suitable for general electronic equipment. Single point grounding is suitable for low-frequency circuits. To prevent power frequency currents and other stray currents from generating ground potential differences between points on the signal ground wire, the signal ground wire is isolated from the power supply and safety ground wire and connected at a single point at the ground connection of the power supply wire.
4. Shielding and shielding is one of the important measures to improve the electromagnetic compatibility performance of electronic systems and equipment, which can effectively suppress various electromagnetic interference propagated through space. Shielding can be divided into magnetic field shielding, electric field shielding, and electromagnetic shielding according to its mechanism. The following points should be noted for electric field shielding:
a. Choose materials with high conductivity and have good grounding.
Choose the correct grounding point and reasonable shape, preferably with the shield directly grounded. Magnetic field shielding usually only refers to the shielding of direct current or very low frequency magnetic fields, and its shielding effectiveness is far inferior to electric field shielding and electromagnetic shielding. Magnetic shielding is often the focus of engineering. When magnetic shielding:
a. Choose ferromagnetic materials.
b. The magnetic shield should be kept away from magnetic components to prevent magnetic short circuits.
c. Double or even triple shielding can be used.
d. Pay attention to the direction of the opening on the upper side of the shielding body, and try to make the long side of the slot parallel to the magnetic flux flow direction as much as possible to minimize the increase in magnetic circuit length. Generally speaking, magnetic shielding does not need to be grounded, but to prevent electric field induction, grounding is better. When an electromagnetic field passes through a metal or a barrier that has a attenuation effect on the electromagnetic field, it will experience a certain degree of attenuation, resulting in a shielding effect on the electromagnetic field. In the actual rectification process, the selection of shielding and the shape, size, and grounding method of the shielding body depends on the specific needs.
5. Change the wiring structure of the circuit board
Some frequency points are determined by the distribution parameters of the wiring on the circuit board, and the aforementioned method is not very useful. This type of rectification changes the circuit parameter structure by adding small inductors, capacitors, and magnetic beads in the wiring to move it to frequency points with higher limit requirements. To fundamentally solve the impact of such interference, rewiring is necessary.
Summary
In short, the previous methods have benefits for improving electromagnetic compatibility, but the most widely used methods are the methods of changing the structure of ground wires and sorting wires and cables. These methods not only save costs, but also are the most effective rectification methods. Although shielding may increase costs, its shielding effectiveness is sometimes incomparable to other methods. Therefore, in actual rectification, the main method should be to change the structure of the ground wire, classify and organize wires and cables, and shield them, supplemented by other methods.
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