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basic methods of emc design
release time: 2018-01-30views: 431

grounding

grounding is a very important problem of electronic equipment. there are three grounding purposes:

(1) grounding makes all unit circuits in the whole circuit system have a common reference zero potential to ensure that the circuit system can work stably.

(2) prevent the interference of external electromagnetic field. the grounding of the shell can make a large amount of charges accumulated on the shell due to electrostatic induction discharge through the earth, otherwise the high voltage formed by these charges may cause spark discharge inside the equipment and cause interference. in addition, for the shielding body of the circuit, if the appropriate grounding is selected, good shielding effect can also be obtained.

(3) ensure safe work. when direct lightning electromagnetic induction occurs, the destruction of electronic equipment can be avoided; when the input voltage of the power frequency ac power supply is directly connected to the chassis due to poor insulation or other reasons, the operator’s electric shock accident can be avoided. in addition, many medical devices are directly connected to the patient’s body. when the chassis is equipped with 110v or 220v voltage, fatal danger will occur.

therefore, grounding is the main method to suppress noise and prevent interference. grounding can be understood as an equipotential point or equipotential surface, which is the reference potential of the circuit or system, but not necessarily the earth potential. in order to prevent the possible damage caused by lightning and the personal safety of workers, the chassis of electronic equipment and the metal components of the machine room must be connected with the earth, and the grounding resistance is generally very small, which cannot exceed the specified value.

there are basically three kinds of circuit grounding methods, namely, single point grounding, multi-point grounding and mixed grounding. single point grounding refers to that in a line, only one physical point is defined as the grounding reference point. all other points that need to be grounded are directly connected to this point. multipoint grounding refers to that each grounding point in a system is directly connected to the nearest grounding plane to minimize the length of the grounding lead. the grounding plane can be the bottom plate of the equipment, the ground conductor that runs through the whole system, or the structural framework of the equipment in a relatively large system. hybrid grounding is to connect those who only need high-frequency grounding points with the grounding plane by using bypass capacitors. however, the resonance phenomenon composed of bypass capacitance and lead inductance should be prevented as far as possible.

shield

shielding is the metal isolation between two space areas to control the induction and radiation of electric field, magnetic field and electromagnetic wave from one area to another. specifically, shielding is used to surround the interference sources of components, circuits, assemblies, cables or the whole system to prevent the outward diffusion of interference electromagnetic fields; enclose the receiving circuit, equipment or system with a shield to prevent them from being affected by external electromagnetic fields.

because the shield plays the role of absorbing energy (eddy current loss), reflecting energy (electromagnetic wave reflected at the interface of the shield) and counteracting energy (electromagnetic induction generates reverse electromagnetic field on the shielding layer, which can counteract part of the interfering electromagnetic wave) to the interfering electromagnetic wave and internal electromagnetic wave from the outside of wires, cables, components, circuits or systems, etc., the shield has the function of reducing interference.

the selection principle of shielding material is:

(1) when the frequency of the interference electromagnetic field is high, the eddy current generated in the metal material with low resistivity is used to counteract the external electromagnetic wave, so as to achieve the shielding effect.

(2) when the frequency of interfering electromagnetic waves is low, materials with high permeability should be used, so that the magnetic lines of force are limited inside the shield to prevent diffusion to the shielding space.

(3) in some cases, if it is required to have a good shielding effect on both high-frequency and low-frequency electromagnetic fields, different metal materials are often used to form a multi-layer shield.

wave filtering

filtering is an important measure to suppress and prevent interference. the filter can significantly reduce the level of conducted interference, because the interference spectrum components are not connected to the frequency of the useful signal, and the filter has good suppression ability for these components that are different from the frequency of the useful signal, thus playing a role that other interference suppression is difficult to play. therefore, the use of filter network is a powerful measure to suppress interference sources and eliminate interference coupling, or to enhance the anti-interference ability of receiving equipment. using resistance capacitance and inductance capacitance decoupling network can isolate the circuit from the power supply, eliminate the coupling between circuits, and avoid interfering signals entering the circuit. clcm π filter composed of two capacitors and one inductor (high-frequency choke) can be used for high-frequency circuit. there are many kinds of filters, and choosing an appropriate filter can eliminate unwanted coupling.

correct selection of passive components

practical passive components are not “ideal”, and their characteristics are different from ideal characteristics. practical components themselves may be an interference source, so the correct selection of passive components is very important. sometimes, the characteristics of components can also be used to suppress and prevent interference.

circuit technology

sometimes the requirements of restraining and preventing interference can not be met after shielding is used. circuit technologies such as shielding and balancing measures can be combined. balanced circuit refers to the two wires in a two-wire circuit and all circuits connected to these two wires, which have the same impedance to ground or other wires. the purpose is to make the interference signals detected by the two wires equal. at this time, the interference noise is a common state signal, which can disappear automatically on the load. in addition, other circuit technologies can be used, such as contact network, shaping circuit, integrating circuit and gating circuit, etc. in short, the use of circuit technology is also an important measure to suppress and prevent interference

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