Principle and Generation of Electromagnetic Interference EMI
Before describing the principle of electromagnetic interference, we now understand the causes of EMI:
1. Causes of EMI
Various forms of electromagnetic interference are the main reasons that affect the compatibility of electronic equipment. Therefore, understanding the cause of electromagnetic interference is an important prerequisite for suppressing electromagnetic interference and improving the electromagnetic compatibility of electronic products. The generation of electromagnetic interference can be divided into:
Internal interference Mutual interference between internal electronic components
1) The working power supply generates interference caused by leakage through the distributed power supply and insulation resistance of the line.
2) The signal is coupled with each other through the impedance of the ground wire, power supply and transmission wire, or the influence caused by the mutual inductance between the wires.
3) Some components inside the equipment or system generate heat, which affects the stability of the components themselves and other components.
4) The magnetic field and electric field generated by high-power and high-point-voltage components affect the interference caused by other components through coupling.
External interference – the influence of factors other than electronic equipment or systems on circuits, equipment or systems.
1) External high voltage and power supply interfere with electronic circuits, equipment or systems through insulation leakage.
2) External high-power equipment generates a strong magnetic field in space, which interferes with electronic circuits, equipment or systems through mutual inductance coupling.
3) Space electromagnetic interference to electronic circuits or systems.
4) The temperature of the working environment is unstable, causing interference caused by changes in the parameters of electronic circuits, equipment or internal components of the system.
2. Transmission route of electromagnetic interference
When the frequency of the interference source is high, and the wavelength of the interference signal is smaller than the structure size of the interfered object, the interference signal can be considered as a radiation field, which radiates electromagnetic energy in the form of plane electromagnetic waves and enters the path of the interfered object. In the form of coupling and coupling, through the insulating dielectric, the coupling of the common impedance enters the interfered system. Interference signals can enter the system through direct conduction.
3. Measures to improve electromagnetic compatibility
To improve the electromagnetic compatibility of electronic products, grounding, shielding and filtering are the basic methods to suppress EMI.
1) Grounding
Grounding is the electrical conduction path between electrical and electronic components in a system to the ground reference point. In addition to providing the safety protection ground of the equipment, the ground also provides the signal reference ground necessary for the operation of the equipment. The ideal ground plane is a physical body with zero potential and zero impedance, which can be used as a reference point for all signal reviews in the circuit, and any interfering signal passing through it will not produce a voltage drop. However, an ideal ground plane does not exist, which requires us to consider and analyze the distribution of ground potential, conduct ground design and research, and find out a suitable ground potential. Grounding methods can be divided into: floating ground, single-point grounding, multi-point grounding, and hybrid grounding. For the circuit system, there are options: circuit grounding, power grounding, and signal grounding.
2) shielding
Shielding is to use the closed surface of conductive or electromagnet to electromagnetically isolate the inner and outer spaces. Mainly suppress the radiation interference in the space. Divided into electromagnetic shielding, electric field shielding and magnetic field shielding.
The shielding design can be aimed at both the interference source and the interfered object. For the interference source, the design of the shielding part can reduce the impact on other surrounding equipment; for the interfered object, it can reduce the impact of external interference electromagnetic waves on the equipment.
Active shielding: Place the interference source inside the shielding body to prevent electromagnetic energy and interference signals from leaking into the external space.
Passive shielding: placing sensitive equipment in a shielding body so that it is not affected by external interference.
3) Filtering
The meaning of filtering refers to a technique for extracting useful signals from original signals mixed with noise or interference. EMI Filters are components to achieve filtering.
In fact, when the device is working, it will also generate various noises. Switching power supply is a very strong interference source, and the EMI signal it generates not only occupies a wide frequency range, but also has a relatively large amplitude. With the propagation of the signal, these noises interfere with the next-level components, and the accumulation of such interference may eventually lead to the abnormal operation of the entire circuit. Assuming that the output signal of the device with large noise and obvious interference to the lower-level device is filtered to filter out the noise signal, the interference to the lower-level device will be reduced, and the system will work stably.
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Post time: Dec-27-2022
