EMC technology application in switching power supply - Power Circuit - Circuit Diagram

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Generation and transmission of electromagnetic interference

There are two ways to transmit electromagnetic interference: one is a conductive transmission method, and the other is a radiation transmission method. Conducted transmission is a complete circuit connection between the interference source and the sensitive device, and the interference signal is transmitted to the receiver along the connection circuit to cause electromagnetic interference.

Radiated transmission is a form of interference that interferes with the propagation of a signal through the medium in the form of electromagnetic waves. There are three common types of radiation coupling: 1) an electromagnetic wave emitted by one antenna is accidentally received by another antenna, which is called antenna-to-antenna coupling; 2) a spatial electromagnetic field is coupled by wire inductance, called field-to-line coupling. 3) The coupling of high frequency signals between two equal wires is called inductive coupling of wire-to-wire.

2 The mechanism of electromagnetic interference

From the perspective of the sensitive device being interfered, the interference coupling can be divided into two types: conduction coupling and radiation coupling.

● Conductive coupling model

Conductive coupling can be divided into three basic coupling modes: resistive coupling, capacitive coupling and inductive coupling according to its principle.

● Radiation coupling model

Radiation coupling is another way of interference coupling. In addition to intentional radiation from sources of interference, there is a large amount of unintentional radiation. At the same time, the traces on the PCB board can be the effect of the antenna, such as the power line, the signal line, the clock line, the data line or the control line, so that the interference wave can be radiated and the receiving function can be performed.

3 electromagnetic interference control technology

1 transmission channel suppression

● Filtering: Pay attention to frequency characteristics, withstand voltage performance, rated current, impedance characteristics, shielding and reliability when designing and selecting filters. The correct installation of the filter has a great influence on its insertion loss characteristics. Only when the installation position is correct and the installation method is correct, the expected filtering effect can be exerted on the interference. The mounting position should be considered when installing the filter, and the wiring on the input and output sides must be shielded and isolated, as well as high frequency grounding and bonding methods.

● Shielding: Electromagnetic shielding can be divided into electric field shielding, magnetic field shielding and electromagnetic field shielding according to the principle. The electric field shield contains electrostatic shielding and alternating electric field shielding; the magnetic field shielding includes low frequency magnetic field shielding and high frequency magnetic field shielding. Different types of electromagnetic shielding have different requirements for the shield. In practical shielding, the electromagnetic shielding effectiveness depends to a greater extent on the structure of the shield, ie the continuity of the electrical conduction. Due to the requirements of manufacturing, assembly, maintenance, heat dissipation, observation and interface connection, the actual shielding body generally has different kinds of holes with different shapes and sizes. These holes play an important role in the shielding effectiveness of the shielding body. Therefore, measures must be taken to suppress the electromagnetic leakage of the hole.

● Grounding: There are two types of grounding: safety grounding and signal grounding. At the same time, grounding also introduces ground impedance and ground loop interference. Grounding techniques include selection of grounding points, circuit combinations, grounding design, and proper application of grounding suppression measures.

● Lap: The lap is to guide the low-impedance connection between the bodies. Only a good lap can make the circuit complete its design function, so that various suppression measures of interference can work. The lap joint method can be divided into two types: permanent lap and semi-permanent lap, and the lap type is divided into direct lap and indirect lap.

● Wiring: Wiring is the key to the electromagnetic compatibility design of printed circuit boards. You should choose a reasonable wire width and adopt the correct wiring strategy, such as thickening the ground wire, closing the ground wire into a loop, reducing the wire discontinuity, and adopting more Laminates, etc.