How to suppress the EMI phenomenon of the switching power supply adapter.ac power adapter price

　　EMI interference sources of switching power supplies are concentrated in power switching tubes, rectifier diodes, high-frequency transformers, etc. The external environment's interference to switching power supplies mainly comes from the jitter of the power grid, lightning strikes, and external radiation. Regarding the research of switching power supply EMI, some start from the mechanism of EMI, and some start from the influence of EMI, and put forward many practical and valuable solutions.

　　The mechanism of electromagnetic interference of switching power supply

　　The interference generated by the switching power supply is classified according to the type of noise interference source, which can be divided into spike interference and harmonic interference; if it is classified according to the coupling path, it can be divided into two types: conducted interference and radiated interference. Now explain separately according to the noise interference source:

　　1. Interference caused by the reverse recovery time of the diode

　　The rectifier diode in the high-frequency rectifier circuit has a large forward current flowing when it is turned on by the reverse bias voltage. Because there are more carriers accumulated in the PN junction, the current carrying For a period of time before the electron disappears, the current will flow in the reverse direction, causing the reverse recovery current for the disappearance of the carrier to decrease sharply and a great current change (di/dt) occurs.

　　2, the harmonic interference generated when the switch tube is working

　　The power switch tube flows through a larger pulse current when it is turned on. For example, the input current waveform of the forward, push-pull, and bridge converters is similar to a rectangular wave under resistive load, which contains rich high-order harmonic components. When using zero-current, zero-voltage switching, this harmonic interference will be very small. In addition, during the cut-off period of the power switch, the current sudden change caused by the leakage inductance of the high-frequency transformer winding will also cause spike interference.

　　3. Interference generated by AC input circuit

　　The rectifier at the input end of the switching power supply without a power frequency transformer will cause high-frequency attenuated oscillations and interference during the reverse recovery period. The spike interference and harmonic interference energy generated by the switching power supply, which are transmitted through the input and output lines of the switching power supply, are called conduction interference; while the energy of harmonics and parasitic oscillations, when propagating through the input and output lines, will be in the space. Generate electric and magnetic fields. This kind of interference produced by electromagnetic radiation is called radiated interference.

　　4, other reasons

　　The parasitic parameters of the components and the schematic design of the switching power supply are not perfect. The printed circuit board (PCB) traces are usually arranged manually, which has great randomness. The near-field interference of the PCB is large, and the installation of the components on the printed circuit board, Unreasonable placement and orientation will cause EMI interference.

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