New reference suggestions for EMI suppression measures in adapters

　　New reference suggestions for EMI suppression measures in adapters

　　Mechanism of Adapter Electromagnetic Interference

　　EMC

　　At present, many universities and research institutes have carried out the research of adapter EMI (Electromagnetic Interference). Some of them start from the mechanism of EMI generation, and some of them start from the influence of EMI, and they have put forward many practical and valuable solutions. Several effective schemes are analyzed and compared here, and new reference suggestions are put forward for the suppression measures of adapter EMI.

　　Mechanism of Adapter Electromagnetic Interference

　　The interference generated by the adapter is classified according to the type of noise interference source, which can be divided into spike interference and harmonic interference. If divided according to the coupling path, it can be divided into conduction interference and radiation 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 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. 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 caused by AC input circuit

　　The rectifier at the input end of the adapter 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 adapter, the interference formed by propagating through the input and output lines of the adapter is called conduction interference; while the energy of harmonics and parasitic oscillations, when propagating through the input and output lines, will generate electric fields in space. And magnetic field. 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 12-volt power adapter 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 components on the printed circuit board Unreasonable installation, placement, and orientation will cause EMI interference.

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