1. Classification of switching power supply interference
The high frequency switching action of power switching devices is the main cause of electromagnetic interference (EMI). The increase in switching frequency on the one hand reduces the size and weight of the power supply, on the other hand it also leads to more serious EMI problems.
When the switching power supply is working, its internal voltage and current waveforms rise and fall in a very short time. Therefore, the switching power supply itself is a source of noise. The interference generated by the switching power supply can be divided 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. The fundamental way to prevent the interference generated by the power supply from causing harm to the electronic system and the power grid is to weaken the source of noise, or to cut off the coupling path between the power noise and the electronic system and the power grid.
Two, the four main reasons for switching power supply interference
1. 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 waveforms of forward, push-pull and bridge converters are approximately rectangular waves when resistive loads, which contain rich high-order harmonic components. When using zero-current, zero-voltage switching, this harmonic interference will be 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 produce spike interference.
2. Interference caused by AC input circuit
The rectifier at the input end of the Shenzhen switching power supply without a power frequency transformer will cause high-frequency attenuation oscillations during the reverse recovery period to cause interference. The spike interference and harmonic interference energy generated by the switching power supply are transmitted through the input and output lines of the switching power supply and the interference formed is 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 radiation interference.
3. Interference caused by the reverse recovery time of the diode
The AC input voltage is changed into a sine pulsating voltage through the power diode rectifier bridge, and then becomes a direct current after smoothing by the capacitor, but the waveform of the capacitor current is not a sine wave but a pulse wave. It can be seen from the current waveform that the current contains high-order harmonics. A large number of current harmonic components flow into the power grid, causing harmonic pollution to the power grid. In addition, because the current is a pulse wave, the input power factor of the power supply is reduced. 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.
4. Other reasons
The parasitic parameters of the components and the schematic design 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 and placement of the components on the printed circuit board, And the unreasonable position will cause EMI interference. This increases the difficulty of the extraction of PCB distribution parameters and the estimation of near-field interference.
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