Four main reasons for switching power supply interference
one, switching power supply interference classification
The high-frequency switching action of power switching devices is the main cause of electromagnetic interference (EMI). The increase of switching frequency reduces the size and weight of the power supply on the one hand, but also leads to more serious EMI problems on the other hand. 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 into two types: peak interference and harmonic interference according to the type of noise interference source; if it is divided according to the coupling path, it can be divided into two types: 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 the noise, or cut off the coupling path between the power supply noise and the electronic system and the power grid.
Two, four main reasons for switching power supply interference
1. Harmonic interference generated when the switching tube is working
A large pulse current flows through the power switch tube when it is turned on. For example, the input current waveforms of forward, push-pull and bridge converters are approximately rectangular waves in resistive loads, which contain abundant 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 tube, the current mutation caused by the leakage inductance of the high-frequency transformer winding will also produce spike interference.
2. Interference generated by the AC input circuit
The rectifier tube at the input end of Shenzhen switching power supply without power frequency transformer will cause high frequency attenuation oscillation and interference during reverse recovery. The peak interference and harmonic interference energy generated by the switching power supply are transmitted through the input and output lines of the switching power supply. Generate electric and magnetic fields. This kind of interference generated by electromagnetic radiation is called radiated interference.
3. Interference caused by the reverse recovery time of the diode
The AC input voltage is changed to a sinusoidal pulsating voltage by a power diode rectifier bridge, and then becomes a DC after being smoothed by a 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 higher harmonics. A large number of current harmonic components flow into the grid, causing harmonic pollution to the grid. In addition, since the current is a pulse wave, the input power factor of the power supply is reduced. When the rectifier diode in the high-frequency rectification circuit is forward-conducting, a large forward current flows through it. When it is reverse-biased and turned to cut-off, because there are more carriers accumulated in the PN junction, the current-carrying For a period of time before the disappearance of the carriers, the current will flow in the reverse direction, causing the reverse recovery current of the carrier disappearance to decrease sharply and a large current change (di/dt) will occur.
4. Other reasons
The parasitic parameters of the components, the schematic design is not perfect, the printed circuit board (PCB) wiring is usually arranged manually, which has a lot of 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 orientation will cause EMI interference. This increases the difficulty in the extraction of PCB distribution parameters and the estimation of near-field interference.
