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The introduction of the power supply surge current suppression module of the switching power supply, because of its simple external circuit, the small size brings great convenience to the switching power supply designer. In this paper, the application of the surge suppression module in the switching power supply is explained in detail by analyzing two kinds of power-on surge current suppression circuits.
Power surge current At present, considering the factors such as volume and cost, most AC/DC converter input rectification and filtering adopts capacitive input filtering mode. The circuit principle is shown in Figure 1. Since the voltage on the capacitor cannot jump, at the beginning of the rectifier power-on, the filter capacitor voltage is almost zero, which is equivalent to a short circuit at the rectifier output. If the power is turned on in the most unfavorable case (the voltage instantaneous value at power-on is the peak value of the power supply voltage), an input surge current is generated that is much higher than the normal operating current of the rectifier, as shown in Figure 2. When the filter capacitor is 470μF and the internal resistance of the power supply is small, the first current peak will exceed 100A, which is 10 times the peak value of the normal operating current. 
Figure 2 Input surge current after power-on
The surge current will cause the power supply voltage waveform to collapse, which will make the power supply quality worse, and even affect the operation of other power-consuming equipment and the protection circuit; because the surge current hits the input fuse of the rectifier, it will be powered on several times. The surge current surges instead of the overload fuse. In order to avoid such phenomena, it is necessary to use a fuse with a higher rated current, but the fuse will not be blown when the overload occurs, and the function of the rectifier and the electric circuit cannot be protected; the excessive surge current is opposite. Rectifiers and filter capacitors cause irreparable damage. Therefore, the surge input current with capacitor filtering must be limited.
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