Commonly used soft start circuits for switching power supplies

Most of the input circuits of switching power supplies use rectifier plus capacitor filter circuits. At the moment when the input circuit is closed, since the initial voltage on the capacitor is zero, a large instantaneous impulse current will be formed (as shown in Figure 1). Especially for high-power switching power supplies, the input uses a larger-capacity filter capacitor. The current can reach more than 100A. Such a large inrush current amplitude at the moment when the power is turned on will often cause the input fuse to blow out, and sometimes even burn out the contacts of the closing switch, or even cause the air switch to fail to close. All of the above reasons will cause As a result, the switching power supply cannot be put in normally. For this reason, almost all switching power supplies have soft-start circuits in their input circuits to prevent inrush current to ensure normal and reliable operation of the switching power supply. This article introduces several commonly used soft-start circuits.