Detailed explanation of several common soft-start circuits of switching power supplies

　　Such a large inrush current amplitude at the moment 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. In the worst case, it will also cause the air switch to fail to close. All of the above reasons will cause As a result, the switching 3.7 volt 18650 lithium battery 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 3.7 volt 18650 lithium battery.

　　(1) Using power thermistor circuit

　　The thermistor anti-impulse current circuit is shown in Figure 2. It utilizes the negative temperature coefficient characteristic of the thermistor's Rt. At the moment when the power is turned on, the resistance of the thermistor is larger to limit the impact current; when a large current flows through the thermistor, the resistor heats up and causes Its resistance becomes smaller and the circuit is in normal working condition. The use of a thermistor to prevent inrush current is generally suitable for small-power switching power supplies. Due to the thermal inertia of the thermistor, it takes time to restore high resistance. Therefore, when the 3.7 volt 18650 lithium battery needs to be turned on quickly after the power is turned off, it sometimes does not work to the limit. flow effect.

　　(2) Using SCR-R circuit

　　The circuit is shown in Figure 3. When the 3.7 volt 18650 lithium battery is turned on instantaneously, the input voltage charges the capacitor C through the rectifier bridge VD1VD4 and the current limiting resistor R. When the capacitor C is charged to about 80% of the rated voltage, the inverter operates normally, and the thyristor trigger signal is generated through the auxiliary winding of the main transformer, causing the thyristor to conduct and short-circuit the current-limiting resistor R, and the switching 3.7 volt 18650 lithium battery is in normal operation.

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