Introduction to the principle of switching power supply starting circuit
The input circuit of switch mode power supplies mostly adopts rectification and capacitor filtering circuits. At the moment when the input circuit is closed, a large instantaneous surge current (as shown in Figure 1) will be formed due to the initial voltage on the capacitor being zero. Especially for high-power switching power supplies, the input uses a larger capacity filtering capacitor, and the surge current can reach more than 100A. At the moment of power on, such a large surge current amplitude often causes the input fuse to burn out, and sometimes even burns out the contacts of the closing switch. In mild cases, it can also cause the air switch to fail to close. All of the above reasons can cause the switching power supply to fail to operate normally. For this reason, almost all switching power supplies are equipped with soft start circuits in their input circuits to prevent surge currents, ensuring the normal and reliable operation of the switching power supply. This article introduces several commonly used soft start circuits.
(1) Using power thermistor circuit
The anti surge current circuit of thermistor is shown in Figure 2. It utilizes the negative temperature coefficient characteristic of the Rt of the thermistor. At the moment of power on, the resistance value of the thermistor is relatively high, which limits the impact current; When a large current flows through a thermistor, the resistor heats up and its resistance decreases, and the circuit is in normal working condition. The use of thermistors to prevent surge currents is generally suitable for low-power switching power supplies. Due to the thermal inertia of thermistors, it takes time to restore high resistance. Therefore, for situations where the power supply needs to be quickly connected after a power outage, it sometimes does not have a current limiting effect.
(2) Adopting SCR-R circuit
The circuit is shown in Figure 3. When the power is instantly turned on, the input voltage charges capacitor C through rectifier bridges VD1-VD4 and current limiting resistor R. When capacitor C is charged to about 80% of its rated voltage, the inverter operates normally. The triggering signal of the thyristor is generated through the auxiliary winding of the main transformer, causing the thyristor to conduct and short-circuit the current limiting resistor R. The switching power supply is in normal operation.
