Basic topology of BUCK circuit

The BUCK circuit achieves stable output voltage regulation by adjusting the duty cycle using a PWM controller through the cycle of switch on (inductor energy storage), switch off (inductor freewheeling) and free-wheel oscillation.

S1 closed

When the switch tube S1 is closed, the inductor L is magnetized and stores energy, and the current flowing through the inductor increases linearly. At the same time, the capacitor C is charged and energy is provided to the load RL. At this time, the Vout voltage rises slowly. If S1 is closed all the time, Vout will eventually be approximately equal to the Vin voltage (S1 has a loss voltage drop);

S1 disconnected

When the switch tube S1 is turned off, the energy storage inductor L discharges through the freewheeling diode D, the inductor current IL decreases linearly, and the output voltage Vout is maintained to slowly decrease by the discharge Ic of the output capacitor C and the reduced inductor current IL. If S1 remains turned off, Vout will eventually drop to 0V;

according to the above description, the Buck output voltage Vout curve is approximately as shown in Figure 1 below:

It can be seen that by controlling the duty cycle of the switch tube S1, the output voltage Vout can be controlled.

Generally, the DCDC integrated chip samples the output voltage to the feedback terminal through the voltage divider resistor, and generates a PWM signal to drive the power tube after comparing it with the reference voltage through the operational amplifier to achieve voltage closed-loop control.