This DC power supply controller is controlled by pulse width modulation PWM, generated by the circuit around timer IC2 7555 according to the formula:
Duty cycle = ((R1 + (VR1/2)/(R1+R2+VR1)) x 100%
Diode D1 is used to bypass resistor R2 and half the effective resistance of the “lower” section of VR1, allowing the duty cycle to be varied from 0.01% to 99.9%.
Read the rest of this DC power controller circuit.