Abstarct: In this thesis, a fractional-order proportional-integral-derivative controller is designed to control the voltage of the pseudo-resonant step-down converter. The controller coefficients are determined by the gray wolf optimization algorithm, taking into account the criteria of settling time and overshoot. The advantages of this converter include low losses, high efficiency, small size and weight. Due to the fact that the quasi-resonant converter has uncertainty and also the presence of noise and changes in the system parameters over time cause instability in the output voltage, a fractional order controller with five degrees of freedom has been used. Fractional order controller is a robust controller that leads to optimal output control.