A digital control method combining primary-side sensing, observer and model-predictive-control techniques is proposed. A conventional isolated Flyback converter is chosen for demonstrating the method. The only measured signal is the drain-source voltage over the switch. Following a procedure of signal processing, state estimation and constraint problem formulation, the controller determines the optimal duty cycle ratio. The advantages of the proposed method include minimal overshoot and fast stabilization, converter state restriction, and measurement network simplification.