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A simple auto-tuning technique, based on the relay feedback, for voltage-mode synchronous half bridge isolated POL converter is here reported. By means of the relay control, small oscillations on the output voltage are generated during converter soft-start. Based on the measurements of the frequency and amplitude of the oscillations, the gain and phase of the converter transfer function at the oscillation frequencies are derived. Instead of estimating the converter transfer function at different frequencies, the paper proposes an innovative iterative procedure, where the tuning of PID singularities is obtained directly by including the controller in the relay feedback. Conversely, compensator gain tuning is performed by means of a technique based on crossover frequency analysis, in order to obtain a specified closed loop bandwidth. The proposed algorithm is very simple, requires small tuning times and it is fully compliant with the cost/complexity constraint of integrated digital ICs. Even if aimed to an integrated digital controller, experimental investigation has been performed using discrete components, implementing the digital control in a Field Programmable Gate Array (FPGA).