Auto-Tuning of PID Controller with Phase Margin Specification for Digital Voltage-Mode Buck Converter

This paper presents an application of an auto-tuning method based on the modified relay-feedback test (MRFT) to a digitally-controlled voltage-mode DC-DC buck converter. PID controller parameters are calculated using the proposed tuning rules that guarantee a specified phase margin. The inputs to the PID tuning rules are the measured frequency and amplitude of the oscillations produced by the MRFT. It is mathematically proven in the paper that the specified phase margin is guaranteed - provided certain given constraints on the controller parameters are satisfied. Another contribution of this paper is the implementation of the MRFT on a DC-DC buck converter. The transient performance of the auto-tuned PID controller is experimentally tested, and is compared to the response of an optimal but non-auto-tuned controller. The results show good performance of the proposed method. Finally, some practical considerations regarding the application of the MRFT-based auto-tuning to switching converters are discussed.