Frequency Domain Design Techniques in Digitally Voltage and Current Mode Controlled DC-DC Converters with Fast Transient Performance

Digital voltage mode control (DVMC) and current mode control (DCMC) techniques find increasing adoption in mainstream power electronics products; however, it still remains a challenge in identifying suitable small-signal based design methods in shaping load and reference transient performance. This paper considers both continuous-time (CT) and discrete-time (DT) small-signal models (SSMs) in a synchronous buck converter. Using both the modeling approaches, (i) stability analysis and controller design are carried out; their (ii) advantages and shortcomings are highlighted using design case studies; (iii) a combined CT and DT design approach is identified to retain design simplicity and closed-loop stability in achieving fast transient performance. Taking the benefits of both the CT and DT SSMs models, controller design algorithms are developed for both DVMC and DCMC. Further, a new digital PID controller design method is developed to achieve load-insensitive closed-loop output impedance, which results in superior load transient performance compared to a traditional loop-shaping approach. Both DVMC and DCMC case studies are considered, and analytical predictions are verified using simulation results. Finally, a buck converter prototype is made, and experimental results are presented to demonstrate a stable design with fast load transient and reference transient performance.