Unified Discrete-Time Large/Small-Signal Modeling for Stability and Performance Analysis of Digitally Controlled Boost PPC under CCM, CrM and DCM

Digital control is predominantly used in commercial PFC products, because of using real-time controller reconfiguration, algorithms for loss optimization, EMI reduction and protection. Fixed frequency average current mode digital control and variable frequency constant on-time digital techniques are often used while operating in continuous conduction mode (CCM), critical conduction mode (CrM) and discontinuous conduction mode (DCM). The primary challenge is to ensure fast-scale stability under large duty ratio variations and finite sampling delay while driving various linear and nonlinear loads, such as constant resistive, constant current, constant power loads, etc. The simplified discrete-time (DT) models are not accurate enough for closed-loop stability analysis, and cannot be readily extended to various digital control methods. A generalized DT large-signal model is developed in this paper, and the model accuracy is verified using simulation results for all three CCM, CrM and DCM configurations. Thereafter, DT small-signal model is developed for fast-scale stability analysis of a closed-loop digitally controlled PFC converter under constant on-time digital current mode control with event-based sampling. The experimental result of 300 W CrM boost PFC is presented to verify the accuracy of the analytical predictions using the proposed framework under the CrM configuration.