Factors affecting validity of PVG-power settling time estimation in designing MPP-tracking perturbation frequency

Abstract

An open-loop and closed-loop operating boost-power-stage converter with relatively low damping factor exhibit resonant behavior in transient conditions. Such an undamped transient characteristic introduces overshoot to the control-to-output-variable transfer function, which is also visible in the inductor current transient behavior. Therefore, due to the either too large duty ratio or voltage-reference step change, the inductor current can move from continuous conduction mode to discontinuous conduction mode. That transforms the second-order system into an equivalent first-order dynamic system extending the PV-power settling time significantly and reducing power tracking performance of the system. This paper introduces design guidelines to determine maximum perturbation step size for duty ratio and input-voltage reference under open-loop and closed-loop operation, respectively. Two different closed-loop design examples are considered in this paper, based on the application of pure integral controller with phase margin (PM) close to 90 degrees and proportional-integral-derivative controller with PM close to 40 degrees, respectively. The closed-loop system dynamics is known to be characterized by the dominating poles and zeros, which locate closest to the origin. This means that the closed-loop system can be usually characterized by the well-known second-order transfer function. Therefore, the minimum and maximum overshoot of the inductor current can be well approximated as demonstrated by deterministic analysis and experimental results.