Dynamic Modeling and Analysis of Buck Converter Based Solar PV Charge Controller for Improved MPPT Performance

Maximum power point tracking (MPPT) control is a key functionality in solar photovoltaic (PV)-based power conversion systems. A variety of perturbative MPPT control schemes are available in the literature, many of which are voltage-based techniques wherein the PV bus voltage is perturbed and set to the required level by an appropriate converter control that achieves the MPP tracking. However, a comprehensive plant model of the PV-fed converter system and a systematic control design of the PV bus voltage loop that facilitates the design of the MPPT control is not available in the literature. In this article, a detailed small-signal model is proposed for a single-stage PV-fed buck converter that acts as a battery charge-controller. The effects of parasitic storage elements present in the PV source as well as the interconnecting cable from rooftop are discussed in detail, along with the impact of series-parallel connection of the PV modules for power scaling. A full-order as well as a reduced-order model of the system is proposed and the various relevant transfer-functions are analytically derived. Based on this model and the control to converter input bus voltage transfer-function ṽ
_in
/d̃, a systematic design procedure for the PV bus voltage controller is proposed. It is shown that such a design facilitates the selection of perturbation period of a typical MPPT control algorithm for an improved tracking performance. The steps involved in the system model development and the control design are generalized and can be extended to other converter topologies as well. Active transfer-function measurements and experiments conducted on a 1-kW charge-controller hardware prototype validate the accuracy of the proposed model and the tracking performance of the MPPT control.