Analysis and design of a PV-fed battery charger with soft-switching tracking and MPPT control

The flourishing development and the increasing interest in renewable energy sources need to look for new approaches to convert and distribute power in a highly efficient way. In line with this objective, this work presents the analysis, design, and PCB implementation of a novel topology of resonant low-voltage battery charger fed by a 48-cell photovoltaic (PV) module. The work aims to go beyond the existing research about PV-fed low-voltage battery chargers, by proposing a new solution from both the topological and control points of view. The DC-DC power converter designed in this work consists in a single-stage, frequency-modulated battery charger that exploits both topological and control-based solutions to achieve Zero-Voltage Switching (ZVS) and Zero-Current Switching (ZCS) of the converter MOSFETs. An ad-hoc control algorithm is designed to real-time track the soft-switching conditions in almost any irradiance and battery voltage operation. The control is based on the acquisition of a single voltage at every switching cycle to update the timing scheme of the MOSFETs gate drivers. In parallel, the implemented frequency modulation technique allows to track the absolute maximum power point (MPP) of the PV module under various uniform-irradiance conditions, thanks to a Perturb & Observe approach. The topology improves the conversion efficiency of an already existing constant-power battery charger, in addition including the novel soft switching-tracking control and MPPT, absent in the original converter. With the goal of comparing different solutions to achieve the highest efficiency, two topologies are considered, differing in the inductance value of the resonant inductor (exploited to assist the soft switching) and in the nature of the output filter (inductive and capacitive). In a first phase, the two topologies were analysed in Matlab and PLECS environments to derive the ideal voltage/current waveforms within a switching cycle, and mathematical models were developed to describe the static characteristics at stationary operation. In a second phase, the topologies were simulated in LTSpice and PSIM with a preliminary components selection, to derive the overall conversion efficiency and the losses contributions from each component, with the aim to tune the definitive bill of materials. The circuits were tested in standardized conditions specified in IEC 62509: 2011 (efficiency tests for lead-acid batteries charge controllers) and EN 50530:2010 (standardized tests for conversion and MPPT efficiency for low-voltage grid microinverters, here applied to a battery charger). The simulations showed that the analysed topologies exhibit EURO efficiencies above 97%, and peak charging efficiencies around 98%. Since the conduction losses become more impactful at large operating powers for the larger resonant inductance topology, the other battery charger was selected to realize a 100W PCB prototype. This work proposed the design procedure for the components selection and illustrates the PCB design in Altium environment. The PCB prototype is intended to be tested in laboratory to validate the expected performances. The experimental results are meant to be put in relation to the simulations and with the performances of existing topologies analysed in the literature review.