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A Pilot Project: a digitally controlled battery charger for photovoltaic applicationsμ-Lab was applied to the design and verification of the control code for a photovoltaic (PV) battery charger. The target application is a low cost digital supervising system for stand alone photovoltaic generators, presenting the following features: i) automatic adjustment of the PV generator operating point, so as to achieve the maximum power point tracking (MPPT); ii) automatic battery charge current limitation and load disconnection, as needed to prevent battery overcharge or excessive battery discharge; iii) three stage battery charge strategy (trickle, bulk and constant voltage operation), to maximize the battery lifetime; iv) temperature sensing and voltage threshold adjustment. The control code is run by a low cost 8 bit microcontroller unit (MCU). The other fundamental components of the system are: a 35 W peak photovoltaic module, with a maximum power point voltage of about 17 V and a maximum power point current of about 2 A (in standard conditions, i.e. with a 1000 W/m2 irradiation and 25 °C operating temperature) and a highly efficiency (90% nominal) battery charger circuit, implemented by a 40 W peak, step down switching converter. μ-Lab allows for the simulation of the above system as a whole. Differently from the usual procedure, where the digital and analog components of the system are developed and tested separately, a complete model of the system is developed here. The model includes both the analog and continuous time components, i.e. the photovoltaic generator, the battery charger and the battery itself, and a realistic model of the digital controller, capable of replicating the real time operation of the MCU. Thanks to the availability of SPICE libraries, the analog part of the system can be simulated with very good detail. At the same time, a newly developed software, interfaces the SPICE model with the MCU real time simulator, taking care of all the interactions required by the application. It is worth noting that the time scale of the different modelled processes is extremely variable, ranging from the microsecond scale, typical for MCU operation, to the several hour scale, typical for a battery charge process. Nevertheless, an efficient simulation is implemented by μ-Lab. As a result, the designer is allowed to test the final application code in all its interactions with a detailed model of the controlled physical system. This way, the code development and verification are greatly enhanced and can be completed even before a prototype of the system is available. The project was developed by S.Buso, F.Marcuzzi and G. Moro from the University of Padova. A detailed description of the application is available on request. |
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