Design and implementation of a sepic battery charger for automotive pv applications

The fossil fuels such as coal and gas pollute the air, water and nature. According to World Wildlife Fund (WWF), generating electricity from fossil fuels causes serious acid rain and damage in forest areas. Demand for electricity is growing day by day. At the same time, the stock of conventional sources such as petroleum, natural gas, coal etc. is limited, and decreasing. Moreover, there is an excessive dependence on imports of conventional sources, because the number of countries which have considerable supplies for fossil fuel are also limited. In addition, there is a continuous increase in their prices. Hence electricity crises have been already started all over the world. Humans are searching for a new energy source because of the undisputed meaning of energy for the continuity of life. Solar energy is a good alternative to fossil fuels because of being renewable, safe to use, and less pollutant. According to National Renewable Energy Laboratory in the United States of America, the annual energy needed by all the people worldwide can be met by the sunlight received by the earth in one hour. As a consequence, governments, businesses, researches, scientists, and communities have been started giving more importance of utilizing renewable energy resources especially solar energy despite the cost of photovoltaic (PV) systems. In fact solar technology is being improved day by day which leads to a decrease in total system cost. A study conducted in 2011 in the United States of America shows that the cost of a PV system redeems itself even in 20 years. Due to being a solid state device, PV cells are robust, easy to design, and require very little maintenance. Furthermore, the size and generation capacity of a solar PV system is a function of number of PV modules. Hence, the applications of solar technology can be easily scaled from microwatts to megawatts. PV systems are mobile, and are able to be used in remote areas. Additionally PV systems have a better life-cost correlation, if the amount of sunlight is enough. On-roof-tops of individual buildings is one of the common, and very first application example of PV systems. On-roof-tops are used to supply power to cool or heat the building, and to provide lighting which decrease fossil fuel dependency significantly. Over time though, the technology of PV systems have been improved, and huge solar PV systems have been beginning to be built on the wastelands. PV cell generates direct current (DC) from solar energy by means of photovoltaics. It can be converted to alternative current (AC) with an inverter to be able to use it at home, or it can be directly stored in a battery to be able to use it when needed. The output characteristics of a PV system depends on the atmospheric conditions such as intensity of solar irradiance, and temperature, due to the nature of PV cell. That is why, the efficiency of PV cell is low, when it is directly connected to the load. PV module utilization is improved by operating PV module around its maximum power point (MPP), where it works with the maximum efficiency. With the objective to operate a PV cell, or PV module around its MPP, maximum power point tracking (MPPT), i.e. maximum power point tracker is used. MPPT is a DC/DC converter which extracts maximum power from PV module under any environmental condition, and transfers it to the load efficiently by providing impedance matching between PV module and load. One of the common usages of PV modules in automotive is solar-powered recreational vehicles (RVs) which are mostly used in boondocks for camping. Solar-powered RVs have at least one 12V leisure battery, which is mostly lead-acid due to being cheap, reliable, well-understood, and easy to find. 12V leisure battery is used for 12V equipment and appliances such as a few lights, tablet, phone charger, laptop etc. which is charged from PV module during camping. The number of leisure batteries depends on the amount of power generated from PV module. PV modules can be portable or mounted on the top of the vehicle such as van, trailer etc. A DC/DC charger is used to take the power generated by PV modules and charge the battery in the most efficient way by using this power. According to the researches, a PV system setup as a backup power source for an RV can amortize itself in 2-4 years. For this thesis, a single ended primary inductance converter (SEPIC) battery charger is designed, implemented, and tested for automotive PV applications, which charges a 12V battery with maximum 8A charging current at 14V for an input voltage range, i.e. a PV module output voltage range from 12V to 25V, and with 85% efficiency. The practical application of this thesis is charging a 12V/80Ah leisure battery of a solar-powered RV, which has a 180W PV module, with maximum 8A charge current at 14V output voltage. In order to achieve thesis's purpose, in the beginning a comprehensive literature review on MMPT in PV applications, DC/DC converter topologies used as MPPT, SEPIC converter design, and SEPIC based battery charger design for PV systems is done. The literature review points out that, an intermediate DC/DC converter is needed to increase the efficiency of PV system, and SEPIC is the most promising topology for target application of this thesis. That is why, SEPIC topology is selected for charger design. Afterwards, theoretical background of SEPIC topology is deeply studied. According to design considerations, and by using theoretical equations, all necessary parameters of components are calculated, switching frequency is defined, and hence a basic SEPIC charger circuit design is completed. A simulation setup is prepared with ideal components, and simulations are performed with LTspice by using an ideal voltage source as an input supply instead of PV module model and a resistor as an output load instead of battery model. The simulation results are interpreted, and compared with theoretical calculations. After verifying design with ideal components, proper off-the-shelf components and controller of charger are selected. Their spice models are gathered from manufacturers if available, else proper models are developed in order to prepare a simulation setup in LTspice. The SEPIC design with real component models i.e. non-ideal component models are performed by again using an ideal voltage source as an input supply instead of PV module model and a resistor as an output load instead of battery model. The simulation results are interpreted, and compared with simulation results of the circuit with ideal components and theoretical calculations. After verifying design with non-ideal components, a 180W PV module model is developed and used instead of ideal voltage source as an input supply and a 12V battery model is used instead of a resistor load as output load in the simulation circuit with non-ideal components with the intention of performing system simulations in LTspice. The simulation results are interpreted, and compared with other simulation results, and theoretical calculations. After the correctness of theoretical calculations and selected components are confirmed by performing simulations in LTspice, SEPIC charger implementation is started. By using Altium Designer 18, schematic and printed circuit board (PCB) of SEPIC charger are designed, after completing the creation of libraries which include all necessary components' symbols, and footprints. Afterwards, all components included in the bill of material report of design are ordered through distributors, and PCB is manufactured. When bare PCB arrives, components are soldered onto PCB. Hence prototype of designed SEPIC charger is obtained, and made ready for testing. All necessary tests are performed in two different test environments, which are laboratory and field tests, in order to validate SEPIC charger design. In the beginning, tests are performed in the laboratory by using a power supply as an input source, and a resistor block as a load. The tests are performed in two main phases in the laboratory. In both phases, a power supply is used as an input source. In the first phase, a resistor block is used instead of battery as a load. In the second phase, a 9Ah battery is used as a load. Eventually, field tests are performed with a 130W PV module used as an input source and a 12V/12Ah battery used as a load. As a consequence, it is achieved to design, and implement a SEPIC charger as intended which provides maximum 8.32A output current at 14.14V output voltage for an input voltage range from 12V to 25V. Implemented SEPIC charger has an efficiency minimum 87.6% at 12V input voltage and maximum 88.6% at 25V input voltage. Additionally in field tests, designed and implemented SEPIC charger charges battery by getting power from PV module properly.