Due to increase consumption of non-renewable energy for example, petroleum and the urgency of improving the ecological environment, energy harvesting, and solar energy, has become a hot topic in the world. More products choose solar as the power resource. The typical system powered by solar cell includes solar panel, energy storage element, similar to supercap or NiMH battery and the DC/DC device for charging the energy storage element from the solar panel, and others DC/DC to regulate output voltage. The result is specifically designed to the system powered by solar energy (less than 5 W). The Buck CC/CV feature ensures that the energy storage similar to super-cap or NiMH battery can be charged well. This result can nearly realize MPPT (Maximum Power Point Tracking) by using bi-directional buck or boost feature in TPS61094. And TPS61094 integrates a 60-nA ultra-low Iq boost converter to regulate output voltage no matter that the solar energy is strong or weak.
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A solar cell is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. The common single junction silicon solar cell can produce a maximum open-circuit voltage of approximately 0.5 V to 0.6 V. Individual solar cell devices are often the electrical building blocks of photovoltaic modules. The most commonly known solar cell is configured as a large-area p-n junction made from silicon.
Open circuit voltage and short circuit current are the most important parameters of solar panels. In general, its operating voltage and current vary with the load resistance (Energy Harvesting From Single Cell Solar Panel for Li-Ion Battery Reference Design). Figure 1-1 shows the operating current and voltage corresponding to different loads, which is the VI/VQ curve of the solar cell.
Under different lighting conditions, the same solar panel has different maximum output power according to different loads. It’s important to make the solar panel work at the maximum power point through adjustment. Maximum power point tracking is a technique used with variable power sources to maximize energy extraction as conditions vary. It maximizes energy extraction when conditions change. The central problem addressed by MPPT is that the efficiency of power transfer from the solar cell depends on the amount of available sunlight, solar panel temperature and the load’ s electrical characteristics. As these conditions vary, the load characteristic that gives the highest power transfer changes. The system is optimized when the load characteristic changes to keep power transfer at highest efficiency. This optimal load characteristic is called the maximum power point (MPP). MPPT is the process of adjusting the load characteristic as the conditions change. Circuits can be designed to present optimal loads to the photovoltaic cells and then convert the voltage, current, or frequency to suit other devices or systems.
There are many ways to realize MPPT, such as perturb and observe, incremental conductance, current sweep, temperature method and constant voltage (A survey of maximum PPT techniques of PV systems). Different ways have their own advantages and disadvantages, and have different requirements for hardware and algorithms.
And the constant voltage way is one of the earlier strategies. It takes advantage of the fact that the maximum power point of the solar cell is almost on the same vertical line under the condition of constant temperature. A controller is used between the solar panel and the load to make the output voltage constant to realize simple MPPT function. It is suitable for applications with stable external environment (Simulation and Hardware Implementation of New Maximum Power Point Tracking Technique for Partially Shaded PV System Using Hybrid DEPSO Method). This application note is also using constant voltage to realize MPPT in system.