In recent years, small devices that use battery power have developed rapidly, such as tablet computers, handheld game consoles, video players, digital photo frames, and so on. In general, these devices use rechargeable lithium-ion (Li-Ion) batteries as a power source. Some common charging solutions include wall adaptor class chargers and Universal Serial Bus (USB) class chargers. Although these charger solutions are a low-cost solution for charging lithium-ion batteries, these chargers also have a common disadvantage: they rely on the main power supply to operate. This dependence on the main power source increases the user's electricity bill and also increases greenhouse gas emissions. And because of their dependence on the main power supply, the portability of these charging solutions is also greatly reduced. To extend battery life in an environmentally friendly way, solar chargers that use solar panels to collect natural light energy may be an ideal solution. Another benefit of a solar charger is that it provides a mobile charging solution.
    In this article, we will explain some important considerations when developing a solar charging solution. The main reason to consider these factors is that with the different lighting environment, the voltage and current also change, then the solar panel will become a high output impedance power supply. The wall power adapter or USB power supply is a low output impedance power supply with a pre-specified output voltage and current. The factors that need to be focused on in the solar charging solution we will discuss include: maximum power point tracking (MPPT), reverse leakage protection, charge termination method tips, and solar panel crash protection.

Maximum power point tracking
    The maximum power point (MPP) is the working area of the solar cell where the maximum power can be obtained. The graph in Figure 1 shows this region. The graph shows the comparison of typical output current and output power with the voltage curve of the MPP two-section solar panel. The MPP on the curve is obvious because it is the voltage and current corresponding to the maximum power output of the solar panel. MPP is related to ambient temperature and light, so it changes over time. This shows that chargers using solar power must have the corresponding circuit to keep track of MPP as environmental conditions change. There are a wide range of MPPT solutions, including simple open-loop technologies (where the panel voltage is maintained at a fixed open-circuit voltage) and complex microcontroller-like technologies (where the input and output power is measured and then the panel voltage is properly regulated).