A solar charging circuit based on LP3947 is designed, and the charging of lithium battery is intelligently controlled by pulse width modulation, so as to improve the output power of solar battery and the use efficiency of lithium battery, and achieve the purpose of prolonging the service life and time of battery.
Keywords: solar energy; LP3947 lithium battery
1. Introduction
As a new resource, solar energy has attracted more and more attention, and a series of industries brought by it have gradually become industries with great development potential. Solar photovoltaic power generation is one of the main industries of solar energy application. In China, solar energy resources are extremely rich, and the annual solar radiation energy received by the land is quite amazing. If we make full use of these solar energy, we can not only save a lot of conventional energy, but also effectively reduce the environmental pollution caused by conventional energy.
At present, the application of photovoltaic power generation in small electrical circuits has gradually matured. With people's life becoming more and more inseparable from a series of digital products such as mobile phones, mp3 players and digital cameras, the charging problem has also become one of the issues that users are extremely concerned about. Designing a charger that uses photovoltaic charging principle to charge these digital products can solve various problems in many aspects. Solar charger is portable, beautiful and fashionable, and can even charge a series of digital products such as mobile phones without power supply.
2. Types and working principles of solar panels
Solar cells are devices that directly convert light energy into electric energy through photoelectric effect or photochemical effect. At present, the mainstream is the solar cell working on the principle of photoelectric effect, and its basic raw material is semiconductor. When the P-N junction is illuminated, the intrinsic absorption and extrinsic absorption of photons by the sample will produce photogenerated carriers, which will cause photovoltaic effect and produce a photogenerated electric field in the opposite direction to the built-in electric field of the P-N junction. Its direction is from p area to n area. This electric field reduces the potential barrier, that is, the photogenerated potential difference, P is positive and N is negative, so the junction current generated by it flows from the P region to the N region, forming unidirectional conduction, which plays the same role as the battery.
Because the output voltage of the solar panel is unstable, a voltage stabilizing circuit is added, and the load battery can be charged through the voltage stabilizing circuit and the charging circuit, and at the same time, the internal battery can be charged for emergencies; When the lighting condition is poor, the output voltage of the solar panel is low, which can't reach the working voltage of the charging circuit. Therefore, the booster and regulator circuits are added to provide stable working voltage for the charging circuit. In the case of extremely poor lighting conditions such as cloudy night, the battery inside the system can charge the subsequent equipment through the booster circuit. In addition, the charger also designed a lighting lamp. At night, when the light is dim, the battery supplies power to the lighting for emergency use.
3. The charger design
3. 1 battery charging principle
If overcharge, overdischarge or overcurrent occurs during charging and discharging, it will damage the battery or reduce its service life. Fig. 3 is the charging curve of lithium battery, which is divided into three stages: pre-charging state, constant current charging and constant voltage charging. Take a battery with a capacity of 800 mAh as an example, its terminal charging voltage is 4.2V, and the battery of110c (about 80 mA) is used for constant current precharge. When the terminal voltage of the battery reaches the low voltage threshold V(min), it is charged at a constant current of 800 mA (charging rate is 10C). At first, the battery voltage rises with a large slope. When the battery voltage is close to 4.2V, it is changed to 4.2 V constant voltage charging, and the current gradually decreases.
Charging curve of mobile phone battery
3.2 charger design concept
The design idea of solar cell phone charging control circuit is based on the constant current/constant voltage charging control of mobile phone lithium ion secondary battery, with lithium ion battery. In rainy weather or when the sun is insufficient at night, the mobile phone is charged with a lithium-ion battery to ensure uninterrupted operation under any circumstances. That is to say, the design of the system is mainly based on solar energy charging. Under the condition of sufficient sunshine and sufficient power supply capacity of the battery, the system can mainly use solar energy to charge the mobile phone, and the battery can supplement the battery; When there is no sunshine or the sunshine is weak, the mobile phone is mainly charged with batteries and charged with solar energy.
3.3 Design of Charging Control Circuit
3.3. 1 Boost Circuit Design
Due to the different intensity of sunlight at different times and places, the output power of solar panels is unstable, so it is necessary to increase the corresponding control links such as boosting and stabilizing voltage. DC boost is to boost the lower DC voltage provided by the battery to the required voltage value.
3.3.2 Design of voltage stabilizing circuit
The design of the voltage stabilizing circuit is based on the three-terminal integrated regulator W7800, which belongs to the series voltage stabilizing circuit, and its working principle is the same as that of the series voltage stabilizing power supply with discrete components. It consists of starting circuit, sampling circuit, comparison and amplification circuit, reference link, adjustment link and overcurrent protection link. In addition, there are overheat and overvoltage protection circuits, so its voltage stabilizing performance is better than that of the series voltage stabilizing circuit of discrete components. Moreover, after the regulated power supply is started, the starting circuit of the three-terminal integrated regulator is in a normal state, and the starting circuit is not in contact with other circuits inside the regulated power supply, so the input voltage change will not directly affect the reference circuit and the constant current source circuit, and the output voltage remains stable.
3.3.3 Design of Charging Circuit
Lithium batteries are widely used in many digital products because of their small size, large capacity, light weight, no memory effect, no pollution and many battery cycles (long life). However, lithium batteries have strict requirements on the use conditions, such as high charging control accuracy.
Poor overcharge endurance. Therefore, in order to protect the lithium battery, the charging circuit includes two parts: the battery charging control circuit and the battery charging detection control circuit. The battery charging control circuit is used to control the boosting or voltage stabilizing circuit to charge the file battery, which is also a lithium battery.
The charging circuit of. The battery charging detection circuit is used to detect the charging amount. When the battery is fully charged, the full indicator lights up, and the logic circuit controls the charging circuit to disconnect and stop charging.
4 conclusion
With the development of modern science and technology, electronic products can be almost popularized, but the batteries of electronic products have been bothering us. The purpose of my research is not to increase the capacity of battery, but to install solar charger on the surface of electronic products, which can greatly increase the service life of battery.