Design of solar lawn lamp

With the development of economy and the progress of society, people have put forward higher and higher requirements for energy. Finding new energy has become an urgent issue facing mankind. Since solar power generation has the incomparable cleanliness, safety, extensiveness and abundance of resources that thermal power, hydropower and nuclear power cannot match, solar energy is considered to be the most important energy source in the 21st century. The storage of solar energy is the key to the development of solar energy products. Currently, various batteries are mainly used. However, the long charging time, short life and environmental pollution of batteries have always been the bottleneck of the development of solar energy products. Supercapacitors are supercapacitors.

As a fast-charging, long-life, green and environmentally friendly energy storage element, it has brought new vitality to the development of solar energy products. This article introduces in detail the design and implementation method of a supercapacitor solar lawn lamp. This lawn lamp combines the advantages of solar energy and supercapacitors. It can actively emit light without installing other power supplies, and can automatically control the switch of the light according to the intensity of the ambient light. It is easy to install, does not require wiring, works stably and reliably, is maintenance-free, environmentally friendly and pollution-free, and has a long service life. It can be widely used in square green spaces, community lawns and other places.
1 Design selection
1.1 Selection of light source
Since LED technology has achieved a key breakthrough, the performance-price ratio has also been greatly improved. The current LED life can reach more than 100,000 hours, and the operating voltage is low, which is very suitable for use in solar lawn lamps. In addition, LED is powered by low-voltage DC, its light source control cost is low, it can adjust the brightness, and can be switched on and off frequently, and it will not have a negative impact on the performance of the LED. Therefore, considering the reliability, cost-effectiveness, color temperature and luminous efficiency, a super bright LED with a rated voltage of 3.3 V and an operating current of 6 mA can be selected as the light source during the design. Since the lawn lamp should not only have a decorative effect, but also have a certain lighting function, 8 LEDs can be selected for use.
1.2 Selection of solar cells
Solar cells are semiconductor devices that convert sunlight energy into electrical energy based on the photovoltaic effect principle of semiconductor PN junctions. They are the core components of supercapacitor solar lawn lamps. The performance of solar cells directly determines the energy conversion efficiency and the stability of the output voltage, and also directly determines the performance of supercapacitor solar lawn lamps. Therefore, monocrystalline silicon solar cells with better cost performance should be used in the design.
Since the total annual solar radiation and the average peak sunshine hours in various regions on the earth are different, the design of solar lawn lamps is related to the geographical location of the lamp. The relationship between the rated output power of the solar cell module and the input power of the lamp is about 2 to 4:1. The specific ratio depends on the daily working hours of the lamp and the lighting requirements for continuous rainy days. The power of the solar cell of this system is 3.3V×0.006×8=0.1584 W. Assuming that it works 12 hours a day, the efficiency of the solar cell is 40%, and the effective working time is 5 hours a day, a 3 W/6 V solar cell can be selected.
1.3 Selection of supercapacitors
Since the input energy of solar cells is extremely unstable, and lawn lights only light up when the surrounding light is weak, a power storage system must be configured to work effectively. At present, lead-acid batteries, Ni-Cd batteries or Ni-H batteries are commonly used, but there are many defects in using batteries as power storage systems: first, the number of charging times of rechargeable batteries is limited (less than 1,000 times) and the service life is short; second, due to the influence of its chemical structure, it cannot be charged with a large current; third, the battery needs to have control circuits such as overcharge prevention, over-discharge prevention and temperature compensation, and the control circuit is relatively complex; fourth, rechargeable batteries mainly use chemical reactions to charge and discharge, and the waste in the battery will pollute the environment, and it is not an environmentally friendly product.
Therefore, supercapacitors are selected as energy storage components in this product. This capacitor has a large capacitance of Farad level, strong charge retention ability, and very small leakage current, with a voltage drop rate of less than 5% in 8 hours; no special charging circuit and controlled discharge circuit are required, and it can be charged quickly, and can be charged at a state just higher than its leakage current (typical value is about 1 mA), so even on cloudy days, solar cells can charge supercapacitors; compared with batteries, overcharging and overdischarging do not have a negative impact on its life, high reliability and long service life (charge and discharge cycle life is more than 100,000 times); in addition, it has excellent temperature performance and can be used normally in an ambient temperature of -40℃ to 75℃; it is pollution-free and a green power source; it can be welded, and there is no problem of loose contact like batteries.
This solar lawn lamp product uses a 5.0V/225F capacitor module composed of 6 2.5V/150F supercapacitors produced by Jinzhou Kaimei Energy Co., Ltd. The ESR (DC) of a single capacitor product is only 20mΩ, with a diameter of 25mm and a height of 48mm. The charging time of a 5 V/225 F supercapacitor is (at a charging current of 450 mA):

Where: C-capacitor rated capacity; △U-capacitor operating voltage change; I-capacitor charging current; t-capacitor charging time.
The supercapacitor discharge time is:

Where: ESR is the DC internal resistance of the capacitor. 5 V/225 F supercapacitor can be discharged from 5 V to 0.6 V.
2 Design of system control circuit
2.1 Charging circuit
The charging circuit in this system consists of overvoltage protection and reverse charging protection circuits. Figure 1 shows its charging circuit. ZD1 in the figure is a 5.6V voltage regulator diode. When the battery voltage is higher than 5.6 V, Q2 is turned on, and all the current of the silicon battery is consumed through resistor Q2. When the silicon battery voltage drops below 5.6 V, Q2 is turned off, and the silicon battery charges the supercapacitor and protects the supercapacitor at the same time.

The anti-reverse charging control circuit can ensure that when the solar cell input voltage is lower than the supercapacitor voltage, the supercapacitor will not reverse charge the solar cell to avoid unnecessary energy loss. The reverse charging control can be completed by the diode D1 in Figure 1. This diode is a Schottky diode because the conduction voltage drop of the Schottky diode is lower than that of an ordinary diode.
2.2 LED driving voltage stabilization circuit
Since the rated voltage of the selected LED lamp is 3.3 V, and the voltage of the supercapacitor is 5.0 V, and the voltage of the supercapacitor will continue to decrease during the discharge process, a voltage stabilization circuit is required to drive the LED.
This article uses a VFM switching DC/DC boost regulator manufactured with CMOS technology and extremely low static current. The chip includes a VFM control circuit, an LX switch drive transistor, a reference voltage unit, an oscillator, an error comparison amplifier, a voltage sampling resistor, an LX switch protection circuit, etc. Only an inductor, an output capacitor and a Schottky diode are required outside the chip to provide a stable low-noise output voltage. The regulator has an extremely low input current of 8 μA, low ripple, low noise, an efficiency of 80%, an extremely low start-up voltage of 0.6 V, and an output voltage accuracy of ±2.5%. This part of the circuit is shown in Figure 2.

Among them, the inductor and diode outside the 331C will affect the conversion efficiency. In addition, the capacitor and inductor will also affect the output ripple. Therefore. When designing, you should choose the appropriate inductor, capacitor and Schottky diode to obtain higher conversion efficiency and lower ripple and noise. According to the data sheet of 331C, this design selects an inductor of 47μH and less than 0.5 Ω and a tantalum capacitor with low ESR of 47μF. The diode used for rectification has a great influence on the efficiency of DC-DC. Although ordinary diodes can also make the DC-DC circuit work normally, it will reduce the efficiency of the DC-DC converter by 5% to 10%. Therefore, D3 uses a Schottky diode lN5817 with a lower forward conduction voltage and a shorter reaction time.
2.3 Light control circuit
Supercapacitor solar lawn lights require a light control switch circuit to control the switch of the LED. So that the LED can be turned on when the ambient light becomes dark, and the LED can be turned off when the light becomes brighter. Some light control switch circuits use photoresistors to switch LEDs, and solar cells can also be directly used as light switches because the characteristics of solar cells are better than photoresistors. This part of the circuit is shown in Figure 3.

In Figure 3, U1 is a six-phase inverter 74HC14 with a Schmitt trigger, in which the multivibrator is composed of the second inverter, R5 and C1. In this way, when the light becomes brighter during the day, the voltage of the solar cell increases, the 1st pin of the 74HC14 is high, the 2nd pin of the 74HC14 is low, the 8th pin also outputs a low level, the transistor Q1 is cut off, and the LED is turned off. When the light becomes darker, when the voltage of the solar cell drops to a certain level, the 1st pin of the HC14 is low, the 2nd pin of the HC14 is high, the multivibrator starts to oscillate, and outputs high and low levels through Y4, Q2 is sometimes turned on and sometimes cut off, at this time, just adjust the values ​​of R6 and C1 to adjust the flash frequency to a level that the human eye cannot distinguish, and the biggest advantage of doing so is that it can extend the working time of the lawn lamp. According to actual tests, the circuit can work continuously for more than 10 hours a day, which can fully meet the user's needs.
3 Conclusion
The design of this supercapacitor solar lawn uses supercapacitors and integrated DC-DC chips, so the entire circuit design is simple, the reliability is high, and it is a green and environmentally friendly product. At present, this design has been successfully applied to actual products.