Design of a photovoltaic charger for LED street lights

A new design scheme of intelligent high-power Light Emitting Diode (LED) street lamp photovoltaic charger is proposed. The working characteristics of white light LED and solar cell as well as the main circuit topology of the photovoltaic charger are given. The control strategy and maximum power point tracking (MPPT) principle based on Microchip's PIC16F874 chip are analyzed. Finally, the working principle block diagram and control principle block diagram of the charger are given. The actual operation shows that the LED street lamp photovoltaic charger system has significant advantages.

1 System Configuration

1.1 LED operating characteristics

The working principle of light emitting diode (LED) is to apply a forward voltage to the semiconductor pn junction, so that its electrons and holes recombine (that is, the junction area becomes narrower). This recombination is that the electrons release energy from the high-energy level conduction band and return to the valence band to recombine with holes. The released energy appears in the form of photons, that is, light is emitted.

According to the formula in semiconductor physics: λ=1.24/Eg, where: Eg is the bandgap width between the conduction band and the valence band of the semiconductor material, and λ is the wavelength. It can be seen from the formula that for semiconductors of different materials, since their Eg is different, their wavelengths are also different, so the colors of the light are different. Obviously, most LEDs are single-color lights, such as red light, green light, yellow light, blue light, etc. The so-called white light is a mixture of multiple colors of light. In the form of white light that the human eye can see, at least two or more lights must be mixed. Generally, there are the following two mixing methods: two-wavelength light—a mixture of blue light and yellow light; three-wavelength light—a mixture of red light, green light and blue light. At present, most of the commercialized white light LED products are two-band blue light single crystals plus YAG yellow phosphor; three-wavelength light is an inorganic ultraviolet light crystal plus R, G, B three-color phosphors. In addition, organic single-layer three-wavelength white light LEDs also have the advantages of low cost and easy production.

1.2 Working characteristics of solar cells

Figures 1 and 2 show the relationship curves between the working voltage, current and sunshine (W/m2) when the temperature of the solar cell is 25°C, and the curves between the output power of the solar cell and sunshine, voltage.

From the I/U relationship in Figure 1, it can be seen that the solar cell array is neither a constant voltage source nor a constant current source, but a nonlinear DC power supply. The battery output current is quite constant in most of the working voltage range, and finally after a sufficiently high voltage, the current drops rapidly to zero. As shown in Figure 2, the working efficiency of the solar cell is equal to the ratio of the output power to the power projected on the solar cell area. Therefore, in order to improve the working efficiency of this system, the solar cell must be operated at the maximum power point as much as possible, so that the maximum power output can be obtained with the solar cell with the smallest power possible. This is the significance of maximum power point tracking. As shown in Figures 1 and 2, points A, B, C, D, and E in the figure correspond to the maximum power points at different sunshine times.

1.3 Working characteristics of lead-acid batteries

At present, lead-acid batteries are widely used in photovoltaic charger systems. Its working principle is to rely on the active material lead dioxide (PbO2) of the lead-acid positive electrode and the active material sponge lead (Pb) of the negative electrode to react with the electrolyte sulfuric acid (H2SO4) to generate lead sulfate (PbSO4). During this working process, sulfuric acid (H2SO4) will be reduced, and water (H2O) will be continuously generated on the positive plate, thereby causing the density of the electrolyte to decrease. During charging, the lead sulfate (PbSO4) on the positive plate is oxidized to lead dioxide (PbO2), and the lead sulfate (PbSO4) on the negative plate is reduced to lead (Pb), and sulfuric acid (H2SO4) is generated at the same time, which consumes the water (H2O) in the battery, increases the density of the electrolyte in the battery, and completes the charging process.

2 How the system works

2.1 Introduction to the system’s main control chip

The hardware block diagram of the charger system is shown in Figure 3.

The main control chip uses Microchip's PIC16F874, which uses a RISC instruction system, Harvard bus structure, low power consumption, and high speed. It integrates modules such as ADC, SPI, and Flash program memory, and has functions such as 10-bit A/D conversion, PWM output, and LCD drive. In addition, it also has a 128-byte E2PROM memory, which can easily write adjustment values ​​for later use. PIC16F874 can achieve seamless connection with the CAN controller MCP2510 through the SPI interface, and the synchronous serial module (SSP) lays the foundation for networking with industrial computers in the future. PIC16F874 has rich I/O resources, with a total of five I/O ports A, B, C, D, and E. In addition to basic uses, each I/O port has some special functions. Rich resources and powerful functions make it very suitable as the control core chip of the control system.

2.2 Analysis of the system’s working process

The control block diagram of the charger system is shown in Figure 4.

As shown in Figure 4, during the battery charging stage, the control circuit voltage loop is only composed of the solar cell voltage. At this time, the output of the voltage loop is the given value of the current loop. By comparing the charging current of the battery in the main circuit with the given current, the output pulse width of SG3525 is changed to make the voltage of the solar cell track the given voltage. As shown in Figure 1, when the solar cell voltage drops, in steady state, the solar cell voltage is equal to the given voltage, the given value of the current loop is also a stable value, and the charging current of the battery is equal to the given current; on the contrary, when the solar cell voltage is less than the given voltage, the output pulse width of SG3525 acts on the drive circuit to drive the power device, so that its conduction duty cycle is reduced, the battery charging current becomes smaller, the working voltage increases, and the solar cell voltage is equal to the given voltage when the circuit reaches steady state. In the overcharge stage, both circuits are in effect. The voltage loop is composed of the circuit formed by the solar cell voltage and the circuit formed by the battery. At this time, the sum of the battery voltage and the given solar cell working voltage is greater than the solar cell voltage. The deviation signal is added to the current input terminal of SG3525 after PI adjustment, which reduces the output pulse width of SG3525 and the battery charging current. As shown in Figure 1, the actual working voltage of the solar cell gradually increases until it works in an open circuit state in a steady state, and the battery charging current is zero, thereby achieving overcharge protection.

In addition, the Modbus communication standard can be used to enable the module controller to communicate in a master (i.e., upper computer)-slave (i.e., lower computer) manner. The operation of the photovoltaic charger and the operation of the LED lamp can be monitored through several controllers or other Modbus devices through the RS485 bus to form a Modbus network, which can successfully realize a networked remote monitoring system.

3 Maximum Power Point Tracking (MPPT) of Solar Cells

For the photovoltaic charging system, the system first uses the solar cell array to charge the battery, and stores the solar energy in the battery in the form of chemical energy. In this process, since the volt-ampere characteristics of the photovoltaic array are strongly nonlinear, the self-optimal control method is usually used in the control strategy to make the solar cell work at the maximum power point. The entire control process can be decomposed into two stages:

(1) Determine the output voltage value Uref when the solar cell operates at the maximum power point;

(2) Change the charging current of the solar cell to the battery so that the output voltage of the solar cell is stable at Uref.

These two stages are realized by the control circuit through detecting the output voltage and current of the solar cell and using the successive comparison method. Its search starting point should be close to the array open circuit voltage. When it is at the steady-state working point, the working voltage value of the photovoltaic array should be searched near the maximum power voltage value. The smaller the search amplitude, the higher the accuracy of the MPPT. Its maximum power point tracking control process is shown in Figure 5. It ensures that the system always makes the solar photovoltaic array work at the maximum power point regardless of the sunshine and temperature conditions, so that the charger system can obtain higher stability and output efficiency when working.

4 Conclusion

The actual operation shows that the LED street light photovoltaic charger system has the following significant advantages:

(1) LED solid light source is shock-resistant, impact-resistant, high in light efficiency, long in life and pollution-free;

(2) The circuit has the characteristics of simple structure, stable and reliable operation, high cost performance and strong real-time performance;

(3) The use of a double closed-loop control strategy improves the working state of the battery and ensures the charging quality;

(4) Realize the automatic charging process of the LED street light photovoltaic charger, thereby increasing the service life of the battery;

(5) Use of universal batteries facilitates the promotion and application of existing ordinary light source street lamps;

(6) It utilizes inexhaustible and pollution-free solar energy, eliminating the need to dig pits and lay cables, thus embodying the green energy and environmentally friendly utilization.