Solar LED street light solution with STM32 MCU

With the declining fossil energy sources and the excessive emissions of greenhouse gases, global warming has become more and more important. On the one hand, people are actively developing various types of renewable new energy, on the other hand, they are also advocating green energy conservation and emission reduction. Environmental technology. As an inexhaustible and clean energy, solar energy has become an important representative of many renewable energy sources. In the field of lighting, long-life, energy-saving, safe, green, colorful, and miniaturized LED solid-state lighting has also been Recognized as an important way of energy conservation and environmental protection in the world. The solar-LED streetlight combines the best of both worlds with clean energy and high-efficiency LEDs for green lighting.

The solar-LED street light scheme introduced in this paper can automatically detect ambient light to control the working state of street lamps. Maximum power point tracking (MPPT) ensures maximum solar panel efficiency, constant current control LED, and battery status output and user can set Set the LED working time and other functions.

System structure and implementation principle

At present, street lamps generally use a high-voltage sodium lamp structure powered by a mains supply. The electronic drive part of the high-pressure sodium lamp needs to convert the mains from AC to DC, and then inverter to AC to drive, resulting in low system efficiency; Mains need to lay complex and expensive pipelines. Solar-LED street lights do not have the above problems. Because solar panels output DC power, and LEDs are DC-driven light sources, the combination of the two can improve the efficiency of the entire system; the use of solar energy also eliminates the need to lay cables and The cost of its related works.

Figure 1 is a schematic view of the structure of a solar-LED street light. When the solar panel is illuminated by sunlight, its internal PN junction will form a new pair of electrons and holes, and a DC current will be generated in one loop; this current will flow into the controller and will charge the battery in some way. The battery will be recharged during the day and energy will be supplied to the LED at night. The operation of the LED is performed by the controller. While ensuring the constant current operation of the LED, the controller also monitors the state of the LED and controls the working time. In the case of continuous rainy days and insufficient battery power, the controller will send a control signal to activate the external mains supply system (not included in the controller) to ensure the normal operation of the LED. The external mains supply system is only used as a backup energy source and will only be used if the battery is not fully charged. Charging the battery is done entirely by solar energy to ensure maximum use of solar energy.

Figure 2: Block diagram of the controller structure

2 is a block diagram showing the structure of the controller. When the solar panel comes in, it will first be connected to the DC/DC converter (battery charging circuit) through a switch MOS tube KCHG. The output of this converter is connected to the two ends of the battery (the actual circuit will be connected to the battery through a fuse first). ). In addition, KCHG has two functions: one is to prevent the reverse charging flow from the battery when the output of the solar cell is low; the other is to protect the circuit when the polarity of the solar panel is reversed. The DC/DC converter adopts a buck topology, and the topology selection must consider not only the maximum power point voltage of the solar panel and the maximum voltage of the battery, but also the efficiency and cost. A DC/DC converter ( LED drive circuit) is also used between the battery and the LED. The constant current control mode is adopted for the LED. Considering the fluctuation range of the battery voltage and the operating voltage range of the LED, a flyback topology is used in the design circuit. Structure to ensure constant current output. The efficiency of the flyback topology is generally not as simple as the boost or step-down circuit. If you want to improve the efficiency of the system, you can use the boost or step-down circuit by optimizing the relationship between the battery voltage and the LED voltage to improve efficiency and possibly further reduce cost.

The control of the whole controller is realized by an MCU. The main work of the MCU includes the following points: First, the MPPT algorithm is used to optimize the working efficiency of the solar panel; second, the appropriate charging mode is adopted for different states of the battery; The constant current output of the drive circuit; the fourth is to judge the day and night to switch the battery charging and discharging mode; the last is to provide monitoring protection, temperature monitoring, status output and user control input detection (DIP1~4). The choice of MCU is mainly to meet the needs of ADC, GPIO and external interrupts, and does not need to pursue speed alone. Table 1 lists the use of MCU peripherals in the actual circuit. Considering the need for future expansion, the main control chip uses STM32F101RXT6 (the latest STM32 series MCU from STMicroelectronics, using the Cortex-M3 core).

Table 1: MCU peripheral assignments.

The auxiliary power supply of the controller is directly changed from the battery. The battery input is 12V through the linear power supply (L78L12), and the logic circuit and PWM switch signal are amplified. 3.3V is connected to the switching power supply (L5970D) through 12V, mainly for the MCU and peripheral circuits. For power supply, the reason why the switching power supply is used is to improve the conversion efficiency (reduce the battery power consumption) and to provide sufficient load when expanding the system in the future. Of course, in order to reduce the cost, it can be realized by a linear power supply.

Controller main function

The main functions of the controller include two aspects: battery charging and battery powering the LED.

1. Battery charging

When the system detects sufficient ambient light, the controller enters charging mode. There are two important voltage values for battery charging: deep discharge voltage and float charge voltage. The former represents the state in which the battery power is used up under normal use, while the latter represents the highest limit voltage for battery charging. These parameters should be found in the battery product manual. In the design circuit, for the 12V battery, set the deep discharge voltage to 11V and the float charge voltage to 13.8V (both voltage values at room temperature, the two values in the software increase the corresponding temperature compensation), specific charging The mode is shown in Table 2.

It can be seen from Table 2 that the trickle charge mode and the constant current charge mode will use the MPPT algorithm. There are many ways to implement the MPPT algorithm. There are many papers in the industry to discuss this. In general, each has excellent Inferior, the design circuit uses a relatively simple perturbation observation method (Perturbance and Observation). The basic idea of this control method is to increase or decrease the duty cycle by increasing or decreasing the PWMCHG duty cycle of the charging circuit switching signal and then observing whether the output power is getting larger or smaller. Since the output of the solar panel changes relatively slowly and is a single pole, this method can still receive better results.

2. Battery discharge

When the system detects that the ambient light is insufficient, it will enter the battery to supply power to the LED. The LED current is sampled back to the MCU through the high current sense detection chip (TSC101AILT), and the MCU obtains a constant output current by adjusting the duty cycle of the switching signal PWMDRV. In order to achieve energy saving, the constant current value of the LED is adjusted according to the ambient light intensity detected by the system: when the ambient light is dimmed, the output current of the system will be correspondingly small to large; when the ambient light is completely dark, The output current of the system also reaches the preset maximum value. In addition to controlling the output of the LED by ambient light, the user can also turn on the time control function by setting the state of the switch DIP1~4. The system will control the LED from 5 minutes to 12 hours according to the setting combination of DIP1~4.

In addition, in order to improve the reliability of the system, the design circuit adds protection functions for a series of hardware and software such as solar panels, batteries and LEDs. Based on this system platform, system performance can be further optimized from the aspects of adding intelligent LED operation mode, adding communication modules and adopting wind and solar complementary systems.

Conclusion of this paper

Solar-LED streetlights can not only use the clean and free solar energy and high-efficiency and environmentally-friendly LEDs to bring lighting to the road, but also reduce greenhouse gas emissions and achieve green lighting. The streetlight system has been successfully implemented at the entrance of the STMicroelectronics building. All streetlight systems have been in operation for half a year and work normally. As the price of solar panels is further reduced and the cost performance of LEDs is improved, it is believed that this system will be more and more widely used.

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