Solar energy based on STM32 MCU: LED street light solution

As fossil energy is decreasing and the problem of global warming caused by excessive greenhouse gas emissions is becoming more and more important, people are actively developing various types of renewable energy on the one hand, and advocating green environmental protection technologies for energy conservation and emission reduction on the other. Solar energy, as an inexhaustible clean energy, has become an important representative of many renewable energy sources; in the field of lighting, LED solid-state lighting with long life, energy saving, safety, green environmental protection, rich colors and miniaturization has also been recognized as an important way to save energy and protect the environment in the world. Solar-LED street lights integrate the advantages of both, using clean energy and high-efficiency LEDs to achieve green lighting. The solar-LED street light solution introduced in this article can automatically detect ambient light to control the working state of the street light, the maximum power point tracking (MPPT) ensures the maximum efficiency of the solar panel, constant current control of the LED, and has battery status output and user-settable LED working time and other functions.
System structure and implementation principle
Currently, street lamps generally use a high-pressure sodium lamp structure powered by mains electricity. The electronic drive part of the high-pressure sodium lamp needs to convert the mains electricity from AC to DC and then invert it to AC to drive, resulting in low system efficiency; and because it uses mains electricity, it is necessary to lay complex and expensive pipelines. Solar-LED street lights do not have the above problems. Since solar panels output DC power and LEDs are also 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 cost of laying cables and related projects.


Figure 1: Solar-LED street light system structure principle. 

Figure 1 is a schematic diagram of the structure of a solar-LED street light. When a solar panel is exposed to sunlight, its internal PN junction will form new electron-hole pairs, which can generate DC current in a circuit; this current flows into the controller and charges the battery in some way. The battery will be charged during the day and provide energy to the LED at night. The operation of the LED is carried out by the controller. While ensuring the constant current operation of the LED, the controller will also monitor the status of the LED and control the working time. In case of continuous rainy days and insufficient battery power, the controller will send a control signal to start the external mains power supply system (not included in the controller) to ensure the normal operation of the LED. The external mains power supply system is only used as a backup energy source and will only be used when the battery power is insufficient. The battery is charged entirely by solar energy to ensure the maximum use of solar energy.

Figure 2: Controller structure block diagram 
Figure 2 is the controller structure block diagram. After the solar panel comes in, it will first pass through a switch MOS tube KCHG to connect to the DC/DC converter (battery charging circuit), and the output of this converter is connected to both ends of the battery (in the actual circuit, it will first pass through a fuse and then connect to the battery). Adding KCHG has two functions: one is to prevent the reverse charging current from the battery when the solar cell output 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. The selection of the topology must not only consider the maximum power point voltage of the solar panel and the maximum voltage of the battery, but also take into account efficiency and cost. The battery and LED are also connected through a DC/DC converter (LED drive circuit). Constant current control is used for the LED. Considering the fluctuation range of the battery voltage and the working voltage range of the LED, the flyback topology is used in the design circuit to ensure constant current output. The efficiency of the flyback topology is generally not as high as that of a simple boost or buck circuit. If the efficiency of the system is to be improved, a boost or buck circuit can be used by optimizing the relationship between the battery voltage and the LED voltage to improve efficiency and possibly further reduce costs.
The control of the entire controller is achieved through an MCU. The main tasks of the MCU include the following: first, the MPPT algorithm is used to optimize the working efficiency of the solar panel; second, the appropriate charging mode is used for different battery states; third, the constant current output of the LED drive circuit is ensured; fourth, the day and night are judged and used to switch the battery charging and discharging mode; and finally, monitoring protection, temperature monitoring, status output, and user control input detection (DIP1~4) are provided. The selection of the MCU is mainly to meet the needs of ADC, GPIO, and external interrupts, without the need to pursue speed alone. Table 1 lists the usage of MCU peripherals in the actual circuit. Considering the need for future expansion, the main control chip uses STM32F101RXT6 (the latest STM32 series MCU of STMicroelectronics, using the Cortex-M3 core). [page]

Table 1: MCU peripheral allocation. 
The auxiliary power supply of the controller is directly converted from the battery. The battery input obtains 12V through a linear power supply (L78L12) to supply the logic circuit and PWM switch signal amplification; 3.3V is obtained through 12V to connect the switching power supply (L5970D), which mainly powers the MCU and peripheral circuits. The reason for using a switching power supply is to improve the conversion efficiency (reduce battery power consumption) and to provide sufficient load when the system is expanded in the future. Of course, in order to reduce costs, a linear power supply can be used to achieve this.
Main functions
of the controller The main functions of the controller include two aspects: battery charging and battery powering LEDs.
1. Battery charging
When the system detects that the ambient light is sufficient, the controller will enter the charging mode. There are two important voltage values ​​for battery charging: deep discharge voltage and floating charge voltage. The former represents the state where the battery power is exhausted under normal use, while the latter represents the maximum limit voltage of the battery charging. These parameters should be found in the battery product manual. In the design circuit, for the 12V battery, the deep discharge voltage is set to 11V and the floating charge voltage is set to 13.8V (both are voltage values ​​at room temperature, and the two values ​​in the software have added corresponding temperature compensation). The specific charging mode is shown in Table 2. Table 2

: Battery charging mode 
From Table 2, it can be seen that the MPPT algorithm will be used in the trickle charging mode and the constant current charging mode. There are many ways to implement the MPPT algorithm. There are many papers in the industry that have discussed this. In general, each has its own advantages and disadvantages. The relatively simple perturbation observation method (Perturbance and Observation) is used in the design circuit. The basic idea of ​​this control method is to increase or decrease the duty cycle of the charging circuit switch signal PWMCHG, and then observe whether the output power increases or decreases, so as to decide whether to increase or decrease the duty cycle in the next step. Since the output of the solar panel changes relatively slowly and is a single pole, this method can still achieve relatively good results.

2. Battery discharge
When the system detects that the ambient light is insufficient, it will enter the battery power supply mode for LED. The LED current is sampled by the high-current detection chip (TSC101AILT) and sent back to the MCU, which adjusts the duty cycle of the switch signal PWMDRV to obtain a constant output current. In order to achieve the purpose of energy saving, the constant current value of the LED will be adjusted according to the ambient light intensity detected by the system: when the ambient light changes from bright to dark, the output current of the system will also change from small to large accordingly; when the ambient light is completely dark, the output current of the system will also reach 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 a series of software and hardware protection functions for solar panels, batteries and LEDs. Based on this system platform, the system performance can be further optimized by adding intelligent light-emitting diode working mode, adding communication modules and adopting wind-solar complementary systems.
Conclusion of this article
Solar-LED street lights can not only use clean and free solar energy and efficient and environmentally friendly LEDs to illuminate the roads, but also reduce greenhouse gas emissions and achieve the goal of green lighting. This street light system has been successfully implemented at the entrance of the STMicroelectronics building. All street light systems have been running for half a year and are working normally. As the price of solar panels continues to drop and the cost-effectiveness of LEDs increases, I believe this system will be more and more widely used.