Design of Solar LED Street Light Controller Based on Single Chip Microcomputer

　　Faced with the reality of the deteriorating ecological environment and the increasing shortage of resources, governments around the world have adopted many policies and measures to vigorously support and develop energy-saving and environmental protection industries. Solar LED street lights are a comprehensive application of solar energy development and utilization and energy-saving technologies in the field of lighting, with the dual advantages of environmental protection and energy saving. According to statistics, lighting consumption accounts for about 20% of the total electricity consumption, and reducing lighting electricity consumption is an important way to save energy. Solar energy is clean, environmentally friendly and renewable, and LED lighting is the most advanced lighting technology in the world. It is the fourth generation of light sources after incandescent lamps, fluorescent lamps, and high-intensity gas discharge lamps. It has simple structure, high efficiency, light weight, good safety performance, no pollution, maintenance-free, long life, and strong controllability. It is considered to be the best way to save electricity and reduce energy consumption in the lighting field. Statistics show that the energy saving of LED street lights alone can save China about the electricity generated by a Three Gorges Dam every year. Due to the energy-saving and environmentally friendly advantages of LED lighting fixtures, the annual growth rate of its global output value has remained above 20% in recent years. China has also launched green lighting projects, semiconductor lighting projects, and the "Ten Cities, Ten Thousand Lamps" plan to promote the development of the industry.

　　The solar LED street light controller designed in this paper first detects parameters such as solar cell output and battery power to determine the system working status, and uses the maximum power point tracking MPPT algorithm to maximize the collection of electrical energy. After the energy reserve is completed, the PWM technology is used to adjust the brightness of the LED to further save energy, thereby realizing automatic control and intelligent energy management of the entire system, which is more conducive to the application and promotion of solar street lights.

　　1 Introduction to Solar LED Street Light System

　　1.1 Composition of Solar LED Street Light System

　　The solar street light system consists of the following parts: solar panels, LED lamps (including LED light sources, lamp poles and lamp housings), controllers, and battery packs, as shown in Figure 1.

Figure 1 Solar street light system

　　1.2 Basic principles of solar LED street light system

　　Solar panels made using the photovoltaic effect receive solar radiation during the day and convert it into electrical output, which is then stored in batteries through a charge-discharge controller. At night, when the illumination gradually decreases, the charge-discharge controller detects this change, and the battery begins to discharge the LED street light. After the battery discharges for about 10 hours, the charge-discharge controller operates, and the battery discharge ends.

　　According to the sunshine characteristics of Sanya, Hainan and the urban road lighting design standards, the component parameters selected for this system are as follows: 1 set of LED street lights (32 W, 24 V, 1.4 A; LED 1 W light source; 4 sets in parallel, 8 in series in each set); 2 solar panels (rated output voltage of each set is 18 V, working current is 5.6 A, open circuit voltage is 21.2 V, short circuit current is 6.1 A, peak power is 80 W); battery (12 V, 200 Ah; overcharge voltage is 14.8 V, floating charge voltage is 12.3 V, over discharge voltage is 10.8 V).

　　2 Hardware Design

　　Although the solar LED street light controller is the smallest part in the whole system, it is the core control part of the whole system.

　　A controller with advanced design, in addition to completing the most basic charge and discharge control functions, can also control the solar cell array to absorb solar energy as much as possible to improve efficiency; it can prevent the battery from overcharging and deep discharge, extending the battery life; it can adjust the brightness of the LED light source according to the environment, especially in the second half of the night, it can also achieve half-power lighting of the load, thereby saving energy as much as possible. Since the output power of photovoltaic panels has a lot of uncertainty, the charge and discharge characteristics of batteries are nonlinear, and the other two are greatly affected by the environment, designing a good charge and discharge controller has a great impact on system performance. This article is a useful exploration of controller design.

　　The controller designed in this paper uses the STC12C5410AD microcontroller as the main control device. The device has 4 built-in PWM channels, 8 10-bit ADC channels, and an operating frequency of up to 35 MHz. The instructions are compatible with 51 microcontrollers but the speed is 8 to 12 times faster, which is very suitable for this design requirement. Since the two groups of solar cells are connected in series, the output voltage is 36 V, the battery voltage is 12 V, and the working voltage of the LED street lamp is 24 V, the charging circuit uses a DC/DC step-down conversion circuit (Buck), and the discharge circuit uses a DC/DC step-up conversion circuit (Boost). The control strategy of charging and discharging is realized through software, so as to ultimately achieve the purpose of improving efficiency and energy saving (as shown in Figure 2). This article focuses on the charging and discharging circuit and its control strategy.

Figure 2 Block diagram of solar LED street light control system based on SIC [page]

　　2.1 Charging circuit and control strategy

　　The charging circuit is a step-down Buck circuit composed of inductor L1, power MOSFET tube T1 and freewheeling diode D2, as shown in Figure 3. The output voltage of the solar panel can be changed by changing the pulse width (Pulse Width Modulation, PWM) applied to the MOSFET control gate. By detecting the output voltage and current of the solar panel, the voltage and current of the battery, the charge state of the battery is determined, and the appropriate charging method is selected to optimize the charging of the battery. When the battery voltage exceeds a certain voltage, T1 is turned off to prevent the battery from overcharging. When the system detects that the ambient light is sufficient, the controller will enter the charging mode.

Figure 3 Buck main charging circuit

　　However, the charging efficiency is closely related to the characteristics of the charging power source (solar cell), load (battery) and environment. The output power of the solar cell is a nonlinear function of the sunlight intensity and the ambient temperature, as shown in Figure 4.

　　That is to say, when the sunshine intensity increases, the maximum output power increases accordingly; when the temperature increases, the output power decreases; but under certain conditions, there is always a maximum output power point. When the temperature effect is ignored, the intersection points A, B, C, D, E (operating points) of the output characteristics under different lighting conditions and the load curve L are obviously not all maximum power points. If direct matching is used, it will inevitably lead to a loss of output power.

Figure 4 Output characteristics of solar photovoltaic cells

　　The maximum power point tracking (MPPT) control strategy can convert the collected solar energy into electrical energy as much as possible and store it in the battery pack. The MPPT control strategy mainly includes interference observation method, admittance increase method and fixed parameter method. The interference observation method is used here. Its idea is: the controller changes the output voltage or current of the photovoltaic cell with a smaller step size in each control cycle - "interference", and the direction of change can increase or decrease; compare the output power of the photovoltaic cell before and after, if the output power increases, continue the interference process in the direction of the previous cycle; if the output power decreases, change the direction of the interference, and finally reach stability at the maximum power point. At this time, the step size can be reduced to further approach the maximum power point.

　　In addition, under current conditions, lead-acid batteries are relatively economical and practical storage devices. The capacity and life of lead-acid batteries are important parameters of batteries, which are greatly affected by the charging method. The acceptable ideal charging curve is a curve in which the charging current decays exponentially over time, but the polarization phenomenon restricts the life of the battery and the charging mode of the photovoltaic battery power generation system. Therefore, it is necessary to adopt a staged charging strategy based on the battery charging characteristic curve to improve the charging efficiency and extend the battery life. The battery charging strategy here is three-stage charging (fast charging, overcharging and floating charging).

　　(1) The output mode of the charging circuit in the fast charging stage is equivalent to a current source. The output current of the current source is determined according to the maximum acceptable current of the battery. During the charging process, the battery terminal voltage is detected. When the battery terminal voltage rises to the conversion threshold value, the charging circuit switches to the overcharge stage. The output current is fixed and the output voltage is controlled by the MPPT algorithm.

　　(2) In the overcharge stage, the charging circuit provides a higher voltage to the battery and detects the charging current at the same time. When the charging current drops below the switching threshold, the battery is considered to be fully charged and the charging circuit switches to the floating charge stage.

　　(3) After the battery pack is fully charged during the floating charge phase, the best way to maintain the charge level is to provide the battery with an accurate floating charge voltage with temperature compensation function.

　　2.2 Discharge circuit and control strategy

　　The load of the discharge circuit is a high-power LED street lamp, which is a green light source formed by connecting high-brightness LEDs of 1 W or above according to a certain topology. The luminous intensity of a high-power LED street lamp is proportional to the current flowing through it. Since the current and voltage parameters of a high-power LED have typical PN junction volt-ampere characteristics, a small change in its forward voltage drop will cause a large change in the forward current. Unstable operating current will affect the life and light decay of the LED, so the driving circuit of the high-power LED must provide a constant current. Its control circuit mainly adopts a DC/DC boost drive circuit (Boost), and the control strategy adopts pulse width modulation (PWM). The Boost charging circuit is shown in Figure 5.

Figure 5 Boost discharge circuit.

　　Inductor L2, power MOSFET tube Q2 and D3 form a boost DC/DC converter, which obtains a stable output voltage through the microcontroller control output PWM2; constant current control of 2-way LED lighting is performed through PWM3 and PWM4 channels, and completely shutting down these 2 loads can also be used for half-power point control; R7 and R10 provide current feedback sampling for the LED lighting drive circuit; other time control functions, temperature compensation circuits and battery over-discharge protection circuits will not be discussed in detail here.

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　　The software design mainly assists the hardware circuit to complete the control strategy of the controller, which consists of the main program and charging and discharging subroutines, as shown in Figures 6 to 9. The charging subroutine completes the three-stage charging conversion according to the voltage and current of the battery. The MPPT algorithm is used in the fast charging stage to maximize the output power of the photovoltaic cell. The discharge subroutine adjusts the load current through PWM technology, which can completely cut off the load in the second half of the night to achieve half-power lighting of the load.

Figure 6 Main program flow

Figure 7 Charging subroutine flow

Figure 8: MPPT algorithm is used to track the maximum power flow in the fast charging stage

Figure 9 Discharge electronic program flow

　　4 Conclusion

　　The solar LED street lighting system is a perfect combination of solar energy development and utilization and a new generation of green light source LED. After multiple hardware and software debugging, the intelligent controller designed with STC12C5410AD microcontroller as the core in this paper realizes the three-stage charging control function as a whole, and can effectively prevent the battery from overcharging; at the same time, it can also realize the timing and half-power point load cut-off, and the load will be cut off when the battery voltage is less than the over-discharge voltage, thereby protecting the battery from over-discharge. The system has certain practical value in terms of energy utilization and working reliability. Considering that the wind resources in Sanya are also relatively rich, the next research direction will be to make full use of the complementarity of solar energy and wind energy to ensure uninterrupted lighting throughout the year, so as to take a step towards truly realizing a zero-pollution, zero-emission, green lighting system.

References:

[1]. STC datasheet http://www.dzsc.com/datasheet/STC+_2043151.html.
[2]. STC12C5410AD datasheet http://www.dzsc.com/datasheet/STC12C5410AD+_1135295.html.
[3 ]. R10 datasheet http://www.dzsc.com/datasheet/R10_1193166.html.