Design of Solar LED Street Light Controller Based on STC Microcontroller

According to the sunshine characteristics of Sanya, Hainan and the urban road lighting design standards [5], the component parameters of this system are as follows: 1 set of LED street lights (32 W, 24 V, 1.4 A; LED 1 W light source; 4 groups in parallel, 8 in series in each group); 2 solar panels (each with a rated output voltage of 18 V, an operating current of 5.6 A, an open circuit voltage of 21.2 V, a short circuit current of 6.1 A, and a peak power of 80 W); and a battery (12 V, 200 Ah; overcharge voltage of 14.8 V, float charge voltage of 12.3 V, and over discharge voltage of 10.8 V).

2 Hardware Design

Although the solar LED street light controller is the least valuable 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; can prevent overcharging and deep discharge of the battery, and extend the service life of the battery; 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, so as to save energy as much as possible. Since the output power of the photovoltaic panel has a lot of uncertainty, the charge and discharge characteristics of the battery are nonlinear , and the other two are greatly affected by the environment, designing a good performance charge and discharge controller has a great impact on the system performance. This article is a useful exploration of controller design.

The controller designed in this paper uses the STC 12C5410AD 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 buck conversion circuit (Buck), and the discharge circuit uses a DC/DC boost 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.

2.1 Charging circuit and control strategy

The charging circuit is composed of an inductor L1, a power MOSFET tube T1 and a freewheeling diode D2 to form a step-down Buck circuit, 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.

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 a solar cell is a nonlinear function of the sunlight intensity and the ambient temperature [1], as shown in Figure 4. That is, when the sunlight 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.