Solar LED street light controller hardware structure and control software

Solar LED street light controller hardware structure and control software

The solar LED street light controller is the most important component in the solar LED street light system, and it is also the biggest difference from various street light systems. The performance of the controller design determines the pros and cons of the operation of the solar LED street light, so it is very important to design a smart solar LED street light controller with complete functions and simple structure. The functions that the controller needs to realize are: automatically turn on the lights when it is dark; automatically turn off the lights when the sky is bright; automatically disconnect the load when the battery power is insufficient to prevent the battery from over-discharging; and must have short-circuit protection, reverse connection protection, etc. The controller is not only responsible for the state control of the entire solar LED street light, but also ensures the safe operation of the solar LED street light.

  1. Hardware structure of solar LED street light controller

(1) The hardware structure block diagram of the solar LED street light controller is shown in Figure 1. The main components include the following parts: current and voltage sampling modules. According to the power of the system, resistance components or transformers can be used, such as domestic CT series transformers, GMR current transformers, etc. The primary side current can be designed according to C/5 (C is the battery capacity).

Figure 1 - Hardware structure block diagram of solar LED street light controller
Figure 1 – Hardware structure block diagram of solar LED street light controller

(2) Power module. The power supply is provided by the battery, and the MCU is provided with a stable working power (2.5~6V) through the 78xx and other power chips.

(3) Keyboard input. A standard determinant keyboard can be used (or an interface is reserved in the photovoltaic system, which can be accessed when setting is required), or a special membrane key can be customized.

(4) LCD display. Because the liquid crystal display has the characteristics of low power consumption, small size and light weight, it is suitable for battery-powered systems.

(5) Remote communication interface. The system adopts asynchronous serial communication. There is an asynchronous serial communication port inside the MCU, and software can be used to complete asynchronous serial communication (RS-232 standard asynchronous serial communication).

(6) MOSFET control module. The system logic control signal of the MCU forms the gate control voltage of the MOSFET through the MOSFET control module, and controls the state and logic of the system. In addition, considering that the system can be applied to different powers, there is sufficient margin for the MOSFETs and high-power switches used to meet the requirements of different systems. At the same time, LED indicators are also set up in the system to directly observe the status and problems of the system.

  1. Solar LED street light control software

For the specific solar LED street light control software, the basic main program is to complete the I/O configuration and interrupt setting of the MCU during initialization. During the cyclic waiting process, the state of the system is collected and judged, and the corresponding state processing is entered. The subroutine waits for the keyboard input and the start bit of serial communication at the same time. The flow chart of the solar LED street light control software is shown in Figure 2.

Figure 2 - Flow chart of solar LED street light control software
Figure 2 – Flow chart of solar LED street light control software

Asynchronous serial communication is to complete asynchronous serial communication in the form of software by setting general I/O ports. At the same time, the system controls the display content of the LCD through the input of the keyboard, and the LCD displays the state (charging or discharging state) of the system online. You can also choose to display the battery voltage, capacity and the size of the charging and discharging current. All these data can be transmitted to the host computer through serial communication for further processing when needed to remotely monitor the working status of the solar LED street light. This part of the program flow can refer to the general asynchronous serial communication program and LCD display program. The following points should be paid attention to when programming the software:

(1) Use fewer buttons to achieve many functions, such as the setting of the load working mode, the setting of the load working time, and the self-checking function, etc., and measures to prevent misoperation should be taken.
(2) The keyboard is read in the timed interrupt service program, and the keyboard is de-jittered by the interrupt interval time, and there is no need to write another delay program to improve the utilization efficiency of the CPU. The keyboard value is stored in the data buffer, the content of the data buffer is read in the main program, and the keyboard subroutine function is executed.

(3) Ambient light (lightning, fireworks display) has a significant impact on the sampling voltage of solar cell modules. When identifying daytime and dusk, a software delay should be performed, generally controlled within 2~3min.

(4) The external interrupt is a high-priority interrupt. When a subroutine is programmed to realize load overcurrent and short circuit protection, the load output is cut off after determining the load overcurrent and short circuit. After the load is cut off, try turning on the load switch every time period, such as 20s. When it is found that the overcurrent and short-circuit signals have been eliminated, the output of the load will be restored, otherwise the load switch will remain disconnected.

(5) In order to protect the load (lamp), when the battery over-discharge protection recovers, the application software sets a hysteresis voltage, so that the load switch will not vibrate, which is beneficial to prolong the service life of the lamp.

  1. Working principle of solar LED street light controller

The solar LED street light controller not only ensures the constant current operation of the LED, but also monitors the status of the LED and controls the working time. In the case of continuous rainy days and insufficient battery power, in order to prevent the battery from over-discharging, the controller will send a control signal to cut off the power supply circuit of the LED street light.

The block diagram of the solar LED street light controller is shown in Figure 3. The input of the solar cell is connected to the DC/DC converter (battery charging circuit) through a switch MOS tube (KCHG), and the output of the converter is connected to both ends of the battery ( The actual circuit is first connected to the battery through a fuse). Setting KCHG has two functions: one is to realize battery reverse charge protection when the solar cell output is low; the other is to protect the circuit when the solar cell polarity is reversed.

Figure 3-Structure block diagram of solar LED street light controller
Figure 3-Structure block diagram of solar LED street light controller

The DCIDC converter adopts a step-down topology structure. The choice of the topology structure not only considers the maximum power point voltage of the solar cell and the maximum voltage of the battery, but also takes into account the efficiency and cost. Because the LED needs constant current control, considering the fluctuation range of the battery voltage and the working voltage range of the LED, a DC/DC converter (LED drive circuit) is set between the battery and the LED, and the circuit design adopts a flyback topology to ensure Constant current output. The efficiency of the flyback topology is generally not as high as that of a boost or buck circuit. If the efficiency of the system is to be improved, a boost or buck circuit can be used to improve the efficiency and further reduce the cost by optimizing the relationship between the battery voltage and the LED voltage. The control of the entire controller is realized through the MCU. The main work of the MCU includes the following points:

(1) The MPPT algorithm is used to optimize the working efficiency of solar cell modules.
(2) Appropriate charging modes are adopted for different states of the battery.
(3) Ensure the constant current output of the LED drive circuit.
(4) Judging day and night and using this to switch battery charging and discharging modes.
(5) Provide monitoring protection, temperature monitoring, status output and user control input detection (DIP1~4) and other functions.

The choice of MCU is mainly to meet the needs of ADC, GPIO and external interrupts, and does not need to simply pursue speed. The auxiliary power supply of the controller is directly converted from the battery. The battery input obtains 12V power supply through the linear power converter (L78L12), which is supplied to the logic circuit and PWM switching signal amplifying circuit: 12V power supply input to the DC/DC converter (L5970D) to generate 3.3V The power supply is used to supply power to the MCU and peripheral circuits. The switching power supply is used to improve the conversion efficiency (reduce the power consumption of the battery) and provide sufficient power when the system is expanded later. Of course, in order to reduce costs, a linear power supply can also be used.

  1. Main functions of solar LED street light controller

The main functions of the solar LED street light controller include two aspects: battery charging and discharging and the battery supplying power to the LED.

1) battery charge and discharge
When the system detects sufficient ambient sunlight, the controller enters charging mode. There are two important voltage values ​​for battery charging and discharging: deep discharge voltage and floating charge voltage. The former represents the state of the battery being used up under normal use, while the latter represents the highest limit voltage for battery charging. These parameters can be found in the battery product manual.
In the circuit design, for the 12V battery, the deep discharge voltage is set to 11V and the float charge voltage is set to 13.8V (both are the voltage values ​​at room temperature, and the corresponding temperature compensation should be added to these two values ​​in the software design), The specific charging mode is shown in Table 1.

Table 1 - Battery Charging Modes
Table 1 – Battery Charging Modes

As can be seen from Table 1, the constant current charging mode will use the MPPT algorithm. There are many ways to realize the MPPT algorithm. Generally speaking, each has its own advantages and disadvantages. In the design, a relatively simple disturbance observation method can be used to realize it. The basic idea of ​​this control method is to increase or decrease the duty cycle of the switching signal PWM of the charging circuit, 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 cell module changes relatively slowly and is a single pole, this control method can achieve better results.

2) The battery supplies power to the LED
When the system detects that the ambient light is insufficient, it will enter the battery-powered LED mode. The LED current is sampled and sent back to the MCU through the high-level current detection chip (TSC101AILT), and the MCU obtains a constant output current by adjusting the duty cycle of the switching signal PWM. 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 correspondingly increase from small to large; when the ambient light is completely dimmed, The output current of the system reaches the preset maximum value. In addition to controlling the light output of the LED by the ambient light, the user can also set the turn-on time of the LED street light by setting the state of the switches DIP1~4. The system will control the LED street light to work at 5min~12h according to the setting combination of DIP1~4. within the time frame.

In addition, in order to improve the reliability of the system, a series of software and hardware protection functions such as solar cells, storage batteries and LEDs are set up in the circuit design. Based on this system platform, the intelligent light-emitting diode working mode and communication module are also set to further optimize the system performance. The following items should be paid attention to in the hardware design of solar LED street light controller:

(1) The induction lightning protection circuit should be designed at the entrance of the solar cell lead, and no other components should be arranged within 4mm around the protection circuit.

(2) The diode used to prevent the reverse connection of the solar cell must be a fast recovery diode. This diode has a small on-state resistance and generates little heat during charging. It can be continuously charged without a radiator, and the charging effect is good.

(3) The width of the PCB circuit of the charging and load discharging circuit should be at least 4~5mm, and the circuit is treated with tin lining to increase the conduction current capability. When the high-current wire on the PCB transitions from one layer to another, To place 3 to 5 vias.

(4) The current sampling resistor selected for the overcurrent and short circuit protection circuits should comprehensively consider three factors of current, power and thermal stability. When the resistance increases, the circuit efficiency decreases. This system selects the constantan wire with the resistance of 0.012 and the overcurrent capability of more than 10A as the current sampling resistor to generate the sampling voltage. The sampling voltage does not exceed 0.2V, and the sampling signal is amplified by LM358.

(5) The layout of the components and the wiring of the PCB are modularized, the high-current signal and the small-current signal should be separated, and the circuit of the amplifier circuit should be carefully arranged. Separate digital ground and analog ground, pay attention to the layout of power and ground wires.

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