In recent years, the industry has increasingly focused on street lighting using renewable, clean energy solar energy. A typical solar street lighting system consists of a solar panel, a charge controller, a battery, a light source, and a light pole, as shown in FIG. In terms of lighting sources, it has experienced three important stages, from incandescent lamps to fluorescent lamps and high-intensity discharge lamps (HIDs), so that both front fluorescent lamps and HIDs have been used in solar street lights.
Figure 1: Schematic diagram of a typical solar-powered street lighting system.
In comparison, light-emitting diodes (LEDs) are considered to be the fourth important stage of illumination sources. LEDs have many advantages such as high energy efficiency, long working life, low DC voltage operation, emitting direct light, providing a variety of colors and white light, compact, solid-state devices, and mercury-free. LEDs are used for solar street lighting. And the energy efficiency and light output performance of LEDs have been greatly improved. The publicly announced ability of the strongest white LED has reached 132 to 136 lumens per watt (lm/W), which is higher than traditional fluorescent lamps and HID metal lamps. Especially in 2008, white LEDs have achieved large-scale commercial production, opening the door for LEDs to enter solar streetlight applications on a larger scale.
Improve solar panel energy efficiency with maximum peak power tracking technology
For solar street lights, it is important to improve the photoelectric conversion energy efficiency of solar panels (currently only about 30%). The voltage-current (VI) characteristic curve of a solar panel exhibits nonlinearity and variability, and it is very difficult to extract the maximum amount of electrical energy therefrom. This requires the solar LED street light's charge controller and other related electronic circuits (usually implemented by a microcontroller) to maximize the benefits by using effective control methods to improve energy efficiency.
The basic charge controller is designed to protect the battery from overcharging or undercharging and to prevent reverse current. The Pulse Width Modulation (PWM) type controller controls the amount of charge on the battery and enables trickle charge to protect the battery and extend its life. The latest controllers that support Maximum Peak Power Tracking (MPPT) provide compensation for the changing V/I characteristics of solar cells, optimize solar cell power output, increase energy efficiency, and charge batteries to optimize power.
Specifically, when we can't actually change the load, the MPPT function causes the solar cell to "think" that the load is changing; in this way, the MPPT "spoofs" the solar panel to output the desired voltage and current, allowing more Power is input to the battery.
ON Semiconductor's solution for solar panel battery charge control applications uses a CS51221 enhanced voltage-mode PWM controller that supports maximum peak power tracking with an input voltage of 12 to 24 V and an output current of 12 V@2 A. Protection features such as pulse-by-pulse current limit, input undervoltage lockout, and output overvoltage lockout are adjustable. The controller provides an auxiliary input for remote transmission and monitoring; it can accommodate solar panel applications up to 90 W.
Figure 2: Schematic diagram of solar panel charging control application of ON Semiconductor CS51221 controller
In the application circuit, you need to choose the appropriate topology for the CS51221. The topology chosen should be able to reduce the solar panel output voltage to 12 V in the case of a battery, and in the case of two or more batteries, it can be easily modified to support boosting to 24 V. The CS51221 itself can be configured as a forward, flyback or boost topology. In the reference design introduced by ON Semiconductor for solar panel charging control applications, the flyback topology was chosen.
In applications, maximum peak power tracking is achieved by dynamically adjusting the current limit at the ISET pin. Once the input voltage drops pulse by pulse, the current limit is reduced until the input voltage is restored. This approach eliminates the need to use expensive microcontrollers (MCUs). The charge controller thus implemented will find the peak power point and dynamically adjust it to match the changing power supply characteristics.
By using maximum peak power tracking technology, approximately 30% of the extra charge can be transferred from the solar panel to the battery, which allows the solar street light system to use smaller solar panels. For example, in the case of the same electrical energy, a 60 W power solar panel with MPPT function can be used instead of a 90 W power solar panel with a basic charge controller. Calculated by outputting about $4 worth of solar panels per watt of electrical energy, the solar panel cost savings of 30 W can be as much as $120, resulting in significant cost reductions.