Introduction
Solar photovoltaic (PV) panel by converting solar radiation into DC electricity using
semiconductors that exhibit the photovoltaic effect is suitable for small-scale solar application system because of its implementation flexibility. The output power of solar panel depends on solar insolation level and PV module temperature, as well as load property. The control method of maximum power point tracking (MPPT) enables the solar charge controller to track the MPP under any input and output conditions.Many MPPT methods have been developed and implemented. The methods vary in complexity,sensors required, convergence speed, cost, implementation hardware, range of effectiveness, popularity,and in other respects.Light emitting diodes (LEDs) have been widely used in many products such as liquid crystal display
(LCD) panel backlighting and street lighting.The significant improvements achieved for highpower
or high-brightness LEDs are gradually realizing the possibility of replacing conventional light
sources based on heated filaments and gas discharges with high-power LEDs. The emission intensity of LEDs varies linearly with the forward current for small currents, but it shows a tendency to saturate athigh currents. This phenomenon implies that the efficacy of an LED is lower if operated at high forward currents. Two commonly used driving techniques for LEDs employ DC and PWM current driving with their inherent advantages and disadvantages. As a typical solar powered off-grid application, lighting system converts electricity generated by solar
panel to light load. In conventional solar lighting system, charger and driver are independent controllers and responsible for energy collection and energy utilization by interacting with battery. Many solar powered off-grid lighting systems have the feature of time-separate energy harvest stage and energy utilization stage e.g. solar street lamp and solar landscape lights. In these applications, single solar controller integrating battery charging and discharging functions is attractive because it makes full use of converter hardware and simplifies system configuration. An integrated solar controller for off-grid lighting system is proposed. Based on Sepic-type bidirectional converter (BDC), both battery charging and LED driving are realized with improved flexibility of voltage level matching among solar PV panel, battery, and LED module. A modified perturb & observe (P&O) MPPT control via battery parameters is proposed and implemented to improve control performance with reduced implementation cost. Digital hysteresis constant current control for driving LED module is also analyzed and verified by experiments.
System Architecture Optimization
Typical solar off-grid lighting system consists of solar panel, solar charge controller, battery, and light source. Optimized system architecture brings benefit to customer with decreased cost, improved reliability, and enhanced flexibility.Based on BDC, the system architecture of solar powered off-grid lighting is shown in Fig. 1. During daytime when solar irradiance is available, PV panel charges battery through diode DPV and BDC with
battery switch Sbat closed and LED switch SLED open. Power flows from PV panel to battery. During nighttime when sunlight is not available and light is needed, PV panel doesn’t work, and battery supplies energy to LED module through LED switch SLED with battery switch Sbat closed. Power flows from battery to LED module.Improved flexibility will be obtained by changing the stepping-up or stepping-down topology with BDC such as Sepic topology which has both stepping-up and stepping-down functions. Solar off-grid lighting system based on Sepic-type BDC is shown in Fig. 2, in which battery switch and LED switch are omitted for simplicity.