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 respectsLight emitting diodes (LEDs) have been widely used in many products such as liquid crystal display
(LCD) panel backlightingand 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 at
high 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.