The research of stand-alone solar-powered LED lighting system
Date Issued
2009
Date
2009
Author(s)
Wu, Min-Sheng
Abstract
A stand-alone solar LED lighting system usually consists of a DC/DC converter to convert the floating battery voltage into a constant current to drive the LED. A 15% energy loss of the DC/DC converter will then be introduced. The DC/DC converter may also increase system costs and reduce system reliability due to the additional components. The present study first develops an intelligent feedback solar battery charge controller, which can store 18% more energy in the battery. In addition, a direct feedback PWM driving controller (to drive the LED luminaire directly from the battery output using the PWM constant average current, or the PWM constant average power) were developed. The test results show the performance of the controller, which has a 3% deviation for the direct PWM constant current driven technique. The direct PWM constant power driven technique has a 2.8% deviation when the battery voltage is changed from 25.8V to 23V. Therefore, the direct PWM constant current/power driven techniques that avoid overdrive the LED luminaire when battery voltage is changed. However, the instantaneous current stress may cause several problems when using the direct PWM driven techniques. The first of these is the decrease in overall light efficiency of an LED luminaire when it is driven by a PWM current. The results show that the overall lighting efficiency loss is about 13.9% for the junction temperature at 70oC when using a 24V battery system. Although the lighting efficiency will be reduced by the PWM current, the direct PWM driven technique can extend the lighting period to 35.7 hours for the direct PWM constant current driven technique, and 36.44 hours for the direct PWM constant power driven technique. These time values are much longer than the constant current driven technique (30.4 hours), which usually uses a DC/DC converter to drive the LED luminaire. The second concern is the switching loss when driven the LED luminaire. The test shows the switching loss is less than 1% when the PWM frequency is equal to 125Hz. The third problem considers that the instantaneous current stress may decrease the reliability of the LED luminaire. A reliability test has undergone more than 15,000 hours of examination, and indicates that the PWM driving technique will not damage the LED when the PWM frequency is equal to 125Hz and the peak current is equal to three times the rated current.his study shows that the feedback PWM charge system can remove the MPPT controller by a MOSFET and realize the same performance by adopting the nMPPO. The direct PWM driving technique is feasible and capable of driving an LED directly from the battery output in the stand-alone solar system by a MOSFET instead of a DC/DC converter. These features further increase the system’s reliability and reduce the cost when removing the MPPT controller and the DC/DC converter in the system.
Subjects
Stand-alone solar-powred system
PWM direct driven technique
LED driver
Type
thesis
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