Solar Cell Boost Converter
The boost converter is used to charge batteries from low voltage solar
arrays. Results were obtained using 3X3 cells that deliver about 400
millivolts at 1 amp. The pictured panel array contains 20 cells in series
and generates about 8 watts at 8 volts in bright sunlight and was assembled
on a 12 X 16 picture frame. Efficiency of the converter measured 87% and
delivers almost 600 milliamps into a 12 volt SLA battery. Efficiency drops
to about 72% using 4 single cells in series (pictured above) charging the
same 12 volt battery at around 70mA. The current was a little low due
to a couple broken corners. A third test was made using a single cell at
0.4 volt charging a 6 volt battery, but efficiency was only about 55%.
The cells were purchased from solarcells101.com in slightly damaged
condition with chipped edges and a few tiny cracks, but still perform well
and sell at discount. There are also many good deals on ebay.
The 10Khz oscillator and drive circuit obtain power from the battery under
charge which should be grater than 4 volts. The output stage (mosfet and
inductor) obtain power from the solar array and produce a charging current
through the schotty diode (VSK 330). Efficiency is improved with 220uF
capacitors added across the input and output. A 12 volt zener diode and
120 ohm resistor were added to protect the circuit from excessive voltage
in the event the battery is disconnected during operation. Additional
protection is obtained with the TL431 voltage reference diode which limits
the output voltage to 18 volts. If the output exceeds 18 volts, the cathode
of the TL431 falls, stopping the oscillator until the output falls below
In operation, the duty cycle of the switching waveform is adjusted with
the 100K pot for maximum current into the battery. This adjustment can be
made by monitoring the voltage across the 1.5 ohm resistor and adjusting
for maximum voltage. This should be the optium setting where efficiency
is highest and maximum power is extracted from the solar array. Regulation
is not used so the optimum setting may need adjustment as conditions change,
lighting conditions, temperature, battery voltage, etc. Some loss occurs in
the 1.5 ohm resistor, maybe 5% which could be reduced with a smaller value
resistor, or a milliamp meter with low resistance.
Most parts are fairly common except maybe the 5mH inductor. I used a ferrite
torroid core from a scrap PC switching PS. The core measures 1.5 inch
outside diameter by 5/8 thick. The windings were removed and replaced with
25 turns of the same 18 gauge wire. The resistance measures 25 milliohms.
The inductor value is fairly tolerant and can be most anything from 1mH
or greater if the resistance is low and the core doesn't saturate with the
peak current. Larger inductors will have lower peak currents but higher
resistance for the same size.