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
  16 volts.

  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.