This is a simple NiCd battery charger
powered by solar cells. A solar cell panel or an array of solar cells
can charge a battery at more than 80 % efficiency provided the available
voltage exceeds the ‘fully charged’ battery voltage by the drop across
one diode, which is simply inserted between the solar cell array and the
battery. Adding a step-down regulator enables a solar cell array to
charge battery packs with various terminal voltages at optimum rates and
with efficiencies approaching those of the regulator itself. However,
the IC must then operate in an unorthodox fashion (a.k.a. ‘Elektor
mode’) regulating the flow of charge current in such a way that the
solar array output voltage remains near the level required for peak
power transfer. Here, the MAX639 regulates its input voltage instead of
its output voltage as is more customary (but less interesting).
The input voltage is supplied by twelve amorphous solar cells with a
minimum surface area of 100 cm2. Returning to the circuit, potential
divider R2/R3 disables the internal regulating loop by holding the V-FB
(voltage feedback) terminal low, while divider R1/R2+R3 enables LBI
(low battery input) to sense a decrease in the solar array output
voltage. The resulting deviation from the solar cells’ peak output power
causes LBO (low battery output) to pull SHDN (shutdown) low and consequently disable the chip. LBI then senses a rising input voltage, LBO goes high and the pulsating control maintains maximum power transfer to the NiCd cells.
Current limiting inside the MAX639 creates a ‘ceiling’ of 200 mA for
I out. Up to five NiCd cells may be connected in series to the charger
output. When ‘on’ the regulator chip passes current from pin 6 to pin 5
through an internal switch representing a resistance of less than 1 ohm.
Benefiting from the regulator’s low quiescent current (10 microamps
typical) and high efficiency (85 %), the circuit can deliver four times
more power than the single-diode configuration usually found in simple
solar chargers. Coil L1 is a 100-µH suppressor choke rated for 600 mA.
Author: D. Prabakaran – Copyright: Elektor July-August 2004