Circuit Detail: Circuit for Online Trickle Charging Using Solar Cells

The circuit in this article solves this problem by using a self-oscillating boost circuit that works at a single solar cell voltage and can continuously trickle charge the battery.

Many battery -powered devices have short working time and low standby power consumption, so the total power consumption of these devices is very small. It is very convenient to use solar cells to charge the power batteries in these devices, but for a given battery voltage, the number of solar cells required for charging is close to Vb/0.6, so a group of solar cells have to be connected in series.

The solar cell charges C2 until the voltage on C2 turns Q2 on. At this point, U1a outputs a high level and is maintained by U1b. The output voltage of U1a turns on Q3, which begins to deliver the power from C2 to L1. The timing circuit R4-C4 determines the duration of the high level output of U1a, which determines the time Q3 charges the inductor. This time should be less than one-quarter of the cycle time of the resonant frequency of L1 and C2, so that Q3 can be turned off before the transformer current reaches its peak. After Q3 is turned off, the inductor current is forced to flow through D3 into the large capacitor C1. The capacitance of C1 should be larger than C2 to reduce ripple.

Resistor R8 and capacitor C5 form a filter to filter the charging current to reduce the ripple caused by the switch. If the power battery is disconnected, Zener diode D5 limits the voltage to a safe value. The standby current of the C NAND gate and the 1MΩ resistor is very small, so the circuit is only charged when the current of the solar cell is greater than the leakage current flowing through C2, R7 and Q3.

This circuit provides a trickle charge to the power battery as long as the solar cell current is greater than the leakage current through C2, R7 and Q3.