Design of low-loss circuit for powering solar lamps

　　The solar lamp circuit shown in Figure 1 is a low-loss circuit that uses a 7W four-pin CFL (compact fluorescent lamp) and a 12V, 7-Ahr sealed maintenance-free battery. The inverter has an efficiency of more than 85% and a quiescent current of less than 2mA. It has a shunt charge controller with battery over-discharge protection and over-charge protection. The low quiescent current, over-discharge protection, and over-charge protection ensure a long battery life. The inverter's preheating function prevents the CFL terminals from blackening, thereby extending its service life. This circuit can be used as a reliable, compact portable light source in rural areas and as an emergency light system in cities. The shunt charge controller circuit includes IC1 (low-current 2.5V voltage reference LM385) and IC2 (LM324 comparator). IC2A with resistors R1 to R8 and transistor Q1 prevents the battery from over-discharging.

 

　　When the battery voltage is below 10.8V, the circuit cuts off the load (inverter and lamp) to prevent over-discharge of the battery. Under no-load conditions, the voltage of the battery after discharge is about 12.2V, so to prevent oscillation, the circuit provides an over-discharge reset voltage of 12.3V. Red LED LED1 indicates low voltage status. IC2B with resistors R9 ~ R14 and transistor Q2 prevents overcharging of the battery. When the battery voltage exceeds 14.8V, Q2 turns on and bypasses the solar array to prevent overcharging of the battery. When the battery voltage is below 12.5V, Q2 turns off and the solar array charges the battery. D2 is a reverse blocking diode. It prevents the battery from discharging to the solar cell when the solar cell is not generating power. Yellow LED LED2 indicates that the battery is fully charged. Green LED LED3, together with IC2c and resistors R15 ~ R20, provides charging indication.

　　Tables 1, 2, and 3 list the core and winding data of the magnetic components in the circuit. The inverter uses a D-class push-pull external drive circuit with MOSFET tubes as switching devices. IC3 (SG3524) drives this inverter. The external drive structure can ensure trouble-free starting of the circuit under various environments. The
switching frequency is about 26kHz. Q6, together with resistors R29, R30, R31 and capacitor C10, form a preheating circuit. In addition to the 12V, 7-Ahr sealed maintenance-free battery, the circuit also uses a 10W, 12V monocrystalline silicon solar panel. For a fully charged battery, the recorded backup time is about 8 to 10 hours, and the light output using a 7W four-foot fluorescent lamp is 370 lumens.

Author: Ramsesh Kumar, Bang galore, India