After decades of technical improvement, LED luminous efficiency has been greatly improved. The efficiency of incandescent lamp and halogen tungsten lamp is 12-24lm / W, that of fluorescent lamp is 50-70lm / W, and that of sodium lamp is 90-140lm / W. most of the power consumption turns into heat loss. The light efficiency of LED has reached 120-200 LM / W, and its light has good monochromatic, narrow spectrum, and can directly emit colored visible light without filtering. At present, countries around the world are still stepping up the research on improving LED luminous efficiency, and its luminous efficiency will be greatly improved in the future.
1. Requirements of LED driving circuit
According to the voltage and current variation characteristics of LED, at present, LED lamps produced by many manufacturers adopt constant voltage drive, which is nothing more than using resistance and capacitance step-down, and then adding a voltage stabilizing diode to stabilize voltage, or using switching power supply to supply power to LED, which has great defects in the way of driving LED. The first is inefficiency. Secondly, the ability of stabilizing voltage is poor. Therefore, the best way to drive LED is constant current drive. Constant current power supply can eliminate the current change caused by the change of forward voltage. Therefore, constant LED brightness can be generated.
2. LED driver circuit
2.1 circuit principle
Based on the electrical characteristics of high-power LED and the principle of constant current source circuit, this paper designs a LED driving circuit based on integrated circuit module, and its principle block diagram is shown in Figure 1. According to the block diagram of the system, the chip should have not only oscillation timing circuit and comparator feedback, but also switch and reference voltage.
Fig. 1 structure diagram of illumination acquisition terminal
2.2 IC selection
Although the linear constant current drive circuit is simple, less components, low cost, high constant current accuracy, reliable and so on, it has many inherent disadvantages in linear state, load change adaptation, power supply convenience and so on. The switch constant current drive circuit can effectively solve the above shortcomings, so this paper chooses the bipolar linear integrated circuit mc34063a to design the drive circuit.
3. Mc34063a driving circuit
Mc34063a provides a voltage stabilized power supply suitable for portable electronic products, which has the following characteristics: high output precision (up to ± 0.04V); simple circuit, few peripheral components; high circuit conversion efficiency (up to 74%); can output 5-12v arbitrarily set voltage.
3.1 circuit design
Based on the negative feedback control theory and the principle of switching power supply, a switching constant current circuit is designed in this paper. The circuit principle is shown in Figure 2.
Figure 2 Schematic diagram of LED driving circuit based on mc34063a