LED driver circuit introduction

In addition to meeting the safety requirements of the LED driver circuit, there are two other basic functions. One is to maintain the constant current characteristics as much as possible, especially when the supply voltage is Â±15%, the output current should be kept at Â±10. % range changes. Second, the driver circuit should maintain its own low power consumption, so that the system efficiency of the LED can be kept at a high level.

Traditional low efficiency circuits:

figure 1

Figure 1 is a traditional low-efficiency circuit. The grid power supply is stepped down by a step-down transformer. After bridge rectification and filtering, the three LEDs are stabilized by resistor current limiting. The fatal disadvantage of this circuit is that the existence of resistor R is necessary. The active loss on R directly affects the efficiency of the system. When the R partial pressure is small, the voltage drop of R accounts for 40% of the total output voltage, and the active loss of the output circuit on R has accounted for 40%, plus the transformer. Loss, system efficiency is less than 50%. When the supply voltage fluctuates within Â±10%, the current flowing through the LED will change by â‰¥25% and the power variation on the LED will reach 30%. When the R partial pressure is large, when the power supply voltage fluctuates within Â±10%, the power variation to the LED can be reduced, but the system efficiency will be lower.

figure 2

Figure 2 is based on Figure 1 with an integrated voltage regulator MC7809, so that the output voltage is basically stable at 9V, the current-limiting resistor R can be used very little and will not overload the LED due to the instability of the power supply voltage. However, in addition to ensuring a substantially constant output of the LED, this circuit is still very inefficient. Because the voltage drop across the MC7809 and R1 is still a large percentage, its efficiency is only about 40%.