As a third-generation illumination source, LED has the advantages of low operating voltage, low power consumption, high luminous efficiency and long life. LED is a nonlinear device. When the LED is turned on, as long as the voltage on the LED changes slightly, Excessive current will cause the LED device to be damaged by heat. The working characteristics of the LED are highly dependent on the quality of the power supply, so achieving a high-quality power supply improves the lighting quality, power utilization, and longevity of the LED. It has important significance. The stability of the power supply depends mainly on the LED drive circuit design. The constant current source drive is the best LED drive mode. With constant current source drive, the current flowing through the LED will not be affected by voltage and ambient temperature. As well as the influence of the discreteness of the LED parameters, the current can be kept constant, and the various excellent characteristics of the LED can be fully utilized. Currently, the widely used constant current source has two forms: one is a linear power supply improved constant current source, and the other is Switching power supply constant current source. Linear power supply improved constant current source with large linear loss and small application range; reliable for switching power supply constant current source Poor, small adaptability, and high cost. Therefore, the economical, practical and reliable digital constant current source has been widely used. This paper designs some shortcomings for the driving mode of small power LED in the existing lighting system. An efficient drive system, a corresponding new drive system is proposed.
1 LED characteristics
1.1 LED volt-ampere characteristics
The mathematical model of the volt-ampere characteristics of the LED can be expressed as
IF=V+RS+△VF/△T (T-25°C) (1)
Where V is the LED starting voltage; RS is the volt-ampere curve slope; IF is the LED forward current; T is the ambient temperature; ΔVF/ΔT is the temperature coefficient of the LED forward voltage, for most LEDs, it The typical value is -2V/°C. From the mathematical model of the LED, a small change in the forward voltage of the LED after a forward conduction will cause a large change in the LED current under certain ambient temperature conditions.
1.2 LED temperature characteristics
The magnitude of the LED forward current varies with temperature. The operating current of a white LED is generally around 200 mA. When the ambient temperature exceeds 50 ° C, the allowable forward current of the white LED will decrease and the normal luminance will not be achieved. The required operating current, in this case, if a large current is still applied, it is easy to age the white LED.
1.3 LED optics
The luminous flux of a light source refers to the visible light energy passing through a solid angle of 4Ï€ per unit time. The relationship between white LED current and luminous flux is shown in Figure 1. As the current increases, the luminous flux of the LED increases nonlinearly and gradually becomes saturated. The main reason is that as the current and time increase, the internal temperature of the high-power LED rises, and the probability of carrier recombination occurring in the P/N junction region decreases, resulting in a decrease in LED luminous efficiency.