The relationship curve between the forward voltage drop (VF) and the forward current (IF). From the curve, it can be seen that when the forward voltage exceeds a certain threshold (about 2V), which is commonly referred to as the turn-on voltage, it can be approximately considered that IF and VF is proportional. See the table for the electrical characteristics of current main super bright LEDs. It can be seen from the table that the highest IF of current ultra-bright LEDs can reach 1A, while VF is usually 2 to 4V.
Since the light characteristics of LEDs are usually described as a function of current, rather than a function of voltage, the relationship curve between luminous flux (φV) and IF, the use of constant current source drive can better control the brightness. In addition, the forward voltage drop of the LED has a relatively large range (up to 1V or more). From the VF-IF curve in the above figure, it can be seen that a small change in VF will cause a large change in IF, which will cause a greater brightness. Big change.
The relationship curve between LED temperature and luminous flux (φV). The figure below shows that luminous flux is inversely proportional to temperature. The luminous flux at 85°C is half of that at 25°C, and the light output at 40°C is 1.8 times that at 25°C. The temperature change also has a certain influence on the wavelength of the LED. Therefore, good heat dissipation is the guarantee for the LED to maintain a constant brightness.
Therefore, the use of constant voltage source drive cannot guarantee the consistency of LED brightness, and affects the reliability, life and light attenuation of the LED. Therefore, ultra-bright LEDs are usually driven by a constant current source.