The relationship between forward voltage drop (VF) and forward current (IF). It can be seen from the curve that when the forward voltage exceeds a certain threshold (about 2V), that is, the so-called turn-on voltage, it can be approximated as IF and VF is proportional. See the table for the electrical characteristics of the current main super bright LEDs. It can be seen from the table that the highest IF of the current super bright LED can reach 1A, and the VF is usually 2 to 4V.
Since the light characteristics of LEDs are usually described as a function of current, not as a function of voltage, the relationship between luminous flux (φV) and IF, therefore, the use of constant current source driving can better control brightness. In addition, the forward voltage drop of the LED varies widely (up to 1V or more), and the VF-IF curve in the above figure shows that a small change in VF causes a large, IF change, which causes a comparison of brightness. Big change.
The relationship between the temperature of the LED and the luminous flux (φV) is shown by the following figure. The luminous flux is inversely proportional to the temperature. The luminous flux at 85 °C is half that at 25 °C, and the light output at 18 °C is 1.8 times that at 25 °C. The change in temperature also has a certain effect on the wavelength of the LED. Therefore, good heat dissipation is a guarantee that the LED maintains a constant brightness.
Therefore, the use of constant voltage source drive can not guarantee the consistency of LED brightness, and affect the reliability, life and light decay of LED. Therefore, super bright LEDs are usually driven by a constant current source.