LEDs are current-driven devices that require current limiting when driven by a voltage source. In most applications, it is desirable to drive LEDs with a constant-current source. The current source is used to regulate the current through the LED light regardless of power supply (voltage) variations or changes in forward voltage drops.
LEDs are no longer just used for providing the pretty red and green indicator lights on electronic equipments. When driven properly, a power LED lights can last up to 100,000 h without degradation in the light output. The typical efficacy of power LED, measured in lumens per watt, is between 50 and 80 lm/W. This is several times greater than incandescent light sources and comparable with fluorescent lights. Since the LED is a solid-state device, it can withstand shock and vibration that would damage a filament bulb or glass envelop.
For a highly efficient LED light control, LEDs must be driven with a constant-current source. Most LED without premature failures. An LED could be driven with a linear voltage regulator configured as a constant-current source. However, this approach is not practical for higher power LEDs due to power LEDs due to power dissipation in the regulator circuit. A switch-mode power supply (SMPS) provides a much more efficient solution to drive LEDs.
