Light emitting diodes (LEDs) are electronic devices that change electrical energy into light energy. The light emitted is at specific wavelengths depending on the material of manufacture. Modification is necessary to produce white light. LED drivers control many factors that affect the performance of the LED and are of critical importance in white LEDs.
An LED driver is analogous to the ballast in fluorescent lighting. The driver converts alternating current (AC) to direct current (DC) if required. It manages the incoming voltage and current to the voltage and current level requirements of the LED. Electronically, the driver is a small integrated circuit (IC).
LEDs were initially used as signal indicators; a typical application being a power-on indicator on a television. In order for LEDs to compete as a source of general illumination with incandescent lighting, they must be able to produce a quality, consistent white light and be dimmable. The choice of LED driver is critical for white light applications.
The requirements that must be considered in specifying an LED driver depend on the planned use of the LED. LEDs are current-driven and experience a large drop in illumination with a small decrease in current. A constant-current driver removes variations in the input current by regulating the voltage across a current-sense resistor. The value of the reference voltage and the resistor determines the LED current. LEDs sharing the same driver should be connected in series to maintain a constant current.
Constant-current devices require overvoltage protection. The current output is also constant, and if the resistance increases down circuit from the LED, the constant current can cause the voltage to rise beyond the voltage rating for the LED or other discrete components. Overvoltage protection is provided by zener diodes, which may be thought of as a reverse fuse, in parallel to the LED. When the overvoltage condition exists, the zener diode starts to conduct electricity. An alternative to the zener diode approach is to monitor the output voltage and shut down the power supply when an overvoltage trip point is met.
Efficiency in conversion of energy to light is important in LED use, as that is what differentiates LEDs as a viable light source. The amount of input power to LED brightness is the measure of efficiency in LED drivers. There is an inverse relationship between the reference voltage of the power supply and the LED brightness. Input power, managed by the LED driver, with smaller reference voltages result in lower electrical use and less heat buildup.
Dimming of the LED light may be managed by the LED driver by decreasing the input current. This causes a shift in the output color spectrum and requires an analog control signal, which adds another necessary circuit to the design. Pulse-width modulation (PWM) switches the current on and off at very-high frequencies. PWM is used in incandescent light dimmers wherein the current is removed from the same portion of the AC energy wave during each cycle. In the DC environment of the high-frequency LED, the PWM circuits must work at even higher frequencies.
