Driving high brightness LEDs with switching regulators

The first LEDs produced in the 1960’s were feeble infrared devices that suffered from short lifetimes and poor efficiency. The semiconductor processes used to make LEDs expanded from early silicon carbide (SiC) and gallium arsenic phosphide (GaAsP) devices to gallium phosphide (GaP) and indium gallium phosphide (InGaP) which produced red and later orange, yellow, and greenish-yellow. LEDs at that point were bright enough to be indicators, and even at this early stage they had better luminous efficiency than incandescent light bulbs. Their overall light output was still much too low and their color range too narrow for other uses, however. The concept of general illumination, the lighting of cars and buildings, was limited both by the lumens output and by the lack of a blue LED to complete the RGB spectrum and make white light. Research into SiC led to blue LEDs with luminous flux that was far too weak to be useful. Still, as the number of processes increased, so did the luminous flux, or brightness, the color, and the power dissipation. Gallium aluminum arsenide (GaAlAs) and indium aluminum gallium phosphide (InGaAlAs) LEDs evolved into the first “super bright” LEDs. Early but important steps towards the ideal of general illumination with semiconductor lighting were taken by Nichia Corporation in Japan in the early 1990’s. Research done by Dr. Shuji Nakamura[1] led to the first commercial blue LEDs. Nichia also pioneered efforts to produce a white LED by adding a phosphor to blue LEDs which emits yellow light. Although the white LEDs cast a cool white, they were quickly adopted in clusters as replacements for low light incandescent applications.

LEDs designed for true illumination emerged in the early 1990’s, with devices made by Nichia, Osram Opto-Semiconductor, and Lumileds. These devices were rated in lumens, common to light fixtures, as opposed to candela, which is normally used for single point light sources. The new High Brightness LEDs (HBLEDs) were also packaged like power semiconductors, using surface mount technology and thermal pads. Standard LEDs encapsulated in epoxy suffer from poor junction to ambient thermal resistance and from loss of light due to the gradual yellowing of the material. The new HBLEDs replaced the epoxy with long lasting silicone based materials.

Single-die, white HBLEDs are available today that deliver 30-40 lumens at a dissipation of 1W. RGB LEDs with three or more HBLED dice placed in a single power package deliver as much as 200 lumens. HBLED designs are used or are under development in automotive, industrial, and commercial lighting, as well as in backlighting for LCD monitors and televisions, as the actual pixels in outdoor/stadium video screens, and in optical communication.

Constant current sources
Regardless of type, color, size or power, all LEDs work best when driven by a constant current source. Light output, measured in lumens, is proportional to current, and hence LED manufacturers specify the characteristics (such as lumens, beam pattern, color) of their devices at a specified forward current, IF, not at a specific forward voltage, VF. LEDs are PN junction devices with a steep I-V curve, hence driving an LED with a voltage source can lead to large swings of forward current in response to even the smallest changes in voltage.

Most power supply ICs are designed to provide constant voltage outputs over a range of currents, (Figure 1a) and it is not always straightforward to adapt a voltage regulator to provide constant current. With an array of more than one LED, the main challenge is to match the drive currents through each LED. Placing all the LEDs in a series string is a common way to ensure that exactly the same current flows through each device.

Figure 1a: Constant voltage regulator

Figure 1b: Constant current regulator