Optimizing Your LED Design

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LED lighting has established itself as a disruptive technology in the lighting industry. The low power consumption and high reliability of LEDs have made them ideal choices for both new lighting and replacement applications.

Because of the small size of the individual lighting ‘elements’, new and novel lighting fixtures can be designed that were not physically possible with traditional incandescent, fluorescent, and arc lighting. Fixtures designed around multiple light point sources can also offer enhanced ability to control the distribution of light more easily compared to the legacy fixtures that rely on relatively large lighting elements. This has led to wide adoption in residential, commercial, industrial, motorway and parking lot illumination, as well as for architectural lighting and electronic signage.

In order to understand how LED lighting fixtures can operate so efficiently over extended lifespans, it is helpful to understand how they work and the failure modes for LEDs. Like other diodes, LEDs are made from a semiconductor that has been ‘doped’ with impurities such that electrons can easily flow in one direction through the two regions in the diode. However, for LEDs, as electrons flow from one region to the other, they pass through a ‘band gap’ between the regions and energy is released in the form of light. By altering the construction of the LED, the wavelengths and intensities of the emitted light can be controlled. It may seem that this would make it easy for LED lighting designers to create arrays of equally performing LEDs with a known wavelength output, but there is more to the equation. As temperature increases, two things happen: First, the wavelength of the light increases, which slightly changes the color of the emitted light. The second result is more important, though—the forward voltage (the voltage drop that occurs when electrons cross the band gap) decreases. If the LED circuit is fed by a fixed voltage source with current limiting series resistors, this increases the voltage drop across the current limiting resistor and facilitates a rise in the overall circuit current, resulting in brighter but hotter and less efficient LED operation and a further temperature increase and color shift. This cycle is not boundless, however—the forward voltage of the LED can only change so much over temperature— but the shift can result in a meaningful reduction in operating efficiency and a significant increase in LED junction temperatures. The change in efficiency and junction temperature are particularly important, as LEDs only exhibit high reliability when they remain in a relatively low temperature state compared to their maximum operating temperatures (or maximum junction temperatures).

Having established that maintaining a constant temperature is important for long LED life, there are several methods to ensure that LEDs stay at a relatively low operating temperature. The first method is heat sinking, or removing heat from the LED assembly and dissipating it elsewhere. This can add significant costs to a design, however, and the heat sink assembly needs to take lighting fixture ambient conditions into account—particularly in the instance of HVAC failure or other abnormal operating conditions because the maximum current rating for most LEDs is significantly reduced at higher ambient temperatures. Because LEDs powered from a constant voltage source will naturally cause more current to flow as temperature increases, the removal of heat alone may not be sufficient to ensure high reliability in LED lighting systems in hot ambient conditions.

 A better way to ensure that LEDs operate at peak efficiency and remain at good junction temperatures is to ensure that the current supply to the LEDs always remains constant. This can be achieved using constant current LED drivers. Compared to constant voltage LED drivers, constant current drivers maintain a constant current output and adjust their output voltage as needed to maintain the targeted current. These supplies offer much better control of the current through LEDs, and eliminate current variance due to temperature changes. Control of the current as opposed to the voltage provides more precise control of the power dissipated in the LED and helps designers project failure rates much more accurately than when constant voltage supplies are used. 

The choice of an LED driver type can depend greatly on the intended end-use and other constraints. For signage and other applications that rely on vibrant or contrasting colors, a constant voltage supply can be more cost effective and easier to design, and any color shifts will tend to be relative and have a minimal impact on the effectiveness of the viewed light. Efficiency is also less important in signage, and additional heat sinking is more economical when the application is a single installation as opposed to lighting distributed throughout a site.  

When the intended end-use is for illumination, however, constant current drivers can be a better choice.  These allow more uniform control of the light quality and brightness, and systems can be easily tuned to keep LEDs operating in their most efficient range. Running LEDs in their most efficient range typically requires less heat sinking, and the metal from the lighting fixture is usually sufficient to spread the heat and keep the LEDs operating in an ideal and efficient state.  

Understanding the functional differences in driving LEDs with constant current or constant voltage supplies can help a designer optimize their design to meet their light output, light quality, and design longevity targets.

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