13 July 2008

Heat is the great enemy of LEDs

Heat is the great enemy of Light emitting diodes (LEDs). This is ironic, since conventional bulbs produce light by heating a filament to such a high temperature that it glows. The most common way for LEDs to fail is by gradual decrease of light output and loss of efficiency. However, sudden failures can occur as well. All caused by excessive heat.

Driving power LEDs on constant current
LEDs use a different principle than incandescent or fluorescent sources to create light. LEDs are semi-conductors diodes that emit light when traversed by a current flow. LED diodes have polarity and, therefore, current only flows in one direction. A photo emission is taking place at the diode junction region when a DC low-voltage, constant current power is applied. Driving power LEDs is relatively simple as, unlike fluorescent or discharge lamps, they do not require an ignition voltage to start. Simply put, too little current and voltage will result in little or no light, and too much current and voltage can damage the light emitting junction of the LED. Consequently, to ensure a proper functioning of a LED light source we need some sort of power supply regulation.

When looking at a typical power LED forward voltage vs. forward current chart, we clearly see that, for a given junction temperature, a small variation of the forward voltage produces a large variation in the forward current. Conversely, as the junction temperature increases, the forward voltage across the LED drops as depicted on the forward voltage vs. junction temperature chart.

If we drive power LED light sources with a regulated constant voltage power supply, the forward current passing through the LED will increases as a result of a forward voltage drop, and in turn will generates additional heat in the junction. Ultimately, if nothing limits the current, the LED junction will fail by over heating.

Instead, by driving power LED light sources with a regulated constant current power supply, the light output and lifetime issues resulting from variation of the forward voltage can be eliminated.

Driving power LEDs for clean light
Luminous characteristics of power LEDs are specified for a specific forward current and a 25°C junction temperature. However most LED light sources are operated well above 25°C, and the “true” light output should be based on the anticipated operating junction temperatures.

As illustrated on the relative luminous flux vs. forward current chart, the light output of increases when the forward current increases. However the efficacy of the light source, expressed in lumens per watt, is adversely affected. Conversaly, the light output from LED light sources decrease with increasing junction temperature, as depicted on the relative luminous flux vs. junction temperature chart.

Therefore, when designing for specific light output, efficacy levels, wavelengths or color temperature, it is important to consider the effects of temperature and to maximize the thermal management of the application.

For a specific LED light source, the forward current may be chosen up to the maximum current recommended by the manufacturer. Driving LED light sources above that maximum may result in lower lumen maintenance or, with excessive currents, catastrophic failure.

High forward currents at elevated temperatures can cause diffusion of metal atoms from the electrodes into the junction’s active region, decreasing the radiative capacity through the creation of dislocations and point defects that produce heat instead of light. High-power LEDs are susceptible to current crowding, non homogenous distribution of the current density over the junction. This may lead to creation of hot spots in the junction, and increases the risk of thermal runaway.

When the epoxy resin used in packaging reaches its glass transition temperature, it starts to expand rapidly, causing mechanical stresses on the semiconductor and the bonded contact, weakening it or even tearing it off.

Higher junction temperatures resulting from increased power dissipation or changes in ambient temperature can have a significant effect on light output. Red and Amber AlInGaP phosphors are more sensitive to temperature effects than Blue and Green InGaN phosphors. Depending on the phosphor type, wavelengths can typically increase from 0.03 to 0.13nm/°C. White LEDs often use one or more phosphors. The phosphors tend to degrade with heat and age, losing efficiency and causing changes in the produced light color and slight shifts in color temperature. Similarly, some materials of the plastic package tend to yellow when subjected to heat, causing partial absorption, and therefore loss of efficiency, of the affected wavelengths.

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Form is the visual shape of mass and volume. Light makes form legible. There is no form without light.


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