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.

Appreciating LED useful life time

With Light emitting diodes (LEDs), outright failure is very unlikely. Contrary to conventional lighting sources which typically fail suddenly or burnout, LEDs are solid state electronic components and as such gradually degrade. But because of their long expected lifetimes, conventional light sources’ life testing is impractical to estimate the useful life of LEDs.

Predicted Life Time
Useful life of conventional lighting sources is commonly expressed as the time to failure. This primary metric is based on the time it takes for 50% of lamps to fail. LEDs, being replaceable semiconductors, are using “mean time to failure” (MTTF) to express their failure rate. This is a well-defined statistical reliability metric commonly used in the electronics industry.
Power LED manufacturers typically predict high brightness LED MTTF to be on the order of 50.000 - 100,000 hours, provided LEDs “are properly packaged and used in accordance with manufacturers’ recommendations”.

Translating these durations in plain English, a LED light source would have an average life time between 5 and 12 years if left on all day. For “normal” general lighting usage, even when considering a 12 hours average daily usage, this would translate into an average life time comprised between 12 and 24 years. A very long life times indeed.

Average Lumen Maintenance
Even when operated within the manufacturers’ specifications, both conventional and LED light sources experience loss of light over time. This is known as lumen depreciation, and is typically expressed as lumen maintenance, i.e. the percentage of initial lumens remaining after a specified period of time.
If you have ever changed a light bulb, you have certainly noticed how bright the new bulb is compared to the older bulb, then you have seen the effects of lumen depreciation. Lumen depreciation in incandescent lamps mainly occurs by depletion of the filament over time and accumulation of evaporated tungsten particles on the bulb wall. In fluorescent lamps, it occurs by photochemical degradation of the phosphor coating and the glass tube, and by accumulation of light-absorbing deposits within the lamp over time.

LEDs also experience lumen depreciation, but many factors can influence light degradation, such as ambient temperature and humidity, drive current and thermal management. That said, the primary cause is heat generated at the LED junction. High junction temperatures accelerate degradation in lumen maintenance, but also result in a temporary reduction in luminous flux. Contrary to other light sources, LEDs do not emit heat as infrared radiation, so the heat must be removed from the component by conduction or convection. If a LED application has inadequate means of removing the heat, such as heat sinking, the temperature will rise and light output will decrease.

Let’s translate this into plain English. For general ambient lighting applications, the commonly admitted light output decline is set at 70% of initial lumens. If for example we are considering LEDs that deliver 70% lumen maintenance at a 50,000 hours rated life, it means we should expect to receive 70% of the initial lumens after 50,000 hours.

Lumen depreciation has to be taken into account when appreciating the “useful” life of very long life components such as LEDs. In short, the “useful” life time of a LED results from a combination of the MTTF and the lumen degradation. From an application perspective, a catastrophic failure of the semiconductor or the lumen performance falling below 70% always boils down to a degraded service. Because of the LEDs very high MTTF, LED applications are more likely to falter because of lumen degradation, which in turn is highly dependent on the appropriate thermal management.

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