Protecting High Brightness LEDs from Thermal Stress
There’s no doubt that the ruggedness and long life of LEDs present clear advantages for a host of different applications. When using high brightness, high power LEDs however, heat dissipation is a serious concern and the fact that 20 percent of power input to an LED is converted into light while 80 percent generates heat means that lighting designers can be faced with a serious problem to overcome.
Since even the best possible thermal design practice can be thwarted by an inappropriate lighting installation, the responsibility of maintaining safe LED operating conditions and minimising the impact of thermal effects on LED life must surely fall to the drive electronics.
A Look at the Specs
A quick review of the product specifications provided by manufacturers of high brightness LEDs serves to identify some key design parameters, which need to be taken into account and to illustrate the negative effects of running such components at high temperatures.
The effective life of the LED is inversely related to the power dissipation and temperature of the LED junction. Manufacturers show MTBF figures of about 100M hours at Tj 80ºC. In practical systems LED failure is not likely to be a problem. However, in systems where heat is not adequately removed and Tj rises to 120ºC or beyond, then LED life will be significantly shortened. In extreme conditions the LED could suffer immediate failure.
Thermal design can certainly apply some overcompensation to take into account worst-case installation scenarios, but in certain instances this may not be possible. Consider a downlight installed in a well-insulated ceiling void. The void not only acts to prevent adequate heat dissipation it also doesn’t offer sufficient housing space to fit a suitable heatsink.
Relative light intensity is inversely related to junction temperature. While data sheets vary, manufacturers quote a reduction of light output of up to 30 percent at maximum junction temperature.
inversely related to junction temperature. An LED can typically lose 30 percent of its light output over 50k hours when run at 70ºC junction temperature presumably the reduction is greater at higher temperatures, but figures are not published.
In reality, the reduction in light output over time—whatever the cause—is actually not a great issue. In fact it may not be all that noticeable and in any case LED performance will be comparable with that experienced with alternative light sources.
By Alan Buxton, Marketing Manager and Ho Wong, Product Marketing Manager, Zetex Semiconductors
There’s no doubt that the ruggedness and long life of LEDs present clear advantages for a host of different applications. When using high brightness, high power LEDs however, heat dissipation is a serious concern and the fact that 20 percent of power input to an LED is converted into light while 80 percent generates heat means that lighting designers can be faced with a serious problem to overcome.
Since even the best possible thermal design practice can be thwarted by an inappropriate lighting installation, the responsibility of maintaining safe LED operating conditions and minimising the impact of thermal effects on LED life must surely fall to the drive electronics.
A Look at the Specs
A quick review of the product specifications provided by manufacturers of high brightness LEDs serves to identify some key design parameters, which need to be taken into account and to illustrate the negative effects of running such components at high temperatures.
The effective life of the LED is inversely related to the power dissipation and temperature of the LED junction. Manufacturers show MTBF figures of about 100M hours at Tj 80ºC. In practical systems LED failure is not likely to be a problem. However, in systems where heat is not adequately removed and Tj rises to 120ºC or beyond, then LED life will be significantly shortened. In extreme conditions the LED could suffer immediate failure.
Thermal design can certainly apply some overcompensation to take into account worst-case installation scenarios, but in certain instances this may not be possible. Consider a downlight installed in a well-insulated ceiling void. The void not only acts to prevent adequate heat dissipation it also doesn’t offer sufficient housing space to fit a suitable heatsink.
Relative light intensity is inversely related to junction temperature. While data sheets vary, manufacturers quote a reduction of light output of up to 30 percent at maximum junction temperature.
inversely related to junction temperature. An LED can typically lose 30 percent of its light output over 50k hours when run at 70ºC junction temperature presumably the reduction is greater at higher temperatures, but figures are not published.
In reality, the reduction in light output over time—whatever the cause—is actually not a great issue. In fact it may not be all that noticeable and in any case LED performance will be comparable with that experienced with alternative light sources.
By Alan Buxton, Marketing Manager and Ho Wong, Product Marketing Manager, Zetex Semiconductors
