Without successful thermal management, LEDs’ light output, color and efficacy can quickly degrade.
Thermal design and management
Fortunately, the solid-state industry takes thermal challenges seriously. It’s now producing brighter and bigger LEDs that can carry more electrical power before suffering burnout. The progress stems from increased demands for HB-LEDs, and, also, brighter, large-area LEDs (for numerous applications, including signage) have spurred manufacturers’ engineers to pursue ongoing, technological improvements.
To conquer thermal problems, LED engineers must resolve how to reduce TJ dependence on the operating (drive) current, which, in turn, will lead to increasing LED lamp lifespan.
The junction temperature is measured at the diode’s active region (the compound semiconductor material in inorganic LEDs), which is the hottest point in the entire LED module or luminaire, if the module is integrated in a luminaire.
No doubt, future design improvements in semiconductor epitaxial quality, LED-device design, metallization and other, related processes will lead to substantially lower TJ dependence on operating current. A lower operating current significantly increases LED lamps’ lifetimes; lower currents also simplify thermal management and reduce design and manufacturing costs.
Stronger TJ dependence in drive current may lead to faster and greater degradation of color quality, due to wavelength shifting, which becomes more pronounced over time.
When higher illumination is preferred, it’s often better to increase the number of individual LEDs (provided there’s space) and, simultaneously, reduce the drive current to generate less heat (see ST, April 2009, page 58).
Every LED module, due to its particular design, material properties and fabrication style, has an inherent maximum TJ, labeled TJMAX. Overdriving this figure will lead to catastrophic failure. Oppositely, the best injected current is when the TJ is 20% (typically) below the TJMAX. Additionally, thermal management must apply to the entire fixture or enclosure, because limiting the maximum operating temperature of all module components duly applies to the entire luminaire. You can also extend an LED lamp’s lifespan by allowing significant heat removal from the junction area.
Without thermal management, an LED chip continues to heat up and, over time, the TJ will exceed TJMAX, which leads to burn-out.
When sophisticated thermal management incorporates both structural design, packaging and material technology, a higher TJ can be tolerated — if an efficient thermal conduction path (heat sink) is created between the LED junction area and the circuit board assembly upon which it is mounted.
However, such designs will increase the module and luminaire cost; it almost always increases the luminaire size. Expensive modules and sophisticated thermal management creates higher unit costs for signmakers, but the cost might diminish if the LED modules generated less heat.
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