U.S. Department of Energy - Energy Efficiency and Renewable Energy
Solid-State Lighting
Why Does Thermal Management Matter?
Excess heat directly affects both short-term and long-term LED performance. The short-term (reversible) effects are color shift and reduced light output while the long-term effect is accelerated
lumen depreciation and thus shortened useful life.
The light output of different colored LEDs responds differently to temperature changes, with amber and red the most sensitive, and blue the least. (See graph below.) These unique temperature response rates can result in noticeable color shifts in RGB
-based white light systems if operating Tj differs from the
design parameters. LED manufacturers test and sort (or "bin") their products for
luminous flux and color based on a 15-20 millisecond power pulse, at a fixed Tj
of 25°C (77°F). Under constant current operation at room temperatures and with
engineered heat mitigation mechanisms, Tj is typically 60°C or greater current
operation at room temperatures and with engineered heat mitigation mechanisms,
Tj is typically 60°C or greater.
Therefore white LEDs will provide at least 10% less light than the
manufacturer's rating, and the reduction in light output for products with
inadequate thermal design can be significantly higher.
50% LOSS*
AT 212° F
Continuous operation at elevated temperature dramatically accelerates lumen
depreciation resulting in shortened useful life. The chart below shows the light
output over time (experimental data to 10,000 hours and extrapolation beyond)
for two identical LEDs driven at the same current but with an 11°C difference in
Tj. Estimated useful life (defined as 70% of initial lumen output) decreased
from ~37,000 hours to ~16,000 hours, a 57% reduction, with the 11°C temperature
increase.
However, the industry continues to improve the durability of LEDs at higher
operating temperatures. The Luxeon K2, for example, claims 70% lumen maintenance
for 50,000 hours at drive currents up to 1000 mA and Tj at or below 120°C. (Luxeon
K2 Emitter Datasheet DS51, dated 5/06)
*25°X9/5+32=77°F
100°X9/5+32=212°F
Back to Index
|
