DOE Examines Effect Of Heat On LED Performance
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. 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.
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)
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