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AN1149-4 Datasheet(PDF) 3 Page - Lumileds Lighting Company |
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AN1149-4 Datasheet(HTML) 3 Page - Lumileds Lighting Company |
3 / 14 page 3 Change in Dominant Wave-length (Color) as a Function of Junction Temperature The junction temperature of LEDs also affects their dominant wavelength, or perceived color. The equation for dominant wavelength, λd , as a function of temperature is: Where: λd (T 1)= dominant wavelength at junction temperature T 1 λd (T 2)= dominant wavelength at junction temperature T 2 A rule that is easy to remember is the dominant wavelength will increase one nanometer for every 10 °C rise in junction temperature. In most designs of red automotive signal lamps, this change in color is not important because the allowed color range is very large (approximately 90 nm). However, for some amber automotive signal lamps, this color shift can be a concern and should be accounted for where the allowed color ranges are small (approximately 5 to 10 nm depending on the regional specifications). Temperature-Induced Failures of LEDs LEDs are typically encapsulated in an optically clear epoxy resin. At a certain elevated temperature, known as the glass transition temperature, T g, these epoxy resins transform from a rigid, glasslike solid to a rubbery material. A dramatic change in the coefficient of thermal expansion (CTE) is generally associated with the T g. The Tg is calculated as the midpoint of the temperature range at which this change in CTE occurs, see Figure 4.2. To avoid catastrophic failure of LED packages, the junction temperature, T j , should always be kept below the T g of the epoxy encapsulant. Lumileds specifies a maximum junction temperature, T j (max) , which is below the Tg of the epoxy encapsulant used. For SuperFlux LEDs, T j (max) = 125 °C. If the T j (max) is exceeded, the CTE of the epoxy encapsulant will permanently and dramatically change. A higher CTE causes the epoxy encapsulant to expand and contract more during temperature changes. This causes more displacement of the wire bond within the LED package, resulting in a premature wear out and breakage of the wire. Wire bond breakage results in an open failure. Figure 4.2 Expansion-Temperature relationship for clear, epoxy, LED encapsulants. |
Similar Part No. - AN1149-4 |
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Similar Description - AN1149-4 |
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