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Annals of the Assembly for International Heat Transfer Conference 13

ISBN 1-56700-225-0 / CD 1-56700-226-9

Volumes per year:

various

Year 2006

DOI: 10.1615/IHTC13.p27

NUMERICAL ANALYSIS OF HEAT TRANSFER FROM AN LED DIE IN DUTY OPERATION

J. S. Park
Micro Display Group, Digital Display Research Laboratory, LG Electronics, Korea

K. S. Bok
Micro Display Group, Digital Display Research Laboratory, LG Electronics, Korea

S. G. Lee
Micro Display Group, Digital Display Research Laboratory, LG Electronics, Korea

ABSTRACT

LED (Light Emitting Diode) is a fascinating light source. Not only the wide color gamut, but also the high brightness makes it competitive to other lightings or illuminators. However, there is a problem, that is, the brightness of LED strongly depends on its current and junction temperature. As the current increases, the luminous flux per unit power decreases steeply. And the luminous flux linearly decreases with junction temperature. Consequently, the LED lighting mechanism as well as its thermal characteristic should be examined before employing power LEDs in high bright systems that use high electric current. Generally, a display makes moving pictures by means of mixing red, green and blue colors in a pixel. For an LED rear projection TV, the prescribed time fraction for each color is assigned and each LED is on only in this time schedule (e.g. red : green : blue = 25 % : 50 % : 25 %). Namely, LEDs are in the duty operation.
For the rear projection TV, high operating frequency, which is higher than 240 Hz, is commonly used. When a system repeats on/off very fast, it can be assumed that it is in thermal equilibrium state. However, the chip size of LED is so tiny (∼1 mm x ∼1 mm) that it has much low heat capacity. In the present study, the thermal characteristics of LED are numerically investigated when it is in duty operation. Also, thermal equilibrium conditions are found through the analyses with various operating frequencies (1, 60 and 360 Hz). For the present simulation, the popular Lumileds Luxeon III LED is used. The major results reveal that at 360 Hz, the LED junction temperature is practically predicted by T_junction = T_case + R_j−c × P, where R_j−c is the thermal resistance between junction and case. And P is the dissipated heat. P is equal to duty × i (current) × V (voltage). That is, at high operating frequency, the assumption of the thermal equilibrium of LED die is justified. Also the thermal stress of LED die caused by the fast on/off switching decreases. However, as the LED operating frequency decreases, it is out of the equilibrium state and temperature fluctuation is clearly observed.