1 3 Heat Mass Transfer (2017) 53:1619–1630 DOI 10.1007/s00231-016-1925-z ORIGINAL Thermal performance of direct illumination high‑power LED backlight units with different assembling structures Yiwei Wang 1 · Jiwen Cen 1 · Wenjiong Cao 1 · Fangming Jiang 1 Received: 4 February 2016 / Accepted: 4 October 2016 / Published online: 12 October 2016 © Springer-Verlag Berlin Heidelberg 2016 K Temperature factor of LED chip (mV/°C) L Characteristic size (m) M The horizontal length (or width) of the simu- lated backlight unit (m) N The vertical height of the simulated back- light unit (m) G Gravity acceleration (m/s 2 ) P r Prandtl number Q Heat generation rate of LED (W) R or R′ Total thermal resistance of LED backlight unit (°C/W) t or t′ Temperature (°C) V Forward voltage (mV) Greek symbols α Surface heat transfer coefficient (W/(m 2 °C)) a v Volume expansion coefficient (°C −1 ) λ Thermal conductivity (W/(m °C) 1 ) Δ Thickness (m) ν Kinematic viscosity of fluid (m 2 /s) Δt Temperature difference (°C) Subscripts a Air e End i Initial j P–N junction m Heat transfer medium p Electric power s Spreading v Volume specific x, y, z Spatial coordinates 0, 1, 2, … 6 Identification of physical quantities Abstract This work presents a detailed study about the heat dissipation performance of direct illumination high- power light emitting diode (LED) backlight units with two different assembling structures, one of which is traditional and the other is new. The traditional structure, referred to by structure-1, consists of multiple LEDs being directly welded to the printed circuit board (PCB), where the PCB is used as a physical support, an electrical connector and also as a heat dissipation medium. The new structure, referred to by structure-2, places the LEDs directly on the cooling boss; in this case the PCB plays mainly the role of an electrical connector. Thermal characteristics related to the two backlight units are analyzed in terms of ther- mal resistance network, numerically simulated and experi- mentally tested. The obtained results by different methods accord with each other reasonably well and all indicate that both structures can meet the requirements of heat dissipa- tion for backlight units at an ambient temperature of 30 °C. Among the two structures, the LED junction temperature of structure-1 backlight unit is 7–8 °C higher and the tempera- ture distribution in the back plane of the backlight unit is also more uniform. List of symbols A Area (m 2 ) G r Grashof number * Jiwen Cen * Fangming Jiang fm_jiang2000@yahoo.com 1 Laboratory of Advanced Energy Systems, CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), 2 Nengyuan Rd, Wushan, Tianhe District, Guangzhou 510640, China