Strictly Private and Confidential
Andrew RicePaul Weindorf, Brian Hayden, Kong Lor, Toshiyuki Abe
TFT Computer Aided Engineering (CAE) Thermal Model
Overview
Introduction / Background
Description and Data
Thermal Model Example and Correlation
Conclusion
Questions
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Introduction / BackgroundPower consumption of TFTs is constantly increasing as the luminance level
increases over 1000 cd/.
LEDs utilized for TFT display backlight have efficiencies ranging from 30% to 50%
Radiated power out of the LEDs should be subtracted from the total LED input power to determine actual LED dissipated power
Most of this radiated power if finally absorbed by the backlight and associated films, in the TFT polarizers, color filters and black matrix.
Almost all TFT backlights include a light recycling pre-polarization film
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TFT LCD Cross Section [1] Typical Backlight Enhancement Film Structure
Introduction / Background
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Cross-section of thermal model of 7in TFT
Plastic Inner Frame
Rear polarizer
Al Rear Frame
LEDLED FlexMetal Inner Frame
TFT PCB
Front Polarizer
Light Pipe
Front Glass
Reflector
Color FilterRear Glass
Diffusion Elements
Metal Outer Frame
Description and Data
An Instrument Systems Spectro 320 Scanning Spectrometer with a 150mm integrating sphere was utilized to measure the radiant power
Sequence of measurements was:1. Measure the TFT output power (normally white TFT)2. Remove the TFT and align polarizer laminated to a sheet of glass to the
transmission angle of the DBEF film. Measure the optical output power to measure the absorption loss in the rear polarizer of the display.
3. Measure the optical output power from the backlight with all the BEF films in place.
4. Extract 4 of the LEDs from the TFT and measure the optical output power from the LEDs.
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Description and DataIn order to scale for the port size which is a fraction of the total light output area,
the below table shows the scaling factors that were utilized.
LED power efficiencies can be determined by measurement of the LEDs
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ISP 150 Sphere Port Size 24.5 mm (diam)Port Area 471.43525 mm²
Display Width 154.08 mm
Height 85.92 mmDisp Area 13238.554 mm²
Area Ratio 28.081383 X
LED Data
A thermocouple was soldered to the cathode of LED1 to discover how efficiency changes as a function of LED temperature.
For this LED, about 30% of the LED input power is converted to lighting radiometric power. Other LEDs have been measured at 38% to 47% efficiencies
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Photometric Radiometric Electrical Luminous LEDLED If (mA) Vf lm mW mW Efficiency Temp C
TFT LED 01 20 mA 20.00 2.78 5.71 20.28 55.6 36.5% 29.0TFT LED 01 30 mA 29.98 2.85 8.28 29.57 85.3 34.7% 32.0TFT LED 01 40 mA 40.00 2.91 10.69 38.32 116.3 32.9% 34.0TFT LED 01 50 mA 50.01 2.97 12.94 46.61 148.3 31.4% 36.0TFT LED 01 60 mA 60.04 3.02 15.04 54.42 181.2 30.0% 39.0TFT LED 01 70 mA 70.03 3.07 17.01 61.75 214.8 28.7% 42.0TFT LED 01 80 mA 80.01 3.12 18.85 68.67 249.3 27.5% 46.0TFT LED 02 80 mA 80.00 3.11 18.52 67.66 249.0 27.2%TFT LED 03 80 mA 80.03 3.11 18.61 67.84 249.2 27.2%TFT LED 04 80 mA 80.00 3.08 19.26 69.65 246.0 28.3%
AnalysisAbsorption loss, , within the backlight is 28.8% due to
tremendous amount of recycling and associated losses.
Light coming out of the backlight is elliptically pre-polarized by the backlight DBEF film
Rear polarizer of the TFT will have a transmission rate, , of 64.1% and consequently have an absorption rate, , of 35.9% of the backlight output power .
The color filter absorption rate, , is about 89.3% of the power out the rear polarizer.
The front polarizer has minimal absorption because the light exiting the liquid crystal structure is in a correct polarization for the white state.
Most of the optical power from the LEDs is distributed in the color filter.
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Back
light
TFT
Rear
Pol
arize
r
TFT
Rear
Gla
ss
TFT
Colo
r Filt
er
TFT
Fron
t Gl
ass
TFT
Fron
t Po
lariz
er
LED
PIN
PBL POut
Element T A
% of LED Output Power
Backlight 71.20% 28.80% 28.80%Rear Polarizer 64.10% 35.90% 25.56%Color Filter 10.70% 89.30% 40.76%Front Polarizer 0.00% 100.00% 4.88%Total 100.00%
Thermal Model Example and Correlation
Using the discussed power modeling, the temperatures were compared between a thermal CAE model and the physical part.
Reasonable agreement between the thermal model and measured results is shown.
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30
35
40
45
50
55
60
65
70
TFT BackLower
TFT BackMiddle
TFT Back Top TFT FrontLower
TFT FrontMiddle
TFT Front Top Rear CoverOutside Top
DisplayInternal Air
Rear CoverInside
DisplayOutside Air
LED Flex
Tem
pera
ture
(°C)
Location
Experimental
Thermal Model
Cross Section with Labels
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ReflectorLens LOCA LOCATouch Screen Display Panel
Front polarizerMetal FrameLight Pipe
LED
LED Flex
Metal Inner Frame
TFT PCB
Conclusion
A simple method to account for the LED output light power distribution has been formulated to assist in CAE thermal models.
The TFT color filter plane absorbs a significant amount (41%) of the radiant power from the LEDs which needs to be accounted for in the thermal model.
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References“What is TFT LCD” by toppoly, http://serdis.dis.ulpgc.es/~itis-byp/NotasDeClase/informacion/Material%20Complementario/VIDEO/Toppoly%20-%20LTPS%20Technology.htm
3M Optical System VikuitiTM Dual Brightness Enhancement Film brochure.
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