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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