Before conducting experiments, the placement of sensor ought to be taken into consideration in advance. The measured results of sensing voltage of RGB LEDs and height of sensor above LEDs arrays are shown in Fig. 4-2. The sensitivity at 5 cm height is high because the absolute value of slope is large. In regard to intensity and sensitivity of sensing voltage, 5 cm height above the center of LED array is found preferable than others. As a result, 5 cm is chosen to be the spacing of sensor placement perpendicular to surface of LEDs arrays.
Sensing vlotage VS. Spacing of detector
0
Spacing of detector above the LED's (cm)
Sensing voltage(Volts)
Red LED Green LED Blue LED
Fig. 4-3 Sensing voltage of sensor VS. Height of detector above the LED’s 4.4 Duty Ratio
Because of using PWM signals to drive LED, the duty ratio becomes another factor to investigate. 20 mA, the default driving current of LED, has been chosen according to data sheets of LEDs. Therefore, various duty ratios are applied to evaluate the differences of LED’s performance. The effective driving current is the same as 20 mA. If 200 mA is applied to drive LED at 10% duty ratio, the effective driving current is equivalent to apply 40 mA at 50% duty ratio.
The calculation is listed below:
200 mA @ 10% duty = 40 mA @ 50% duty
Eq. (4-1) 200 × 10% = 40 × 50% = 20
In terms of duty ratio, color coordinate u’, v’ are measured and color difference Δu’v’ are calculated under 120 hours usage. (Figs. 4-3 and 4-4)
Duty Ratio VS Δu'v' Without Feedback
0.001
Fig. 4-4 Color difference Δu’v’ at various Duty Ratios without feedback control system
Duty Ratio VS Δu'v' With Feedback
0
Fig. 4-5 Color difference Δu’v’ at various Duty Ratios with feedback control system The performance of reducing color difference is obviously improved when feedback control system is applied. Moreover, RGB LED arrays show similar trends at specific duty ratios whether feedback control system is applied or not. This experimental result is in consistent with Gu’s research.[23] By comparing Fig. 4-3 with Fig. 4-4, it
is found that Red and Blue LEDs have better performance in color difference Δu’v’ at 90% duty ratio, and the Green is at 80% duty ratio.
4.5 Aging condition
LED aging condition and aging environment were simulated under high current driving to investigate if feedback control system contributed to reduce color difference Δu’v’ over time. According to LED’s specification, RGB LEDs are supposed to be driven by 20 mA. LEDs with 100 mA operating current at room temperature 25 ℃ were experimented to simulate HALT (High Accelerated Life Test). [24][25] Light outputs of Red, Green, and Blue LEDs with 100 mA driving current fall down to 85%, 82, and 80% respectively when feedback control system was not applied. According to the calculation formulas of research about HALT (Eq.
(4-2),(4-3)), the usage period of LEDs can be projected. [26] where α denotes decay constant, t denotes projection usage period in Hour scale, I denotes driving current in Ampere scale, and y denotes relative light output. Therefore, RGB LEDs with 120 hours usage under 100 mA can be projected to 3250, 3969, and 4462 hours usage, respectively.
In Figs. 4-5 to 4-10 straight linear curves denote the linear regression lines of measured data. Their functions are shown next to linear curves. These linear regression lines represent the relationships between color difference Δu’v’ and operation time. The slopes which represent the increasing rate of color differences with time are all positive. It is obvious that the slopes of 100 mA driving current are larger than those of 20 mA driving current. And the slopes of color difference of LEDs without feedback control system are always larger than those with feedback control system as well. Compare the linear regression line functions with each other pair by pair, and the coefficient of slope with feedback control system is smaller than
that without feedback control system by at least one order. Which means feedback control system contributes to reduce the increasing rate of color difference of LEDs.
4.5.1 Δu’v’ of Red LED
Δu'v' of Red LED Without Feedback
ΔEr = 4E-05Thr + 0.0031
Fig. 4-6 Color difference Δu’v’ of Red LED Without Feedback control
Δu'v' of Red LED With Feedback ΔEr = 5E-06Thr + 0.0031
Fig. 4-7 Color difference Δu’v’ of Red LED With Feedback control
According to Fig. 4-6, when feedback control system is applied, the color difference Δu’v’ of red LED is kept within 0.004.
4.5.2 Δu’v’ of Green LED
Δu'v' of Green LED Without Feedback
ΔEg = 4E-05Thr + 0.0025
Fig. 4-8 Color difference Δu’v’ of Green LED Without Feedback control
Δu'v' of Green LED With Feedback ΔEg = 3E-06Thr + 0.0035
Fig. 4-9 Color difference Δu’v’ of Green LED With Feedback control
After comparing Fig. 4-7 with Fig. 4-8, When feedback control system is applied, the color difference Δu’v’ of Green LED is reduced and kept within 0.004.
4.5.3 Δu’v’ of Blue LED
Δu'v' of Blue LED Without Feedback
ΔEb = 5E-05Thr + 0.0015
Fig. 4-10 Color difference Δu’v’ of Blue LED without Feedback control
Δu'v' of Blue LED With Feedback ΔEb = 5E-06Thr + 0.0027
Fig. 4-11 Color difference Δu’v’ of Blue LED With Feedback control Blue LEDs shows consistent result in color difference as well. According Figs.
4-9 and 4-10, when feedback control system is applied, the color difference Δu’v’ of Blue LED is reduced and kept within 0.004.
In order to predict the projection usage time of 0.005 color difference, linear regression lines with feedback control system are extended to calculate.
ΔEr = 5E-06 Thr_Red + 0.0031 Linear regression function of Red LED ΔEg = 3E-06 Thr_Green + 0.0035 Linear regression function of Green LED ΔEb = 5E-06 Thr_Blue + 0.0027 Linear regression function of Blue LED Thr_Red, Thr_Green ,Thr_Blue are 380, 500, and 260 respectively.
Fig.4-12 Linear calculation method of aging projection time
As mentioned before, feedback control system keeps the light output brightness Lv within 10% variation. Moreover, LEDs are regarded as linear aging before their brightness decreased by 10%. [24][27] Linear calculation method of aging projection time is shown in Fig.4-11. As mention previously, Known Aging Projection Hours are 3250, 3969, and 4462 for red, green, and blue LEDs respectively. Consequently, color differences of red, green, and blue LEDs are calculated to reach 0.005 in 10291, 16537, and 9667 aging projection hours respectively.
4.6 Temperature dependency
To examine the temperature dependency of the LEDs, RGB LEDs are experimented in the range of 0 °C to 50 °C. These limits were chosen to represent the typical ambient temperature range for the operation of TFT LCDs according to LED manufacturer’s application note. [28] A temperature and humidity controlled system, GTH-800-40-1P (Fig. 4-12) was utilized to provide environment conditions. Chroma meter was placed to measure color coordinate out of chamber when the door of chamber was open.
Fig. 4-13 A temperature and humidity controlled system, GTH-800-40-1P
The driving currents of RGB LEDs needed to be changed because of color shifts due to temperature changes. Feedback control system reacted by adjusting duty ratio of PWM signal. RGB LEDs were experimented at various ambient temperatures with feedback control system or not. Experimental results are shown in Figs. 4-13~18.
According to Figs 4-13~18, the color differences of RGB LEDs with feedback control are invariant in ambient temperature ranges from 0 °C to 50 °C. Comparing to the experimental results without feedback control, no matter what degree ambient
temperature affects the color shift of RGB LEDs, feedback control system keeps color difference of RGB LCDs within 0.004. Moreover, the smaller the LED driving current is, the smaller the color difference of RGB LEDs is. The difference between 20 mA and 100 mA driving current with 14 bits feedback control system lies in recursive current scale. Recursive current scales of 20 mA are 1.2 μA, and 6 μA for 100 mA.
The calculation is listed below.
20 (mA) ÷ 214 = 1.2 (μA)
Eq. (4-3) 100 (mA) ÷ 214 = 6 (μA)
Δu'v' of Red LED after 120 hours driving( If = 20 mA ) y = 7E-05x + 0.0028
y = 2E-06x + 0.0021 0
0.002 0.004 0.006 0.008
0 10 20 30 40
Ambient Temperature (Tamb)/ ℃
Δu'v'
50 Without Feedback
Feedback
Fig. 4-14 Color difference of Red LED in relation to ambient temperature ( If=20mA)
Δu'v' of Red LED after 120 hours driving( If = 100 mA )
Ambient Temperature (Tamb)/ ℃
Δu'v'
0 Without Feedback
Feedback
Fig. 4-15 Color difference of Red LED in relation to ambient temperature ( If=100mA)
Ambient Temperature (Tamb)/ ℃
Δu'v'
50 Without Feedback
Feedback
Fig. 4-16 Color difference of Green LED in relation to ambient temperature ( If=20mA)
Δu'v' of Green LED after 120 hours driving( If = 100 mA )
Ambient Temperature (Tamb)/ ℃
Δu'v'
0 Without Feedback
Feedback
Fig. 4-17 Color difference of Green LED in relation to ambient temperature(If=100mA)
Ambient Temperature (Tamb)/ ℃
Δu'v'
0 Without Feedback
Feedback
Fig. 4-18 Color difference of Blue LED in relation to ambient temperature ( If=20mA)
Δu'v' of Blue LED after 120 hours driving( If = 100 mA )
y = 9E-05x + 0.0058
y = 1E-06x + 0.0035
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014
0 10 20 30 40 5
Ambient Temperature (Tamb)/ ℃
Δu'v'
0 Without Feedback
Feedback
Fig. 4-19 Color difference of Blue LED in relation to ambient temperature(If=100mA) 4.7 White Light analysis
Fig. 4-20 Simulation of White Point Color Difference
White light can be generated by combining blue, red, and green light. Therefore, after taking the color difference variation of red, green, and blue LEDs as 0.005 into consideration, the maximum variation range of white point can be obtained as 0.005.
The color difference variation range is shown in dotted line in Fig. 4-19.
4.8 Δu’v’ VS. Resolution Bits
Bits of Resolution VS. Δu'v'
0
Resolution of feedback system (Bits)
Δ u' v'
Red LED Green LED Blue LED
Fig. 4-21 Color difference Δu’v’ of RGB LEDs at different Resolution bits
When the least significant bit of Analog to Digital or Digital to Analog conversion is neglected, the relationship between resolution digits and color difference Δu’v’ can be obtained in Fig. 4-20. It is found that 13 bits is required for recursive approach to make color difference Δu’v’ within 0.005. Besides, 14 bits resolution bits can make Δu’v’ within 0.004. Whereas the recursive current unit is 2.4 μA for 13 bits resolution, and 1.2 μA for 14 bits under 20 mA driving current. The calculation is listed below.
20 (mA) ÷ 213 = 2.4 (μA)
Eq. (4-4) 20 (mA) ÷ 214 = 1.2 (μA)
Consequently, the color differences of RGB LEDs with feedback control system are dominated by LED driving currents and resolution bits.
4.9 Summary
In order to examine the reliability and accuracy of recursive feedback control system, ambient temperature dependency and aging condition of LED were simulated by applying 100 mA driving current. Besides, relationship between resolution bits and color difference was also investigated. 14 bits Recursive Approach Feedback Control System makes Color difference Δu’v’ be kept within 0.004 under aging simulation of 6000 hours usage. Moreover, 13 bits Resolution Digits is necessary for Recursive approach to make Δu’v’ within 0.005 in LED light output feedback control. 2.4 μA recursive current (13 bits) contributes to 0.005 of color difference Δu’v’. Since this feedback control system is initially designed for 14 bits, 1.2 μA recursive current (14 bits) can further approach 0.004 of color difference Δu’v’ of LEDs. Moreover, when feedback control system is applied, ambient temperature ranges from 0 °C to 50 °C is independent to LED’s color difference. Besides, led driving current is the key for feedback control system to restrict LED’s color difference. The higher the driving current is, the larger the LED’s color difference will be. Consequently, small current driven LEDs are preferable for this feedback control system.
Chapter 5
Conclusions and Future works
5.1 Conclusions
LED lighting has the ability to change the color and dimming level instantaneously in addition to many other advantages such as long lifetime [29][30].
They therefore have great potential in many applications such as LCD backlighting, and video projection. However, LED still has some issues to be resolved. The major issues to be tackled are the control and maintenance of LED light output. We utilized a recursive approach method to deal with color shift of LED after longtime usage. Our feedback control system possesses some features and advantages listed below.
1. A 14 bits RGB LEDs light output feedback control system has been realized and verified. Finally, we proved that feedback control system is independent to ambient temperature ranges from 0 °C to 50 °C.
2. We have demonstrated that 14 bits Recursive Approach Feedback Control System makes LEDs color difference (Δu’v’) of Red, Green, and Blue LEDs within 0.005 under aging simulation of 10291, 16537, and 9667 hours usage and white point is in 0.005 color difference variation range. Such performance is competitive to the 5000 hours lifetime standard of 5.7 inch TFT LCD with white light LED backlight.
[10]
3. Instead of using many mathematic transformations as prior art mentioned in section 1.3, this feedback control system provide an illustration to resolve LED color shift issue in accurate current control domain. Moreover, this feedback control system has simpler system configuration than prior art and features comparable
performance on reducing color difference of LED at the expense of high accuracy of recursive current unit control and instrument cost.
4. 13 bits resolution is proved necessary for this system to keep Δu’v’ within 0.005, the just noticeable difference of human. The process steps of 13 bits resolution are half of that of 14 bits resolution because recursive current of 13 bits is twice as big as that of 14 bits.
In order to simplify the system configuration, it is trade-off between cost and performance after all. In order to optimize the balance of cost and performance, it is necessary to find the relationship between minimum controllable color difference and corresponding resolution bits of feedback control system. Therefore, by determining desired minimum color difference, the resolution bits of feedback control system can be known. Afterward appropriate and applicable electronic devices can be found and utilized.
5.2 Future works
In this thesis, the light output of 5mm RGB Lamp LEDs is stabilized within just noticeable difference. Moreover, it is proved that its lifetime is competitive to commercial TFT LCD backlight. Besides, this thesis has set an example and rules to design a feedback control system.
In order to maintain the brightness and raise the contrast ratio of LED backlight, local dimming is a popular method to utilize. Obviously, the heat dissipation of LED is a critical issue needed to be resolve. Therefore, the LED arrangement and heat dissipation mechanism design is a following research topic. On the other hand, the circuit design and data processing for local dimming and sensor signal of feedback control system is of great importance as well.
Reference
[1] HTTP://www.ausairpower.net/OSR-0398.html
[2] G. Harbers et al., “High performance LCD backlighting using high intensity red, green, and blue light emitting diodes”, SID’01, pp. 702-706, (2001).
[3] HTTP://campaign.hncb.com.tw/intranet/monthly/mon046/04604.pdf
[4] S. Muthu et al., “Red, green and blue LED based white light generation: issues and control”, 37th Annual IEEEIAS meeting 2002, vol. 1, pp. 327 –333, (2002).
[5] S. Muthu et al., “Red, green and blue LED-based white light source:
implementation challenges and control design”, Proc. IEEE ISA’03, vol. 1, pp.
515-522, (2003).
[6] S. Muthu et al., “Red, green and blue LEDs for white light illumination”, IEEE, journal on selected topics in quantum electronics, vol. 8, no. 2, pp. 333-338, (2002).
[7] US patent 6,507,159, “Controlling method and system for RGB based LED luminary”.
[8] US patent application 20020195541 A1, “Method and system for controlling a light source”.
[9] N. Mohan et al., “Power electronics: Converters, Applications and Design”, 2nd Edition, John Wiley & Sons, New York, (2003).
[10]http://www.ecntaiwanmag.com/article-6766-5000%E5%B0%8F%E6%99%82%E 5%A3%BD%E5%91%BD%E7%9A%84TFTLCD%E6%A8%A1%E7%B5%84-Asia .html
[11] R. Jackson et al., “Computer Generated Color: A Practical Guide to Presentation and Display”, 2nd Edition, John Wiley & Sons, New York, (1993).
[12] G.. Wyszecki et al., “Color Science: Concepts and methods, quantitative data and formulae”, 2nd Edition, John Wiley & Sons, New York, (1982).
[13] D. Malacara, “Color Vision and Colorimetry: Theory and Applications”, SPIE Press, Bellingham, Washington, (2002).
[14] S. Robinson et al., “Polychromatic optical feedback: control, stability, and dimming”, Proceedings of Solid State Lighting VI, SPIE vol. 6337, 633714 (2006).
[15] Wien, “CIE. Colorimetry”, 3rd Edition, Commission Internationale de l’Eclairage, Austria, (2004).
[16] HTTP://www.eedesign.com.tw/article/forum/fo1158.htm
[17] D.L MacAdam, “Color Measurements: Theme and Variations”, 2nd Edition, Springer, New York, (1985).
[18] HTTP://www.datasheetsite.com/datasheet/S9032 [19] HTTP://www.sitikorea.com/DD313.pdf
[20] HTTP://www.siti.com.tw/product/spec/LED/DM413.pdf
[21] R.S. West et al., “LED backlight for large area LCD TV’s”, IDW'03, pp. 657-660, (2003).
[22] HTTP://hk.cgan.net/book/books/print/packcolor/link/2-2.htm
[23] Y. Gu et al., “spectral and luminous efficacy change of high-power LEDs under different dimming methods”, Sixth International Conference on Solid State Lighting, Proceedings of SPIE 6337, 63370J. (2006).
[24] D. L. Barton et al., “Life tests and failure mechanisms of GaN/AlGaN/InGaN light-emitting diodes”, Proc. SPIE, v 3279, pp.17 (1998).
[25] D. Kececioglu et al., “The Arrhenius, Eyring, inverse power law and Combination models in accelerated testing”, Reliability Engineering, (1983).
[26] 劉熙娟 , 溫岩 , 朱紹龍, “白光LED的使用壽命的定義和測試方法” ,光源與照 明, 2001年04期
[27]O. Pursiainen et al., “Identification of aging mechanisms in the optical and electrical characteristics of light-emitting diodes” [J].Appl.Phys.Lett., (2001)
[28] Raimund Zach, “Color Stabilization of RGB LEDs in an LED Backlighting Example”, OSRAM Opto Semiconductors, Application Note, (2004).
[29] M.G. Craford, “LED’s challenge the incandescents”, IEEE Circuits and Devices Mag., vol. 8, pp. 24-29, (1992).
[30] S.A.Steigerwald et al., “Illumination with solid state lighting technology”, IEEE journal on selected topics in quantum electronics, vol. 8, pp. 316-317, (2002).
Appendix
(A) Pin Connection and Description of LED Driver, DD313
(B) Pin Connection and Description of PWM Generator
(C) Recursive Program of Central Processor
{
module run(rst,osc,clk,r1,g1,b1,out1,out2,stb,r2,g2,b2,glin,adc);
input rst,osc;
input [7:0]r1,g1,b1,r2,g2,b2,glin;
output clk,out1,out2,stb,adc;
wire [5:0]gl;
wire [6:0]counts;
count_1 thecount_1 (rst,osc,counts);
clkd theclkd (rst,osc,counts,clk);
rgb thergb
(rst,osc,counts,r1,g1,b1,out1,out2,r2,g2,b2,gl,glin);
stbdat thestbdat (rst,osc,counts,stb);
trig gtlu (rst,osc,adc);
endmodule
//---count---module count_1(rst,osc,counts);
input rst,osc;
output [6:0]counts;
reg [6:0]counts;
always@(posedge osc or negedge rst) begin
if(!rst)counts=0;
else if(counts <'d66) counts=counts+1;
else counts=0;
always@(posedge osc or negedge rst) begin
if(!rst)clk=0;
else if((counts=='d0)||((counts
<'d67)&&((counts%2)==1))||(counts>='d66))clk=0;
else clk=1;
always@(posedge osc or negedge rst)
if(counts<'d1)begin out1=0; out2=0; end
if(counts=='d1)out1=r1[7]; if(counts=='d3)out1=r1[6];
if(counts=='d5)out1=r1[5]; if(counts=='d7)out1=r1[4];
if(counts=='d9)out1=r1[3]; if(counts=='d11)out1=r1[2];
if(counts=='d13)out1=r1[1]; if(counts=='d15)out1=r1[0];
if(counts=='d17)out1=g1[7]; if(counts=='d19)out1=g1[6];
if(counts=='d21)out1=g1[5]; if(counts=='d23)out1=g1[4];
if(counts=='d25)out1=g1[3]; if(counts=='d27)out1=g1[2];
if(counts=='d29)out1=g1[1]; if(counts=='d31)out1=g1[0];
if(counts=='d33)out1=b1[7]; if(counts=='d35)out1=b1[6];
if(counts=='d37)out1=b1[5]; if(counts=='d39)out1=b1[4];
if(counts=='d41)out1=b1[3]; if(counts=='d43)out1=b1[2];
if(counts=='d45)out1=b1[1]; if(counts=='d47)out1=b1[0];
if(counts=='d49)out1=1; if(counts=='d51)out1=1;
if(counts=='d53)out1=gl[5]; if(counts=='d55)out1=gl[4];
if(counts=='d57)out1=gl[3]; if(counts=='d59)out1=gl[2];
if(counts=='d61)out1=gl[1]; if(counts=='d63)out1=gl[0];
if(counts=='d1)out2=r2[7]; if(counts=='d3)out2=r2[6];
if(counts=='d5)out2=r2[5]; if(counts=='d7)out2=r2[4];
if(counts=='d9)out2=r2[3]; if(counts=='d11)out2=r2[2];
if(counts=='d13)out2=r2[1]; if(counts=='d15)out2=r2[0];
if(counts=='d17)out2=g2[7]; if(counts=='d19)out2=g2[6];
if(counts=='d21)out2=g2[5]; if(counts=='d23)out2=g2[4];
if(counts=='d25)out2=g2[3]; if(counts=='d27)out2=g2[2];
if(counts=='d29)out2=g2[1]; if(counts=='d31)out2=g2[0];
if(counts=='d33)out2=b2[7]; if(counts=='d35)out2=b2[6];
if(counts=='d37)out2=b2[5]; if(counts=='d39)out2=b2[4];
if(counts=='d41)out2=b2[3]; if(counts=='d43)out2=b2[2];
if(counts=='d45)out2=b2[1]; if(counts=='d47)out2=b2[0];
if(counts=='d49)out2=1; if(counts=='d51)out2=1;
if(counts=='d53)out2=gl[5]; if(counts=='d55)out2=gl[4];
if(counts=='d57)out2=gl[3]; if(counts=='d59)out2=gl[2];
if(counts=='d61)out2=gl[1]; if(counts=='d63)out2=gl[0];
always@(posedge osc or negedge rst) begin
if(!rst)stb=1;
else begin
if(counts>='d65)stb=0;
else stb=1;
end end
endmodule
//---adc--- module trig(rst,osc,adc);
input rst,osc;
output adc;
reg adc;
reg stop;
always@(posedge osc or negedge rst) begin
if(!rst)begin adc=0; stop=0;end else
begin
if(stop<1)begin adc=1; stop=stop+1; end else adc=0;
end end
endmodule }