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2 11 Intductin t LED Backlight Diving Techniques f Liquid Cystal Dislay Panels Huang-Jen Chiu 1, Yu-Kang L 1, Kai-Jun Pai 1, Shih-Jen Cheng 1, Shann-Chyi Mu and Shih-Ta Lai 3 1 Natinal Taiwan Univesity f Science and Technlgy, Ching-Yun Univesity, 3 Chung-Yuan Chistian Univesity, Taiwan Oen Access Database 1. Intductin Liquid cystal dislay (LCD) is widely used in vaius dislay alicatins such as cellula hnes, PC mnits, televisins (TVs), multimedia ducts, amng thes. An LCD backlight mdule usually includes backlight suces, a light-diffusin late, a eflect, a bightness-enhancement film (BEF) and a light-guide late (LGP). Cnventinally, cld cathde fluescent lams (CCFLs) ae equied t vide sufficient backlighting f LCD anels [1-5]. Gwing cncens abut envinmental issues will inhibit the use f CCFLs that cntain isnus mecuy. Owing t imvement in lng eative life, wide eatin temeatue ange, and the simlicity f dive cicuit wk with lw and safe vltages, light emitting dide (LED) has gadually substituted the CCFL as backlight [6-9]. This chate will intduce sme LED backlight diving techniques f LCD anels. Sme dimming cntl methds will als be intduced and cmaed f egulating the LED cuent and bightness f the LED backlight system. The incial gal f this chate is t ensue that eades becme familia with LED backlight diving techniques f LCD anels. We begin this chate with a lk at thee LCD backlight stuctues: edge-light tye, bttm-light tye and hllw tye. Figue 1(a) shws the bttm-light stuctue. Because f its high-luminance featue, the bttm-light stuctue is cmmnly used f PC mnits and TVs. With egad t the luminance unifmity n an LCD anel, a light-mixed zne is necessay between the diffusin late and the light suces. This zne causes undesiable thickness f lage-sized TV alicatins. Figue 1(b) shws the hllw tye stuctue in which an LGP is used t educe the thickness f the light-mixed zne. The gd qualities f this tye etain t its cmact shae, high luminance and gd themal dissiatin. Figue 1(c) shws the edge-light stuctue cmmnly used in a small-scale LCD anel. This tye is f cmact shae and lw we cnsumtin, s it is suitable f ntebk PCs and esnal digital assistant (PDA) ducts. Thee ae tw tyes f LEDs f backlight suces, the white-light LEDs and RGB LEDs. The white-light LED is cmsed f a blue LED cated with yellw hsh. Simle diving featue make it as a ula chice f new geneatin f LCD backlight suces in table dislay ducts. Its cl filte divides the emitted white light int RGB sub-ixels t esent cl ictues. Thicknesses f Suce: New Develments in Liquid Cystals, Bk edited by: Gegiy V. Tkachenk, ISBN ,. 34, Nvembe 009, I-Tech, Vienna, Austia

3 08 New Develments in Liquid Cystals RGB sub-ixels must be adjusted accding t the cesnding wavelengths t cect the white balance n LCD anel. This esults in the difficulty f manufactuing cess. The white-cl int may vay afte a lng wking time. Thus, the RGB LEDs mixing theecl lights t white light ae me suitable f medium-scale, even lage-size sceens [10-16]. White balance f the LCD anel with RGB LED backlight can be easily cected by egulating the emissin luminance f the RGB LEDs individually. (a) (b) (c) Fig. 1. (a) Bttm-light Tye, (b) Hllw Tye and (c) Edge-light Tye Backlight Stuctues.. RGB LED backlight cicuit Figue shws the blck diagam f an LCD TV we suly with RGB LED backlight design. The LCD TV we vides a 1V utut f the signal-cess bad, a 4V utut f the backlight diving cicuit and an additinal 5V standby-we utut. As shwn in Figue 3, the backlight diving cicuit cnsists f thee we cnvetes. Backlight LEDs ae cnnected in seies and aallel in the RGB LED backlight mdules. LED cuent/ vltage chaacteistic vaiatins cause bightness diffeence. Theefe, dimming cntl is an imtant design cnsideatin f LED backlight alicatins. We studied thee dimming methds f cuent egulatin f the aallel cnnected LED aays: the tanscnductance-amlifie (TA) dimming, the cuent-mi (CM) dimming and the bustmde (BM) dimming.

4 Intductin t LED Backlight Diving Techniques f Liquid Cystal Dislay Panels 09 Fig.. Blck Diagam f an LCD TV Pwe Suly. Fig. 3. RGB LED Backlight Diving Cicuit Figue 4(a) shws the TA dimming cicuit. The LED cuent can be exessed as Equatin (1). I Vd R LED =, (1) Figue 4(b) shws the CM dimming cicuit. The LED cuent can be exessed as Equatin (). V I LED I = Kn VGS VTN ) = -V d GS (, () whee K n and V TN ae the cnductin aamete and theshld vltage f the dimming tansists Q and Q d, esectively. By using the TA dimming and CM dimming cicuits, the R

5 10 New Develments in Liquid Cystals cuent egulatin f aalleled LED aays can be achieved. Hweve, the cnductin lsses f the dimming tansists will be difficult t slve [17]. An adative vltage utut f the DC-DC cnvete is usually designed t etain the minimum dain-suce vltage n the dimming tansists. As shwn in Figue 4(c), the backlight LED cuent can be als cntlled with a BM dimming cicuit. Cnsideing the switching lss f the dimming tansists, the bust-mde fequency f b is designed at 400Hz that ae uneceivable t the human eye. The duty atis f the dimming tansists ae vaied t egulate the LED aveage cuent that can be eesented as Equatin (3). I LED(av) = I m δ, (3) whee I m dentes the eak value f the LED aay cuent. The dimming tansists ae eated as lw-fequency switches, the themal blem n the dimming tansists can be imved significantly. The cuent vaiatins can be minimized by using the TA BM dimming methds while the CM dimming has the simlest cicuit cnfiguatin. Anyway, the TA and CM dimming methds ae unsuitable t be used in the high-we LED backlight design f LCD anels due t the significant cnductin lsses f the dimming tansists unde dimming eatins. The emissin luminance f the RGB LEDs is able t be egulated individually f achieving the white balance f the LCD anel. The luminance f the ed light is always highest and the luminance f the blue light is lwest amng thee cl lights. In actical alicatins, the blue light is mst sensitive t human eyes such that lwe luminance f blue LED is enugh t cmse white light with the ed and geen LEDs. (a) (b) (c) Fig. 4. (a) TA, (b) CM and (c) BM Dimming Methds

6 Intductin t LED Backlight Diving Techniques f Liquid Cystal Dislay Panels Sft-switched LED backlight cicuit Figue 5(a) shws a half-bidge DC-DC Seies-Resnant Cnvete (SRC) tlgy f diving the RGB LEDs. The sft-switched DC-DC esnant cnvete includes we switches Q 1 and Q, esnant induct L, esnant caacit C, tansfme T 1, ectifie dides D f1 and D f, filte caacit C f, and the LED aays eesented by an equivalent esistance R. The chaacteistic imedance and the esnant fequency ae esectively [18-1]. L Z c =, (4) C 1 f =, (5) π LC Fm (4) and (5), L and C can be exessed as L Z c =, (6) π f The tun numbe f the imay winding is N C = 1, (7) π f Z (V c 1) 10 8 N =, (8) 4 f s B A e whee V N is the eak-t-eak amlitude f the tansfme imay vltage v N, f s is the switching fequency, B is the magnetic flux density, and A e is the effective ce aea. T simlify the analysis, the fist-de hamnic aximatin has been alied, and the cicuit elements f the imay side in Figue 5(a) ae eflected t the secnday side, as deicted in Figue 5(b). The equivalent imedances Z 1 and Z ae esectively Z n =, (9) 1 + jωn L jωc Z jωn L m // R =, (10) whee n=n s /N. The vltage divide ule can be used t btain the utut vltage has V L as V ( Z 1 Z + Z 1 L = Vin ), (11) whee the amlitude f V L is assumed equal t V. V in is the has f the fundamental cmnent f v a in Figue 5(a).

7 1 New Develments in Liquid Cystals V n π =, (1) in V s Substituting (9), (10) and (1) int (11), the vltage gain tansfe functin can be exessed as V V s = π nω L m [ R ω C R ( L + Lm )] + [ n ωlm (1 ω LC C R )], (13) (a) Fig. 5. (a) Half-bidge DC-DC SRC Tlgy and (b) Its Equivalent Cicuit. It is clealy seen fm (13) that the switching fequency must be vaied t egulate the utut vltage. The highest switching fequency aeas at the highest inut vltage and the lightest lad. On the the hand, the lwest switching fequency haens at the lwest inut vltage and the heaviest lad. F the SRC t eate in the ze-vltage-switching (ZVS) egin, the lwest switching fequency must be highe than the esnant fequency as exessed in (5). Meve, due t the switching seed limitatins f the we devices, the highest switching fequency is belw a secified value. In the wds, the vaiatins f the inut DC vltage and the lad vaiatins must be cnfined t a small ange. Usually a we fact cect (PFC) is added in fnt f the DC-DC cnvete t aise the inut we fact and educe the inut cuent hamnics. A hase-shift ulse width mdulatin (PSPWM) dimming cntl can effectively cnfine the lad vaiatin f the DC-DC SRC. Cnsequently, the utut vltage vaiatin f the PFC can be limited t a smalle extent. This esults in a bette eating cnditin f the SRC. F the PSPWM dimming stategy, the wking duatins f the shunt LED aays ae ely hase-shifted t cnfine the vaiatin f the utut cuent f the SRC. Figue 6 illustates the cicuit aangement f N shunt single-cled LED aays with PSPWM dimming methd. It is almst the same as the cnventinal ne, excet that the dimming signals ae alied with a secified hase diffeence. With the PSPWM dimming, thee ae always velas between the LED diving cuents. The maximum duty cycle, the vela, is 100 %, cesnding t the highest bightness. T event the DC-DC SRC fm eating at n lad, the minimum duty cycle f the PSPWM dimming signal is 1/N, whee N is the numbe f the shunt LED aays. (b)

8 Intductin t LED Backlight Diving Techniques f Liquid Cystal Dislay Panels 13 Unde this cicumstance, the vela is ze, cesnding t the lwest bightness. Cmaed with the cnventinal dimming scheme, it is aaently ecgnized that the lad vaiatin f the SRC is less with the sed PSPWM dimming functin. T futhe investigate the eating incile f the PSPWM dimming, a me geneal case with N shunt LED aays is discussed as fllws. Figue 7 shws the wavefms f the N diving cuents and the utut cuent f the SRC. As stated ealie, the duty cycle ange f the dimming signal is fm 1/N t 100 %. In tems f the hase angle, if a cmlete eid is 360º, the duty cycle ange is fm 360º/N t 360º. Assuming that the dimming signal f the LED aay 1 stats at 0º, then the dimming signal f the k-th LED aay wuld stat at = 360 ( k 1) k / N φ, (14) If the duty cycle f each dimming signal is φ d, then the aveage diving cuent f ne LED aay is avg = φ d I /, (15) I 360 whee I is the amlitude f the diving cuent f each LED aay. Theefe the aveage utut cuent f the SRC is I I N / 360, avg = φ d, (16) Fig. 6. The PSPWM Dimming Methd. It can be bseved fm Figue 7 that if the end f the dimming signal f LED aay 1 is at φ d, whee φ d is between φ k and φ k+1 and k 1, then the utut cuent f the SRC in the ange f φ k t φ k+1 is i ki f φk φ φd = ( k 1) I f φd φ φk +1, (17)

9 14 New Develments in Liquid Cystals This is als the SRCs utut cuent in each duatin fm φ j t φ j+1, whee j = 1 t N. Theefe, the aveage utut cuent f the SRC is nw I, avg ki = [ φ 360 / N ( k 1) ] + ( k 1) I [ 360 / N k φ ] d d = N φ I / / N d, (18) Fig. 7. Cuent Wavefms f N Shunt LED Aays f the PSPWM dimming. A faved featue is that the lad vaiatin f the SRC is always within ne ste change f I, n matte what the lad level is. Theefe, by caefully designing the duty cycle and the amlitude f the diving cuent f each LED aay, the n lad eatin f the DC-DC SRC may be ecluded. Meve, the utut tansient f the SRC is imved due t the cnfined lad change. The numbe f the LED aay f ne cl, and the eak diving cuent f each LED aay ae fist detemined accding t the secificatins f the LED and the sectum f the white cl. Then a suitable duty cycle is chsen allwing a easnable san f vaiatin f dimming cntl. 4. Single-stage LED backlight cicuit Figue 8 shws a single-stage LED backlight diving system. The backlight diving system cnsists f an AHB DC/DC cell integated with a chage-um PFC cell. The we MOSFETs Q1 and Q, eate with asymmetical duty atis, δ and 1-δ, which equie sht and well-defined dead time between the cnductin intevals. D1, D and C 1 and C ae the bdy dides and the aasitic caacits f we MOSFETs, esectively. The chageum PFC cell is cmsed f esnant induct L, chage-um caacits C 1 and C, inut dides D i1, and D i, claming dides D c1, and D c. The caacit C bus is used as the DC bus caacit between the chage-um PFC cell and the st-stage AHB DC/DC cell. The

10 Intductin t LED Backlight Diving Techniques f Liquid Cystal Dislay Panels 15 tansfme leakage induct L l esnates with the aasitic caacits C 1 and C duing dead-time intevals t achieve ze-vltage switching f the we MOSFETs. The blcking caacit C b is used t assue that the we sent int the tansfme imay winding is a ue AC tye. A DC vltage is sulied t the LED aays thugh the secnday ectifie and filte cicuit that ae cmsed f D3, D4, L and C. Fig. 8. Single-stage LED Backlight Diving System. The aveage ectified inut cuent I in,av can be exessed as fllws. Q = Δ f C V, (19) Iin =,av s 1 Ts whee ΔQ is the chage vaiatin f C 1. Fm Equatin (19), we can see that the aveage ectified inut cuent is tinal t the ectified inut vltage. Thus, high we fact can be achieved. Based n the we balance between the inut and utut f the AC/DC cnvete, the fllwing equatin has t be satisfied. I P in in = V, (0),av in ηvin whee η and P ae the veall efficiency and utut we f the cnvete. Fm Equatins (19) and (0), the design equatins f the esnant induct L and the chageum caacit C 1 can be deived as fllws [-5]. L ηvin =, (1) 8π fs P C P 1 =, () ηfsvin The ZVS cnditins f we switches deend n the esnant inductance cuent I L elated with the inut vltage. At the ze-cssing f inut vltage, the esnant inductance cuent I L will be ignable. Cnsideing the ZVS cnditin duing an entie a

11 16 New Develments in Liquid Cystals line eid, the tansfme leakage inductance L l culd be detemined by using Equatin (3). L n V bus l ( C 1 + C )[ ], (3) min(ns1,ns )I In actical design, an extenal induct L e is usually needed t be added in seies cnnected with L l f satisfying ZVS cnditin [6-8]. The inut cuent has a nea sinusidal wavefm and in hase with the inut vltage. High efficiency and high we fact can be achieved because f single-stage we cnvesin with sft-switching featues. 5. Cnclusin The advantages f LED backlighting ve cnventinal CCFLs ae numeus: fast esnse, bade cl sectum, lnge life san, and n mecuy. Hweve, CCFLs still have cst advantages. F a LED backlighting, luminus efficacy and themal management ae the mst imtant issues need t be slved befe cmmecializatin. Anyway, aid advances in mateial and manufactuing technlgies will enable significant develments in highluminance LEDs f backlighting alicatins. In this chate, we intduced sme LED backlight diving systems f LCD anels. Dimming cntl methds ae then discussed t egulate the LED cuent and bightness f the LED backlight system. 6. Refeences [1] C. H. Lin, The Design and Imlementatin f a New Digital Dimming Cntlle f the Backlight Resnant Invete, IEEE Tans. Pwe Electnics, vl. 0, n. 6, , Nv [] C. G. Kim, K. C. Lee, and B. H. Ch, Mdeling f CCFL using Lam Delay and Stability Analysis f Backlight Invete f Lage Size LCD TV, IEEE APEC 05, Vl. 3, [3] Y. H. Liu, Design and Imlementatin f an FPGA-Based CCFL Diving System With Digital Dimming Caability, IEEE Tansactins n Industial Electnics, Vl. 54, Issue 6, , Dec [4] C. H. Lin, Digital-Dimming Cntlle with Cuent Sikes Eliminatin Technique f LCD Backlight Electnic Ballast, IEEE Tansactins n Industial Electnics, Vl. 53, Issue 6, , Dec [5] Y. K. L, and K. J. Pai, Feedback Design f a Piezelectic Tansfme-based Halfbidge Resnant CCFL Invete, IEEE Tans. Industial Electnics, vl. 54, n. 4, , Oct [6] K. H. Lee, and S. W. R. Lee, Pcess Develment f Yellw Phsh Cating n Blue Light Emitting Dides (LEDs) f White Light Illuminatin, in Pc. Electnics Packaging Technlgy Cnfeence, 006, [7] T. Taguchi, Y. Uchida, and K. Kbashi, Efficient White LED Lighting and Its Alicatin t Medical Fields, Junal f hysica status slidi (a), vl. 01, n. 1, , Set [8] N. Mhan, T. M. Undeland, and W. P. Rbbins, Pwe Electnics, USA: Jhn Wiley & Sns, 003,

12 Intductin t LED Backlight Diving Techniques f Liquid Cystal Dislay Panels 17 [9] H. van de Beck, G. Sauelande, and M. Wendt, Pwe Dive Tlgies and Cntl Schemes f LEDs, in IEEE Pc. APEC 07, 007, [10] C. C. Chen, C. Y. Wu, Y. M. Chen, and T. F. Wu, Sequential Cl LED Backlight Diving System f LCD Panels, IEEE Tansactins n Pwe Electnics, Vl., Issue 3, , May 007 [11] H. J. Chiu and S. J. Cheng; LED Backlight Diving System f Lage-Scale LCD Panels, IEEE Tansactins n Industial Electnics, Vl. 54, Issue 5,.: , Oct [1] G. Pak; T. S. Aum, J. H. Bae, J. H. Kwn, S. K. Lee; M. H. Lee and H. S. Sh, Otimizatin f Diect-tye LCD Backlight Unit, Pacific Rim Cnfeence n Lases and Elect-Otics, Aug. 005, [13] S. Y. Lee, J. W. Kwn, H. S. Kim, M. S. Chi and. S. Byun, New Design and Alicatin f High Efficiency LED Diving System f RGB-LED Backlight in LCD Dislay; the 37th IEEE Pwe Electnics Secialists Cnfeence, June 006,.1-5. [14] S. Muthu, F. J. Schuumans, and M. D. Pashley, Red, Geen, and Blue LED based White Light Geneatin: Issues and Cntl, Annual Meeting. Cnfeence Recd f the Industy Alicatins Cnfeence, Oct. 00, Vl. 1, [15] F. Benitz, O. Schallmse, and W. Swa, Advanced Electnic Dive f Pwe LEDs with Integated Clu Management, Annual Meeting. Cnfeence Recd f the Industy Alicatins Cnfeence, Vl. 5, Oct. 006, [16] C. C. Chen, C. Y. Wu, and T. F. Wu, Fast Tansitin Cuent-Tye Bust-Mde Dimming Cntl f the LED Back-Light Diving System f LCD TV, IEEE Pwe Electnics Secialists Cnfeence, June 006, [17] Dnald A. Neamen, Electnic Cicuit Analysis and Design, e, McGaw-Hill, 001. [18] C. C. Chen, C. Y. Wu, and T. F. Wu, LED Back-light Diving System f LCD Panels, IEEE APEC '06, [19] S. Y. Lee, J. W. Kwn, H. S. Kim, M. S. Chi, and K. S. Byun, New Design and Alicatin f High Efficiency LED Diving System f RGB-LED Backlight in LCD Dislay, IEEE PESC '06,.1-5. [0] M. Ric-Secades, A. J. Calleja, J. Ribas, E. L. Cminas, J. M. Alns, J. Cadesin, and J. Gacia-Gacia, Evaluatin f a Lw-Cst Pemanent Emegency Lighting System based n High-Efficiency LEDs, IEEE Tansactins n Industy Alicatins, Vl. 41, N. 5, Set.-Oct. 005, [1] H. Sugiua, S. Kagawa, H. Kanek, M. Ozawa, H. Tanize, T. Kimua, and H. Uen, Wide Cl Gamut Dislays using LED Backlight- Signal Pcessing Cicuits, Cl Calibatin System and Multi-Pimaies, IEEE ICIP 05, Vl., [] G. Mschuls and P. Jain, Single-Phase Single-Stage Pwe-Fact-Cected Cnvete Tlgies, IEEE Tansactins n Industial Electnics, Vl. 5, Issue 1,.3 35, Feb [3] F. S. Kang, S. J. Pak, and C. U. Kim, ZVZCS Single-Stage PFC AC-t-DC Half-Bidge Cnvete, IEEE Tansactins n Industial Electnics, Vl. 49, Issue 1,.06-16, Feb. 00. [4] J. Qian, and F. C. Y. Lee, A High-Efficiency Single-Stage Single-Switch High-Pwe- Fact AC/DC Cnvete with Univesal Inut, IEEE Tansactins n Pwe Electnics, Vl. 13, N. 4, July 1998,

13 18 New Develments in Liquid Cystals [5] J. Qian, and F. C. Lee, Chage Pum Pwe-Fact-Cectin Technlgies. II. Ballast Alicatins, IEEE Tansactins n Pwe Electnics, Vl. 15, N.1, , Jan [6] F. Benitz, O. Schallmse, and W. Swa, Advanced Electnic Dive f Pwe LEDs with Integated Clu Management, IEEE IAS 06, Vl. 5, [7] S. Muthu and J. Gaines, Red, Geen and Blue LED-based White Light Suce: Imlementatin Challenges and Cntl Design, IEEE IAS 03, Vl. 1, [8] S. Muthu, F. J. Schuumans, and M. D. Pashley, Red, Geen, and Blue LED based White Light Geneatin: Issues and Cntl, IEEE IAS 0, Vl. 1,

14 New Develments in Liquid Cystals Edited by Gegiy V Tkachenk ISBN Had cve, 34 ages Publishe InTech Published nline 01, Nvembe, 009 Published in int editin Nvembe, 009 Liquid cystal technlgy is a subject f many advanced aeas f science and engineeing. It is cmmnly assciated with liquid cystal dislays alied in calculats, watches, mbile hnes, digital cameas, mnits etc. But nwadays liquid cystals find me and me use in htnics, telecmmunicatins, medicine and the fields. The gal f this bk is t shw the inceasing imtance f liquid cystals in industial and scientific alicatins and insie futue eseach and engineeing ideas in students, yung eseaches and actitines. Hw t efeence In de t cectly efeence this schlaly wk, feel fee t cy and aste the fllwing: Huang-Jen Chiu, Yu-Kang L, Kai-Jun Pai, Shih-Jen Cheng, Shann-Chyi Mu and Shih-Ta Lai (009). Intductin t LED Backlight Diving Techniques f Liquid Cystal Dislay Panels, New Develments in Liquid Cystals, Gegiy V Tkachenk (Ed.), ISBN: , InTech, Available fm: htt:///bks/new-develments-in-liquid-cystals/intductin-t-led-backlight-divingtechniques-f-liquid-cystal-dislay-anels InTech Eue Univesity Camus STeP Ri Slavka Kautzeka 83/A Rijeka, Catia Phne: +385 (51) Fax: +385 (51) InTech China Unit 405, Office Blck, Htel Equatial Shanghai N.65, Yan An Rad (West), Shanghai, 00040, China Phne: Fax:

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