Single-Stage AC/DC Single-Inductor Multiple-Output LED Drivers

Transcription

1 Sngle-Stage AC/DC Sngle-Inductor Multple-Output LED Drver 1 Yue Guo, Snan L, Member, IEEE, Albert T. L. Lee, Member, IEEE, Sew-Chong Tan, Senor Member, IEEE, C. K. Lee, Senor Member, IEEE, S. Y. (Ron) Hu, Fellow, IEEE Abtract Varou AC/DC LED drver topologe have been propoed to meet the challenge of achevng a compact, effcent, low-cot and robut mult-trng LED lghtng ytem. Thee LED drver typcally employ a two-tage topology to realze the functon of AC/DC rectfcaton and ndependent current control of each LED trng. The choce of havng two tage converon nvolve addtonal hardware component and a more complcated controller degn proce. Such two-tage topologe uffer from a hgher ytem cot, ncreaed power lo, and large form factor. In th paper, a ngle-tage AC/DC ngle-nductor multple-output (SIMO) LED drver propoed. It ue only one ngle nductor and N+1 actve power wtche (N beng the number of LED trng) wth reduced component count and maller form factor. The propoed drver can acheve both functon of AC/DC rectfcaton wth a hgh power factor and prece ndependent current control of each ndvdual LED trng multaneouly. A prototype of an AC/DC ngle-nductor trple-output (SITO) LED drver contructed for verfcaton. Expermental reult corroborate that prece and ndependent current regulaton of each ndvdual LED trng achevable wth the propoed drver. A power factor of above 0.99 and a peak effcency of 89% at 30 W rated output power are attanable. Index Term Sngle-nductor multple-output (SIMO), color control, lghtng ytem, LED, power factor control. I. INTRODUCTION Lght-emttng-dode (LED) are ncreangly ganng acceptance n lghtng ndutry wth a growng lt of applcaton, uch a general, decoratve and dplay lghtng applcaton [1] [6]. The four major factor upportng ther popularty are () preponderant long lfetme; () mercury free Manucrpt receved Apr 09, Th work upported by the Hong Kong Reearch Grant Councl under Theme-baed Reearch Project: T22-715/12N. Guo Yue wth the Department of Electrcal & Electronc Engneerng, The Unverty of Hong Kong (emal: guoyue3858@163.com). Snan L wth the Department of Electrcal & Electronc Engneerng, The Unverty of Hong Kong (emal: nl@eee.hku.hk). Albert T. L. Lee wth the Department of Electrcal & Electronc Engneerng, The Unverty of Hong Kong (emal: tlalee@eee.hku.hk). S. C. Tan wth the Department of Electrcal & Electronc Engneerng, The Unverty of Hong Kong (emal: ctan@eee.hku.hk). C. K. Lee wth the Department of Electrcal & Electronc Engneerng, The Unverty of Hong Kong (emal: cklee@eee.hku.hk). S.Y. R. Hu wth the Department of Electrcal & Electronc Engneerng, The Unverty of Hong Kong (emal: ronhu@eee.hku.hk) and Imperal College London (e-mal: r.hu@mperal.ac.uk). and envronmental frendly; () hgh lumnou effcency; and (v) flexblty to perform color mxng and dmmng control [7]-[11]. Dependng on the pecfc applcaton requrement, the LED can ether be arranged n ere a a ngle trng (or a ngle LED chp), or n parallel formng a mult-trng tructure (for medum and hgh power applcaton). Many LED drver achevng mall form factor and low cot have been propoed for the ngle LED chp/trng applcaton [12] [14]. However, achevng a compact and low-cot LED drver degn challengng for applcaton where multple parallel LED trng are needed. Th becaue extra functonalte uch a current balancng, ndvdual trng current regulaton, or open/hort crcut fault protecton are typcally demanded n uch mult-trng LED ytem. For ntance, n hgh power applcaton, uch a treetlght and large-cale LCD panel, current harng between trng crucal for provdng an evenly dtrbuted lght output and heat. Mot mportantly, f the current mbalance caue one or more LED trng to exceed ther rated current value, the lfetme of the LED trng wll be dratcally reduced [15] [19]. In color mxng applcaton, uch a RGB LED lamp and LED-backlt LCD dplay, fat and prece current control of the red, green and blue LED hould be guaranteed [20] [22]. Bacally, thee functonalte,.e., current harng, ndvdual trng regulaton, and/or open/hort crcut fault protecton, can be multaneouly acheved f each of the trng current regulated ndependently. In th way, current harng can be mply realzed by agnng a common current reference for all trng, whle ndvdual current regulaton accomplhed by agnng a dfferent reference command for each trng. Several oluton for drvng mult-trng LED ytem wth ndependent current control have been propoed. They can be broadly clafed nto two type, a hown n Fg. 1 and.

2 Fg. 1. Conventonal mult-trng LED ytem of whch the AC/DC tage generate a common output bu voltage and a eparate output voltage for each ndvdual LED trng. Ther major dfference le n the crcut archtecture of the AC/DC tage, whch requred to enable an AC voltage nput and/or perform power factor correcton (PFC) functon. Fg. 1 how an AC/DC tage whch generate a ngle common output bu V o that hared by all the LED trng [14], [23] [26], wherea Fg. 1 how an AC/DC tage whch agn a eparate output voltage for each LED trng [15], [27], [28]. To realze ndependent current regulaton of each LED trng, the output of the AC/DC pre-regulaton tage mut be cacaded wth an addtonal pot-regulator for each LED trng, whch regulate the current of the trng to whch t connected. There are generally two type of pot-regulator: lnear type [23], [24], [28] and DC/DC converter type [14], [15], [25]. The lnear type of pot-regulator gve the mplet hardware confguraton, but mght ncur evere power lo f mproperly degned [23]. On the other hand, the DC/DC converter type of pot-regulator deally lole. However, each DC/DC pot-regulator ntroduce addtonal wtche and pave component uch a nductor to the ytem. Th nevtably lead to a hgher ytem cot and larger form factor that grow a the number of LED trng ncreae. Therefore, there alway tradeoff between effcency and the ytem cot and ze whenever a pot-regulator ued. Another problem wth the two-tage confguraton that two et of controller (one for the AC/DC tage and the other for the pot-regulator) are requred, whch complcate the ytem degn. Addtonally, a two-tage tructure requre the ue of DC-lnk capactor() (typcally electrolytc capactor (E-Cap)) (C o1 for Fg. 1, and C o1 C on for Fg. 1). If the DC-lnk voltage hgh, t hard to elect a proper capactor that ha a long lfetme. The ue of hort lfetme capactor n the LED drver reduce the relablty of the LED drver [29], [30]. In vew of the aforementoned ue, n th paper, a ngle-tage AC/DC ngle-nductor-multple-output (SIMO) LED drver for mult-trng LED applcaton, whch can multaneouly acheve PFC and ndependent current regulaton of each LED trng, propoed. The ytem archtecture of the propoed ngle-tage SIMO drver llutrated n Fg. 3, n whch the functon of a PFC tage and a conventonal DC/DC SIMO topology are ntegrated nto a ngle tage. Therefore, the need for a pot-regulator tage elmnated. A the name ugget, only one nductor needed. The total number of wtche alo reduced a compared wth the conventonal two-tage oluton ung DC/DC type of 2 pot-regulator. Therefore, the propoed LED drver compact and cot-effectve. In addton, t requre only one controller to regulate the wtchng equence of all the power and output wtche. Th made poble by tme-multplexng the control gnal of each trng. Moreover, by enablng one-tage operaton, the ntermedate hgh-voltage E-Cap elmnated. It enable the ue of low-voltage, long-lfetme capactor whch extend the operatng lfe of the propoed LED drver. In order to perform a power lo analy, a non-deal crcut mulaton model, whch nclude the paratc retance, nductance and capactance for the major component, ha been created for the propoed ngle-tage AC/DC SIMO LED drver topology a well a the two pror art, namely the conventonal two-tage AC/DC LED drver (wth three pot-regulator) [14], [23] [26] and the two-tage AC/DC SIMO LED drver [39], [40] for comparon purpoe. Baed on the mulaton reult, the total power lo and the power effcency n each of the three topologe have been compared and tabulated n Table I. TABLE I. COMPARISON OF THE SIMULATED POWER LOSS AND POWER EFFICIENCY OF THE PROPOSED SINGE-STAGE AC/DC SIMO LED DRIVER AGAINST THE CONVENTIONAL TWO-STAGE AC/DC LED DRIVER [14], [23] [26] AND THE TWO-STAGE AC/DC SIMO LED DRIVER. [39], [40]. To ummarze, the propoed ngle-tage AC/DC SIMO LED drver reult n the mallet total power lo, compared wth the pror art. Specfcally, the propoed ngle-tage AC/DC SIMO drver reult n a 32 % reducton n the total power lo, compared wth the conventonal two-tage drver ung three pot-regulator and about 18 % reducton n the total power lo, compared wth the two-tage SIMO. On the other hand, the mulated power effcency of the propoed SIMO drver around 91 %, compared wth % from the conventonal two-tage drver and % from the two-tage SIMO. The propoed ngle-tage SIMO drver can acheve hgher power converon effcency due to the ue of only one buck wtch a well a one freewheelng dode n the power tage. A a further mprovement n power effcency of the propoed drver, we can conder replacng the freewheelng dode n the power tage wth a low-de MOSFET havng a mall R d(on) a n a ynchronou buck converter confguraton.

3 II. AC/DC SINGLE-INDUCTOR MULTIPLE-OUTPUT LED DRIVERS A. Extng AC/DC SIMO LED Drver There growng nteret n ung DC/DC SIMO converter for mult-trng LED applcaton due to ther reduced cot and maller form factor. A ngle-nductor dual-output (SIDO) converter wth tme-multplexng control cheme operatng n DCM frt reported n [31], [32]. Extendng from SIDO, a DC/DC SIMO parallel trng LED drver operatng n DCM recently reported n [33]-[38]. All of thee reported SIMO converter can only realze DC/DC converon, and a table DC nput typcally requred. To accommodate an AC voltage nput, e.g. a 110 V, 60 Hz AC man, a DC/DC SIMO LED drver often cacaded behnd an AC/DC front-end tage [39], [40], a hown n Fg. 2, agan formng a two-tage confguraton, whch mlar to that gven n Fg Unlke extng AC/DC SIMO LED drver that are confgured a hown n Fg. 2, the propoed AC/DC SIMO LED drver can drectly drve multple LED trng off an AC voltage ource n a ngle tage, wthout an ntermedate DC-lnk. Both PFC and ndependent regulaton of trng current are multaneouly vable. Th poble through proper component ntegraton of a PFC tage and a DC/DC SIMO converter. For example, f a DCM buck converter adopted for the PFC tage (Fg. 4), and a buck-type DC/DC SIMO elected for the SIMO tage (Fg. 4), by ntegratng ther man power wtch S a and S a, freewheelng dode D a and D a, and nductor L and L, a ngle-tage buck-type AC/DC SIMO drver can be obtaned a hown n Fg. 4(c). + Fg. 2. Sytem archtecture of the extng two-tage AC/DC SIMO LED drver. In [39], the AC/DC front-tage mply a dode brdge rectfer wth a large capactor. An unregulated DC voltage produced wthout performng any PFC. Such a confguraton only ueful for low-power LED applcaton, of whch the PF requrement le trngent [41], [42]. Alo, the SIMO converter n [40] operatng n contnuou conducton mode (CCM) and uffer from cro-regulaton ue. Therefore, ndvdual current regulaton of LED trng unvable, and only current harng functon performed. On the other hand, n [40], a boot PFC converter mplemented a the AC/DC front-tage converter, provdng a well-regulated DC voltage and a hgh PF. Neverthele, by employng a two-tage confguraton, thee extng AC/DC SIMO LED drver nherently have mlar demert a thoe decrbed n Fg. 1. B. Propoed Sngle-Stage AC/DC SIMO LED Drver Fg. 3 how the confguraton of the propoed ngle-tage SIMO LED drver. Fg. 3. Sytem archtecture of the propoed ngle-tage AC/DC SIMO LED drvng ytem. (c) Fg. 4. Dervaton of a buck-type ngle-tage AC/DC SIMO LED drver. A DCM buck PFC converter. A buck-type DC/DC SIMO converter. (c) The derved buck-type ngle-tage AC/DC SIMO LED drver. By employng a tme-multplexng control cheme, at any ntance n tme, the LED drver depcted n Fg. 4(c) can be operated to act a a ngle-nput ngle-output DCM buck converter. Snce a DCM buck converter naturally an emulated retor at low frequence [43], the averaged nput current of the LED drver over each wtchng perod nherently proportonal to the lne voltage. A a reult, the orgnal DC/DC SIMO converter can be readly turned nto a ngle-tage AC/DC SIMO drver ntegrated wth PFC functon through mnor hardware modfcaton ncludng the addton of the front-end dode rectfer. In contrat to all prevou method that are two-taged-baed, the drver n Fg. 3 and Fg. 4(c) requre no E-Cap between the dode brdge and the SIMO tage. Clearly, the removal of a hort lfetme hgh voltage E-Cap extend the operatng lfetme of the propoed LED drver. Alo, by operatng the propoed SIMO drver n DCM,

4 cro-regulaton can vrtually be elmnated a the ndvdual LED trng are fully decoupled from one another. III. OPERATING PRINCIPLES OF THE PROPOSED SINGLE-STAGE AC/DC SIMO LED DRIVER A. Operatng Mode A ngle-nductor trple-output (SITO) AC/DC buck converter a hown n Fg. 5 ued for the ake of our dcuon. correpond to (1 2), (2 2), and (3 2) n Fg. 6. Mode 3 (t 2 t 3 ): Both S a and D a are OFF. I L reman at zero durng th dle perod. In order to reduce the wtchng lo, for example, S 1 can be turned off wth zero-current wtchng (ZCS) and S 2 can be turned on wth ZCS durng th nterval. Th correpond to (1 3), (2 3), and (3 3) n Fg Fg. 5. Complete crcut dagram wth three LED trng. A hown n Fg. 5, a total of four wtche,.e., one man wtch S a and three output wtche S 1 S 3, are ued n th converter. L f and C f form the nput EMI flter, C d the hgh-frequency flter capactor, D a the freewheelng dode, and L the man nductor. D the branch dode n the th LED trng for preventng revere flow of the branch current. C o and R are the output capactor and enng retor of the th LED trng. The AC nput voltage V ac, the nput voltage to the buck converter repreented by V n and the three output voltage are V o1 V o3. I L the nductor current and I branch1 I branch3 are the branch current that flow through the repectve output wtche. The deal waveform of S a, S 1 S 3, I L, and I branch1 I branch3 are hown n Fg. 6, where T repreent the wtchng perod of the man wtch S a. It can be een that the propoed AC/DC SIMO converter operate n DCM where I L alway return to zero at the end of each wtchng cycle. Fg. 6 depct the control equence of the SITO AC/DC converter under normal operaton. In three wtchng cycle (0 3T ), there are a total of nne control equence whch can be categorzed nto the followng three dtnctve mode of operaton. Mode 1 (t 0 t 1 ): Man wtch S a ON and freewheelng dode D a OFF. The nductor current I L ncreae at a rate of (V n V o1 )/L. The output wtch S 1 ON and S 2 and S 3 are OFF nce only the frt output enabled. Th correpond to (1 1), (2 1), and (3 1) n Fg. 6. Mode 2 (t 1 t 2 ): S a OFF and D a ON. I L decreae lnearly at a rate of V o1 /L. At t 2, I L drop to 0 and Mode 2 end. Th Fg. 6. Tmng dagram of the man wtch S a and output wtche S 1 S 3, nductor current I L and branch current I branch1 I branch3 and control equence of the propoed AC/DC SITO LED drver. The ame proce repeated n the next two wtchng perod for the econd and thrd output n whch S 1 OFF and S 2, S 3 take turn to be ON. The energy tranferred from the nductor to the three output n a tme-nterleaved manner. The ame control equence can alo be caled convenently to N output, where N the total number of LED trng. The output wtch correpondng to each LED trng, namely S 1, S 2,, S N, ON only durng one of the N wtchng cycle. The output wtch OFF durng the remanng (N 1) wtchng cycle.

5 B. Control Scheme The control crcut of the propoed AC/DC SITO buck LED drver a pecalzed tme multplexed controller a hown n Fg. 5. Accordng to the operatng prncple decrbed n Secton A, the on-ntant of S a hould be ynchronzed wth repectve output wtche S 1 S 3. The ynchronzaton realzed by the 75 khz tme ynchronzaton block. A more detaled explanaton wll be gven n Secton C. The averaged current of each LED trng controlled by the repectve control loop that compare the current-ene voltage V (whch equal to the LED current amplfed by 10 tme) to a reference I ref. The error gnal V EA compenated by a PI compenator and modulated by a PWM modulator to gve the on-tme duty rato d and command S a. The gnal that are provded by the three phae clock generator are ued to command S 1 S 3 and elect one of the three channel of the MUX. In practce, there wll be a total of three feedback loop, one for each LED trng. The three PI controller take turn to ue the analog comparator, whch mean that n any ntance, the crcut effectvely ha only one et of PI controller n operaton. In addton, wth reference to [43], by operatng the ytem n DCM, the load eentally an emulated retor connected to the converter nput. Although the emulated retor, whch determned by the duty cycle d, dfferent n three LED trng, n any ntance only one emulated retor wll be connected to the converter nput, whch mean that the PFC can be acheved. Fg. 7 how the tmng dagram of the tme-multplexed PWM control ung three dtnct-colored LED to repreent dfferent loadng condton among the three LED trng. 5 can be controlled eparately by agnng a unque current reference n each LED trng. It can be expected that, wth dfferent loadng condton and current reference command, the nductor current I L for repectve trng wll have dfferent (rng and fallng) lope and duraton. Th phenomenon hown n Fg. 6 and verfed later by expermental meaurement. In addton, current balancng, whch a pecal cae of ndependent current control, can be realzed by ung the ame current reference gnal acro all the LED trng wthout the need for addtonal pot-regulator crcut. C. 75 khz Tmng Synchronzaton Block The tmng of S a and S 1 S 3 ynchronzed ung a 555 tmer operatng n monotable tate. The detaled 75 khz tmng ynchronzaton block llutrated n Fg. 8. Fg tmer operatng n monotable tate to generate lnear ramp V aw and pule tran V pule. The ba voltage of the BJT T et by R T2 and R T3, and R T1 erve to lmt the current flowng through T to charge up capactor C T1. The voltage acro C T1 V = Q = I t (1) T CT1, C C T1 T1 Fg. 7. Tmng dagram for dfferent PWM duty rato ung three dtnct-colored LED. Note that for dfferent loadng condton and/or wth dfferent current reference command, the PI output are dfferent and thu the PWM duty rato for each trng are dfferent. In order to mnmze the hardware reource, the output of the PI compenator are tme-multplexed together whle harng a common PWM modulator. Th enable the ubequent logc element beyond the PI compenator to be tme hared among all the SITO output. In the SITO topology, the ue of tme-nterleavng control wth multple energzng phae mean that each of the LED trng ndependently drven and decoupled from the other trng wth mnmal cro-nterference. The current n each ndvdual LED trng where Q the charge of C T1, and I T the current through the BJT. Under the gven confguraton, the 555 tmer operate to generate a lnear ramp V aw at pn 6. The output pn 3 generate a trgger pule whch dp every tme C T1 dcharged. By nvertng th trgger pule, a pule tran V pule, whch ynchronzed wth V aw, obtaned. V aw fed to the PWM comparator and V pule ued to generate the three phae clock to enable the SITO operaton. IV. PARAMETER DESIGN OF THE SIMO LED DRIVER A. Inductor Degn To mnmze the ze of the nductor and mplfy the controller degn for PFC, the converter hould be operated n DCM. Alo, the current rpple n the nductor L hould be lmted to reduce the current tre of the power wtche. Thu, the buck man nductor hould nether be too large nor too mall.

6 6 In the frt wtchng nterval, the ncreang rate of nductor current I L di L VL () t V V n o = =, (4) dt L L where L the nductance when the th trng condered.the peak-to-average current rpple defned a Fg. 9. An equvalent LED model whch compre of an deal dode D LED, mall gnal retor R LED and threhold voltage V th. Fg. 9 how an equvalent LED model, whch compre a ere connecton of an deal dode D LED, a retor R LED and a threhold voltage V th. Baed on th model, the parameter of the red (R), green (G), and blue (B) LED [44] ued n the experment are tabulated n Table II. TABLE II. PARAMETERS OF THE RGB LEDS. Type Equvalent Retance R LED (Ω) Rated Current I LED (ma) Threhold Voltage V th (V) Forward Voltage V F (V) Rated Power P LED (W) Luxeon Rebel Red Luxeon Rebel Green Luxeon Rebel Blue V V n o IL, pa = dt. (5) 2L In teady-tate condton, the DC component of the buck capactor current hould be zero. Therefore, the DC component of the buck nductor current I L V WV o th = I =, (6) LED WR where W the number of LED n one trng, and I LED, V th and R LED are repectvely the rated LED current, the LED threhold voltage, and the LED equvalent retance n the th trng. If the ytem operate n DCM, then I L < I L, where I L repreent the maxmum nductor current rpple when the buck converter operate n boundary conducton mode (BCM),.e., LED V WV V V o th n o < dt. (7) WR 2L LED where d = V o /V n n BCM. Hence, the mnmum value of L ( V V ) WR ( V V ) WR V L = dt = T, n o LED n o LED o _ mn 2( V WV ) 2( V WV ) V o th o th n (8) and the upper boundary of the man nductance gven by L< mn{ L }, = 1,2,3,... W. (9) _ mn Fg. 10. Inductor voltage waveform of a buck converter n CCM operaton. Fg. 10 how the nductor voltage waveform of a buck converter n CCM at teady tate. Ung nductor volt-econd balance, ( V V ) dt V (1 d) T = 0, (2) n o o where V n the nput voltage of the buck converter, V o the output voltage of the th LED trng, a decrbed n Fg. 5, and d the duty rato of the th LED trng a hown n Fg. 7. The output voltage of the th LED trng V = dv (3). o n On the other hand, the lower boundary can be obtaned by conderng the maxmum allowable nductor current rpple ΔI L_max ung V V n o IL = dt I L_ max L. (10) In DCM operaton, we have 2 V rm P, LED R ( d ) = (11) e_ where V rm the RMS value of V n, R e_ (d) the equvalent retance emulated by the DCM buck converter for the th LED trng gven by [43] 2L Re_ ( d ) =, (12) 2 dt and P LED the power conumed by the LED load n the th trng gven by P = WV I, (13) LED F LED

7 where V F the forward voltage of LED n the th trng. From (11) and (12), the duty cycle d can be repreented by 2L 2LP d = =. (14) R d T V T LED 2 ( ) e_ rm By ubttutng (14) nto (10), the maxmum value of L L _ max V V 2P T = (15) D n o 2 LED ( ), 2 I V L _ max rm and the lower boundary of the man nductance gven by L max{ L }, = 1, 2,3,... W. (16) _ max B. Output Capactor Degn For each LED trng, an output capactor C o eparately requred. The degn of the capactor can be performed ndependently nce the operaton of each trng decoupled. The degn approach the ame a that for a DC-lnk capactor n conventonal AC/DC rectfyng ytem nce the employed output capactor have to perform the ame functon of AC energy torage and wtchng frequency flterng. Th dfferent from that of the DC/DC SIMO LED drver n whch the output capactor degned to handle only wtchng rpple. If ΔV = kv o, where V o the average output voltage n trng, ΔV the peak output voltage rpple, and k the rpple factor that defne the allowable peak voltage rpple, then wth reference to [43], the lower lmt for C o C where T ac =1/(60 Hz). o P T kv (17) LED ac, 2 o 2π 7 where M the DCM converon rato of the th LED trng gven by V 2 o M = = V n 8 L WR 2 d T LED. (19) A mple PI controller ued a the compenator. In Fg. 11, whch how the mall-gnal control block dagram, the tranfer functon of the compenator of the th LED trng gven by k + k p _ nt _ G () =, (20) c where k p_ the proportonal gan and k nt_ the ntegral gan. Here, V M the ampltude of the awtooth carrer waveform and H () the enng gan for the th trng. The output of PI compenatorvˆc fed nto the PWM modulator wth a gan of 1/V M n order to generate a duty rato d. The averaged current n each LED trng determned by the correpondng current reference value I ref (). Fg. 11. Small-gnal block dagram of the th trng n the propoed cloed-loop SIMO converter. The loop gan T () of the ytem can be repreented a 1 T() = G () G () H (). (21) V c Buck _ M C. Small Sgnal Analy and Controller Degn Due to the tme-multplex arrangement of the three controller, only one output effectve at any ntance. Therefore, the controller can be degned ndependently. Take one trng a an example. Fg. 11 how the mall-gnal block dagram of one trng. Eentally, the controlled power plant a buck converter operatng n DCM. A traght-forward way to determne the low frequency mall-gnal control-to-output tranfer functon of the buck converter n the th trng, denoted by G Buck_ (), to let the man nductance L tend to zero. Wth reference to [43], G Buck_ () gven by G Buck _ 2V 1 M 1 o ˆ d 2 M WR o LED () = =, dˆ vˆ g = M (1 M ) WR C LED o (18) By ubttutng (18) and (20) to (21), the loop gan become 2V 1 M 1 o k + k d 2 M WR 1 p _ nt _ LED () = H(). VM T D. Degn Example 1+ 2 M (1 M ) WR C LED o (22) The degn parameter gven n Table III are adopted for llutratve purpoe. By ubttutng the value nto (8), the upper lmt of the nductance for the three dfferent LED trng can be found a L 1_mn = 254 µh, L 2_mn = 336 µh, L 3_mn = 341 µh. Accordng to (9), the upper lmt of nductance wll be L < 254 µh. Next, by ubttutng the ame degn parameter nto (15), the lower lmt of nductance for the three LED trng can be found a L 1_max = 3.52 µh, L 2_max = 4.48 µh, L 3_max = 4.53 µh. From (16), the lower lmt of nductance found a L 4.53 µh. Therefore, the range of nductance 4.53 µh L 254 µh. In order to mnmze the ze of the man nductor to acheve a maller overall form factor of the propoed LED

8 drver, L elected to be 5 μμ. However, for a practcal degn, more degn margn of L are recommended to compenate for the operatng tranent, component tolerance, etc. Then, by referrng to (17), the lower lmt of C o for the three LED trng are C o1 902 µf, C o2 653 µf, C o3 642 µf. For llutraton purpoe, C o1, C o2, and C o3 are all choen to be 1000 μf. To demontrate the controller degn, trng 1 (red LED) choen a an example. The nput voltage V n ha a peak value of V. Wth reference to Table II, t dered to upply a regulated output voltage V o1 = 14.7 V and LED current I LED1 = 350 ma. The frt tep to determne the feedback gan H 1 (). A 1 Ω retor R 1 ued a the current enng retor. The voltage of R 1 wll then be amplfed by a factor of p = 10 ung proportonal amplfer, and compared wth current reference I ref1. Hence, we have H ( ) = R p= 10. (23) By ubttutng the related parameter lted n Table III nto (21), the open-loop tranfer functon of the ytem before compenaton (when G c1 () =1) can therefore be wrtten a 1 56 T () = G () H () =. u1 Buck _1 1 V M (24) By ettng T u1 (jω) = 1, the cro-over frequency f cu1 of the uncompenated loop gan T u1 () can be obtaned a f cu1 = khz. The dered cro-over frequency of the loop gan after compenaton T 1 () choen to be f c1 = (1/10) f o = 2.5 khz, where f o the output wtch frequency. From (24) at 2.5 khz, the magntude of T u1 () 56 j 2π 2.5k Tu1( j 2π 2.5 k) = = db. (25) From (20), to obtan a unty loop gan at 2.5 khz, the compenator hould have a 2.5 khz gan of db, whch mean that j 2p 2.5k k + k G j p k = = db (26) j 2p 2.5k p _1 nt _1 ( ) c1 Fg. 12. Bode plot of loop gan T 1 () before and after compenaton a well a the compenator tranfer functon G c1 (). V. EXPERIMENTAL VERIFICATION A hardware prototype of the propoed ngle-tage AC/DC ngle-nductor three-output (SITO) LED drver ha been contructed. Fg. 13 how a photo of the prototype. By choong k p_1 = 3.5, k nt_1 can be calculated a k nt_1 = Thu, the compenator tranfer functon G c1 () G k + k = = + (27) p _1 nt _1 ( ) 3.5. c1 Fg. 13. Hardware prototype of the propoed ngle-tage AC/DC ngle-nductor three-output (SITO) LED drver. Baed on (24) and (27), the Bode plot of the open-loop gan before and after compenaton a well a the compenator tranfer functon G c1 () can be plotted a hown n Fg. 12. From the fgure, the phae margn 70, whch ndcate that the ytem table. Experment verfcaton are performed baed on the hardware prototype hown n Fg. 13 and the degn pecfcaton provded n Table III.

9 TABLE III. DESIGN SPECIFICATIONS. 9 Degn Parameter Input voltage V ac EMI Flter (L f, C f ) Flter Capactor C d Man Swtch Frequency f Max. Current Rpple Δ Lmax Output Capactor (C o1, C o2, C o3 ) Rated Output Voltage (V o1, V o2, V o3 ) Value 110 V 1 mh, 0.1 μf 0.1 μf 75 khz 8 A Degn Parameter Rated LED Current I LED Voltage Rpple Factor k Senng Retor R Power Inductor L Cro-over Frequency f c 1000 μf Same-colored LED Strng 1: 14.7 V Strng 2: 20.3 V Strng 3: 20.7 V Dtnct-colored LED Value 350 ma 7% 1 Ω 5 μh 2.5 khz Strng 1: 7 Blue LED Strng 2: 7 Blue LED Strng 3: 7 Blue LED Strng 1: 7 Red LED Strng 2: 7 Green LED Strng 3: 7 Blue LED Fg. 14. Meaured waveform of the AC lne nput voltage and current for 30 W output power ung ame-colored LED wth a common current reference of 350 ma. Table IV how a lt of component ued n the experment. The experment nvolve two type of LED load. In the frt cenaro, ame-colored LED are ued for the three trng, that, each trng cont of 7 blue LED. In the econd cenaro, dtnct-colored LED are ued for the three trng, that even red LED are agned to the frt trng, even green LED for the econd trng, and even blue LED for the thrd trng. Note that the current n the three trng n ether cenaro can be controlled ndependently to be dentcal or dfferent. TABLE IV. COMPONENT LIST. Component Model no. Component Model no. Dode Brdge Rectfer GBU10G-BP MUX CD74HC4051E Man Swtch (S a ) IPW50R280CE Comparator AD8561ANZ MOSFET Gate Drver IRS2101PBF Ocllator LM555CN/NOPB Freewheelng Operatonal and Branch MUR1540G Amplfer Dode OP340PA Output Swtche (S 1, S 2, S 3 ) IRFI4227PbF Output Capactor (C o1, C o2, C o3 ) UPX1V102MHD (long lfetme) Fg. 15. Meaured waveform of I L and I branch1 I branch3 employng ame-colored LED wth dentcal LED current of 350 ma. Fg. 15 how the full vew of the nductor current I L and the three branch current I branch1 I branch3 wth ame-colored LED and a common 350 ma reference current. The maxmum current n each LED trng peak at around 7.5 A whch fall wthn the degn pecfcaton lmt (.e., Δ Lmax = 8 A). Fg. 16 to 18 how the cloe-up vew of I L and I branch1 I branch3, and the correpondng drvng gnal of man wtch V drve_man and output wtche V drve_1 V drve_3 under dfferent condton. From Fg. 16, ame-colored LED wth a 350 ma common reference command are ued. It how that () the duty cycle of the PWM gnal that drve the man wtch S a are mlar for dfferent LED trng; and () the peak value of I branch1 I branch3 are the ame. Alo, S a ON n every wtchng cycle, but the output wtch S (where = 1,2,3) ON n every three wtchng cycle. Conequently, I L ramp up and down n each wtchng cycle but the branch current I branch of each LED trng appear every three wtchng cycle. In other word, I L agned to each of the three LED trng n a round-robn fahon. The expermental reult verfy the functonalty of the SIMO topology and the tme-multplexed control method. A. Ccrut Operatng Prncple Fg. 14 how the AC lne voltage and nput current waveform ung a 110 V 60 Hz AC ource and ame-colored LED a the load. It can be een that the AC lne voltage and the nput current are eentally n phae and the power factor meaured a 0.99, thereby verfyng the functonalty of PFC.

10 10 Fg. 16. Cloe-up vew of drvng gnal of man wtch S a and output wtche S 1 S 3 and the correpondng I L and I branch1 -I branch3 wth ame-colored LED and a common 350 ma reference command. Fg. 17 how the dtnct-colored LED cenaro wth a 350 ma common reference. It mportant to note that the PWM duty rato correpondng to each of the three LED trng dfferent. Alo, the peak value of the branch current I branch (alo the peak nductor current) alo dtnct among the three LED trng. On the other hand, Fg. 18 how the ame-colored LED cenaro wth dfferent reference value. Smlar to Fg. 17, the duty cycle of the PWM gnal whch drve S a and the peak value of I branch n three LED trng are dfferent n every wtchng cycle. Fg. 18. Cloe-up vew of drvng gnal of man wtch S a and output wtche S 1 S 3 and the correpondng I L and I branch1 -I branch3 ung ame-colored LED and wth dtnct reference current value (.e., 250 ma, 350 ma, 450 ma) acro the three LED trng. B. Current Balancng and Steady State Independent Current Regulaton The averaged current n each of the three ndvdual LED trng can be ndependently adjuted for the purpoe of color-mxng and dmmng. Alo, n order to acheve brghtne unformty, current balancng of dfferent LED trng requred. The waveform for thee two cenaro are llutrated n Fg. 19. Fg. 17. Cloe-up vew of drvng gnal of man wtch S a and output wtche S 1 S 3 and the correpondng I L and I branch1 -I branch3 wth dtnct-colored LED and a common 350 ma reference command.

11 11 Fg. 19. Output current waveform of the three LED trng ung ame-colored LED and wth 250 ma, 350 ma, 450 ma ndvdual current control and 350 ma current balancng condton. Fg. 19 how the ndvdual current control of output current I LED1 I LED3 n each LED trng n teady-tate condton. It how that the average current value n the frt, econd and thrd LED trng are 250 ma, 350 ma and 450 ma, repectvely, due to dfferent current reference beng appled to each LED trng. Fg. 19 how the current balancng of I LED1 I LED3 n each LED trng. The average current value n each of the three LED trng are dentcal (I LED = 350 ma) wth a peak-to-peak rpple wthn 10% of I LED. Th demontrate the current balancng capablty of the propoed drver. C. Independent Current Control wthout Cro-Regulaton In order to further demontrate the ndependent current control capablty of the propoed AC/DC LED drver, the reference command I ref3 for Strng 3 tep changed from 3.5 V (350 ma) to 2.5 V (250 ma) and then back to 3.5 V (350 ma) hown n Fg. 20, correpondng to 100% to 70% load nterchange. The current reference of the other two trng I ref1,2 are kept contant at 350 ma. A hown, the rng and fallng tranton tme are both around 25 m and there no obervable cro-regulaton ue for the three LED trng. Fg. 20. Tranent current waveform and reference control command for 350 ma to 250 ma n LED trng 3 and 250 ma to 350 ma n LED trng 3. D. Meaured Effcency and Performance The meaured power converon effcency, power factor and total harmonc dtorton (THD) veru output power are repectvely hown n Fg Effcency 95.00% 90.00% 85.00% 80.00% 75.00% 70.00% Fg. 21. Meaured power effcency veru the output power Output Power (W) Power Factor Output Power (W) Fg. 22. Power factor meaurement veru the output power.

12 THD (%) Output Power (W) Fg. 23. Meaured THD veru the output power. From Fg. 21, t can be een that a the output power ncreae, the effcency of the propoed AC/DC SIMO LED drver alo ncreae and peak at 89% (ncludng drver lo) at around 21 W. Fg. 22 how the varaton of the power factor acro dfferent value of the output power. The meaured power factor peak at and the correpondng THD meaured to be 7%, a hown n Fg. 22 and 23. The meaured nput current alo conform to Cla C of the IEC tandard [45], a wll be dcued hortly. It hould be noted that wth an ncreang number of LED connected n ere or wth an ncreaed output power (.e., the output voltage become larger) at a gven AC lne nput voltage, the power factor (PF) could potentally drop below 0.99 due to the larger dtorton n the AC lne nput current I n at the zero-crong pont, where there a hort nterval when the current not conductng. The duraton of th non-conductng nterval of I n drectly related to the output DC voltage. That, the larger the output voltage, the longer th nterval wll be. Hence, when ether more LED are connected n ere or the output power ncreae (.e., hgher output DC voltage), both THD and PF performance wll be degraded. From the above analy, the propoed LED drver can be degned baed on the rated output power o that the power factor can be mantaned to be no le than 0.99 over the entre dmmng range. E. IEC Standard Complance The harmonc current of the propoed LED drver, whch belong to the Cla C Equpment under the IEC tandard [45], are meaured and compared agant the correpondng harmonc current lmt n accordance wth the IEC tandard. Fg. 24 how the meaured harmonc current agant the harmonc current lmt at a 30 W rated output power. Lkewe, Fg. 24 how the meaured harmonc current agant the harmonc current lmt at a 3 W output power (.e. 10 % of the rated output power). The expermental reult clearly how that all the meaured harmonc current fall wthn ther correpondng maxmum harmonc current lmt a defned by the IEC tandard [45]. Fg. 24. Comparon of the meaured harmonc current veru ther correpondng maxmum harmonc current lmt defned by the IEC tandard at 30 W rated output power; 3 W output power (.e., 10 % rated output power). V. CONCLUSIONS Th paper propoe an AC/DC SIMO LED drver whch ntegrate the power factor correcton (PFC) pre-regulaton and LED current regulaton nto a ngle-tage converter. Unlke the extng two-tage drver topologe, the ntermedate DC-lnk tage elmnated n the propoed ngle-tage topology. Th enable the ue of low-voltage, long-lfetme capactor n the propoed LED drver. In addton, the propoed drver employ only one ngle nductor to drve multple ndependent LED trng. It can acheve fully-ndependent current control n each LED trng wth no notceable cro-regulaton. The major beneft of the propoed ngle-tage LED drver nclude a lower component count, reduced BOM cot, mplfed control cheme, and eae of mplementaton. The expermental reult demontrate the effectvene of the propoed ngle-nductor three-output (SITO) LED drver n attanng prece and ndependent current regulaton acro the three ndvdual LED trng. It enable flexble color-mxng and wde-range dmmng for hgh-qualty lghtng applcaton.

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14 [37] K. H. Chen, Y. H. Lee, S. J. Wang, Y. Y. Yang, Y. H. Ln, SIDO power converter and drvng method, US Patent A1, Jul. 05, [38] M. Gllom, Current control for SIMO converter, US Patent 8,736,195, May [39] S. Huynh and C. V. Pham, Sngle nductor multple LED trng drver, US Patent A1, Feb. 23, [40] H. Km, C. Yoon, H. Ju, D. Jeong, and J. Km, An AC-powered, flcker-free, mult-channel LED drver wth current-balancng SIMO buck topology for large area lghtng applcaton, n IEEE APEC, 2014, pp [41] Electromagnetc Compatblty (EMC) Part 3: Lmt-Secton 2: Lmt for Harmonc Current Emon (Equpment Input Current < 16A Per Phae), n IEC , [42] S. L, S. C. Tan, C. K. Lee, E. Waffenchmdt, S. Y. R. Hu and C. K. Te, A urvey, clafcaton and crtcal revew of lght-emttng-dode drver, IEEE Tran. on Power Electronc, (early acce). [43] R. W. Erckon, D. Makmovc, Fundamental of Power Electronc, Second Edton, Sprnger Scence, New York, [44] LUXEON rebel and LUXEON Rebel ES Color Portfolo, Phlp Lumled Lght. Co. (2014). [Onlne]. Avalable: PB68_LUXEON%20Rebel%20Color_ pdf [45] "Electromagnetc Compatblty (EMC) Part 3: Lmt Secton 2: Lmt for Harmonc Current Emon (Equpment Input Current < 16 A Per Phae)," n IEC , [46] Y. Guo, S. L, A. T. L. Lee, S. C. Tan, C. K. Lee, and S. Y. R. Hu, "AC-DC Sngle-Inductor Multple-Output LED Drver", PCT Appl. PCT/CN2015/077290, 23 Apr Albert T.L. Lee (M 13) receved the BSc degree (Hon.) n Electrcal Engneerng from the Unverty of Wconn, Madon n 1994, the MSc degree n Electrcal and Computer Engneerng from the Unverty of Mchgan, Ann Arbor n 1996, and the Ph.D. degree n Electronc and Computer Engneerng at the Hong Kong Unverty of Scence and Technology (HKUST), Kowloon, Hong Kong n Albert joned Intel Corporaton, Hllboro, Oregon n 1996 a a Senor Component Degn Engneer and wa nvolved n the development of Intel P6 famly mcroproceor. In 2001, he erved a a Senor Corporate Applcaton Engneer n the Sytem-Level Degn Group at Synopy Inc., Mountan Vew, Calforna. In 2003, he joned the Hong Kong Appled Scence and Technology Reearch Inttute Company Ltd. and erved a EDA Manager n the Wrelne Communcaton Group. In 2006, he joned the Gant Electronc Lmted a Hardware Degn Manager and became Aocate General Manager n Preently, he a Reearch Aocate n the Department of Electrcal and Electronc Engneerng, The Unverty of Hong Kong, Hong Kong. H reearch nteret are focued n the area of power electronc and control, LED lghtng, and emergng LED drver technologe. Yue Guo receved the BEng degree n Electronc and Informaton Engneerng degree from the Hong Kong Polytechnc Unverty n He preently workng toward h MPhl degree n Electrcal and Electronc Engneerng at the Unverty of Hong Kong. H reearch nteret nclude embedded hardware degn and dgtal gnal proceor (DSP) baed applcaton. Snan L (M 14) wa born n Chna n He receved the B.S. degree n electrcal engneerng from Harbn Inttute of Technology, Chna, n 2009, and the Ph.D. degree n electrcal and electronc engneerng from The Unverty of Hong Kong, Hong Kong, Chna, n He alo one of the foundng member of IEEE-Eta Kappa Nu (HKN) at HKU. He currently a Reearch Aocate wth the Department of Electrcal and Electronc Engneerng, The Unverty of Hong Kong. Dr. L ha publhed more than 20 tranacton paper and conference paper. He alo hold three U.S. patent. H current reearch area nclude the Power Electronc, LED Lghtng, Control, Renewable Energy and Smart Grd. Sew-Chong Tan (M 06 SM 11) receved the B.Eng. (Hon.) and M.Eng. degree n electrcal and computer engneerng from the Natonal Unverty of Sngapore, Sngapore, n 2000 and 2002, repectvely, and the Ph.D. degree n electronc and nformaton engneerng from the Hong Kong Polytechnc Unverty, Hong Kong, n From October 2005 to May 2012, he worked a Reearch Aocate, Potdoctoral Fellow, Lecturer, and Atant Profeor n Department of Electronc and Informaton Engneerng, Hong Kong Polytechnc Unverty, Hong Kong. From January to October 2011, he wa Senor Scentt n Agency for Scence, Technology and Reearch (A*Star), Sngapore. He currently an Aocate Profeor n Department of Electrcal and Electronc Engneerng, The Unverty of Hong Kong, Hong Kong. Dr. Tan wa a Vtng Scholar at Granger Center for Electrc Machnery and Electromechanc, Unverty of Illno at Urbana-Champagn, Champagn, from September to October 2009, and an Invted Academc Vtor of Huazhong Unverty of Scence and Technology, Wuhan, Chna, n December H reearch nteret are focued n the area of power electronc and control, LED lghtng, mart grd, and clean energy technologe. Dr. Tan erve extenvely a a revewer for varou IEEE/IET tranacton and journal on power, electronc, crcut, and control engneerng. He an Aocate Edtor of the IEEE

15 . He a coauthor of the book Sldng Mode Control of Swtchng Power Converter: Technque and Implementaton (Boca Raton: CRC, 2011). 15 Ch Kwan Lee (M 08) receved the B.Eng. and Ph.D. degree n electronc engneerng from the Cty Unverty of Hong Kong, Kowloon, Hong Kong, n 1999 and 2004, repectvely. He wa a Potdoctoral Reearch Fellow n the Power and Energy Reearch Centre at the Natonal Unverty of Ireland, Galway, from 2004 to In 2006, he joned the Centre of Power Electronc n Cty Unverty of Hong Kong a a Reearch Fellow. In he wa a Lecturer of Electrcal Engneerng at the Hong Kong Polytechnc Unverty. He ha been a Vtng Reearcher at Imperal College London nce From January 2012, he ha been an Atant Profeor at the Department of Electrcal & Electronc Engneerng, The Unverty of Hong Kong. He won an IEEE Power Electronc Tranacton Frt Prze Paper Award for h publcaton on Md-Range Wrele Power Tranfer n He a co-nventor of the Electrc Sprng and planar EMI flter. H current reearch nteret nclude wrele power tranfer, clean energy technologe and mart grd. S. Y. (Ron) Hu (M 87-SM 94-F 03) receved h BSc (Eng) Hon at the Unverty of Brmngham n 1984 and a D.I.C. and PhD at Imperal College London n Preently, he hold the Phlp Wong Wlon Wong Char Profeorhp at the Unverty of Hong Kong and a part-tme Char Profeorhp at Imperal College London. He ha publhed over 300 techncal paper, ncludng more than 190 refereed journal publcaton. Over 55 of h patent have been adopted by ndutry. He an Aocate Edtor of the IEEE and IEEE Tranacton on Indutral Electronc, and an Edtor of the IEEE Journal of Emergng and Selected Topc n Power Electronc. H nventon on wrele chargng platform technology underpn key dmenon of Q, the world' frt wrele power tandard, wth freedom of potonng and localzed chargng feature for wrele chargng of conumer electronc. In Nov. 2010, he receved the IEEE Rudolf Chope R&D Award from the IEEE Indutral Electronc Socety and the IET Achevement Medal (The Crompton Medal). He a Fellow of the Autralan Academy of Technologcal Scence & Engneerng and the recpent of the 2015 IEEE Wllam E. Newell Power Electronc Award.