Dual Current-Mode Control for Single-Switch Two-Output Switching Power Converters

Transcription

1 Dual Curren-Mode Conrol for Single-Swich Two-Oupu Swiching Power Converers S. C. Wong, C. K. Tse and K. C. Tang Deparmen of Elecronic and Informaion Engineering Hong Kong Polyechnic Universiy, Hunghom, Hong Kong, China Absrac A novel mehod for programming curren in dc/dc converers operaing in disconinuous conducion mode is described in his paper. The conrol variable is he produc of he square of he duy cyle and he swiching period, i.e., D 2 T, which is direcly proporional o inpu and oupu currens of a disconinuous-mode converer. A mehod of conrolling D 2 T is applied o converers ha uilize one swich (or one se of synchronous swiches) for achieving wo conrol funcions. In paricular a single-swich wo-oupu boos converer, in which a coninuous-mode converer and a disconinuous-mode converer share one acive swich, is sudied. In his sysem, curren-mode conrol is used o regulae he oupu volage of he coninuousmode converer and he proposed D 2 T conrol is used o regulae he oher disconinuous-mode converer. The resul is a generic curren-mode conrolled dual-oupu converer. Keywords Curren-mode conrol, dc/dc converer, dual-oupu converer, disconinuous conducion mode. generaor waveforms p(t) = r(t) r() p() 0 (1D)T T swich driving signal off-ime on-ime off-ime on-ime (a) I. INTODUCTION This paper describes a single-swich wo-oupu regulaor which comprises wo boos converers, one operaing in coninuous conducion mode (CCM) and he oher in disconinuous conducion mode (DCM) [1], [2]. Previously repored conrol mehods ake advanage of he insensiiviy of he CCM converer o swiching frequency, and apply duy-cycle conrol o regulae he CCM converer while regulaing he DCM converer by frequency modulaion [3], [4]. In his paper, we consider applicaion of generic curren-mode conrol o boh converers in order o achieve faser ransien responses [5]. Specifically, we use a sandard curren programming for he CCM converer and a novel D 2 T conrol for regulaing he DCM converer. We will also discuss he local sabiliy of he combined curren-mode and D 2 T conrol scheme, and presen a pracical circui implemenaion of he conroller. II. OUTLINE OF THE POPOSED CONTOL Inspired by is averaged behaviour model, he DCM converer can be conrolled by varying he conrol quaniy D 2 T which is proporional o he oupu curren [6]. In principle, if he quaniy D 2 T is adjused via a feedback mechanism, he oupu load volage can be regulaed. A special bu pracically imporan case is when eiher he on-ime or off-ime duraion is already deermined by anoher conrol law. We shall make reference o a single-swich wo-oupu boos converer laer in he paper. Suppose he duy cycle is D, and he period begins wih he swich urned off. Thus, afer a duraion of (1 D)T, he swich is urned on; and afer anoher duraion of DT, he swich is urned off again, compleing one cycle. Our objecive is o derive a scheme, whereby he off-ime duraion (1 D)T is pre-deermined and he period T is adjused o give he desired value of D 2 T. The proposed I b resar swiching cycle si b v y Cy kv y s = 1 for on ime s = 0 for off ime (b) Fig. 1. (a) Waveforms of he parabolic and ramp generaors in he concepual D 2 T conrol scheme; (b) concepual implemenaion. conroller consiss of a periodic parabola generaor p() and a ramp generaor r(): C z p() = a( 0 ) 2 (1) r() = b( 0 ) (2) where 0 and 0 are arbirary sar insans for he wo generaors, and a and b can be considered as consan for he ime being. Fig. 1 (a) shows hese generaor waveforms. A he sar of he swiching cycle, say =0, he ramp generaor is riggered o sar. A =(1 D)T (which is deermined exernally), he swich is urned on. A he same ime, he parabola generaor is riggered o sar from zero. As soon as he oupus of he generaors are equal, he swich is urned off whereby spawning a new cycle. Clearly, his /04/$ IEEE. 23

2 Fig. 2. i L1 V in i L1 i 1 i 2 m c m 2 m 1 compensaion slope I ref T 2 T 1 Inducor curren waveform for sabiliy analysis. S L 1 L 2 Q curren-mode conrol [CCM] [DCM] D 2 T conrol rese I ref analog C 1 o1 v o1 C 2 o2 v o2 D 2 T analog C f2 f2 C f1 f1 V ref2 V ref1 oupu 1 oupu 2 Fig. 3. Schemaic of dual-oupu boos regulaor under combined currenmode D 2 T conrol. Deails of he D 2 T conrol block is shown in Fig. 4. condiion forces i2 r2 i1 r1 p(dt) =r(t ) a(dt) 2 = bt D 2 T = b/a. (3) Hence, he quaniy D 2 T can be made conrollable by varying b/a. A concepual implemenaion of his conrol scheme is shown in Fig. 1 (b). In his scheme, he off-ime duraion is exernally deermined, and he conrol circui in urn produces a pulse o se he period. Thus, he proposed conrol can be viewed as a frequency modulaor which, for each cycle, mainains a consan D 2 T. If he curren sources (i.e., a and b) are conrollable, he quaniy D 2 T can be modulaed via feedback. III. LOCAL STABILITY OF COMBINED CUENT-MODE AND D 2 T CONTOL Our ineres in his secion is o sudy he local sabiliy of he proposed conrol scheme when he off-ime duraion is pre-deermined by a curren-mode conrol scheme. Essenially, in a ypical coninuous-mode dc/dc converer under currenmode conrol, he urn-off insan is deermined by comparing he inducor curren wih a reference level. Thus, effecively, his convenional scheme is conrolling he on-ime duraion. In our scheme, however, we conrol he off-ime duraion insead. As we will see, his has an imporan implicaion on he sabiliy. In brief, he swich is urned on a he insan he inducor curren descends o a reference level. The urnoff insan, moreover, is deermined by our proposed D 2 T conrol. In oher words, he repeiion period is conrolled by he D 2 T conroller. The sabiliy issue herefore involves he consideraion of he sabiliy of he curren-mode conrol wih off-ime duraion being pre-deermined and he period being conrolled by he D 2 T scheme. Firs, referring o Fig. 2, we can wrie he inducor curren values a he sar and end of a period as i 1 = I ref (m c m 2 )T 2 i 2 = I ref m c T 2 m 1 T 1 (4) where T 1 is he on-ime duraion, T 2 is he off-ime duraion, m 1 is he on-ime inducor curren slope, m 2 is he offime inducor curren slope, and m c is he compensaion slope. Noe ha we consider here he general case where a compensaion slope is included in he curren-mode conrol. Upon differeniaing (4), we ge δi 2 = m 1δT 1 m c δt 2 = m δt c m 1 1 δt 2. (5) δi 1 (m c m 2 )δt 2 m c m 2 Le us now inroduce he D 2 T conrol o he abovemenioned curren-mode conrolled converer. In he seady sae, he aim is o fix he value of D 2 T, i.e., T 2 1 T 1 T 2 = consan, (6) where he consan in he HS is equal o he seady-sae value of D 2 T. Thus, differeniaing (6), we ge δt 1 δt 2 = D 2 D. (7) Hence, from (5), and using m 1 D = m 2 (1 D), we have D 2 D 1 D 2 D δi 2 = m c m 1 = m c m 2 (8) δi 1 m c m 2 m c m 2 Local sabiliy requires ha he magniude of he above expression be less han 1. Thus, we can see ha sabiliy is guaraneed for all D since 0 < 1 D 2 D < 1 2 for all D (0, 1), (9) which implies δi 2 /δi 1 < 1 for all 0 <D<1. I is ineresing o noe ha he sabiliy of he sysem is unaffeced even when he compensaion ramp is zero. In oher words, he D 2 T conrol inherenly sabilizes he curren-mode conrol, eliminaing he need for ramp compensaion. 24

3 Fig. 4. Deailed circui schemaic of he D 2 T conrol. IV. EXPEIMENTAL VEIFICATION In his secion we presen a single-swich wo-oupu boos regulaor, as oulined previously in he Inroducion. Figs. 3 and 4 show he schemaic of he sysem and he deailed circui of he D 2 T conroller, respecively. In brief, his regulaor consiss of a CCM boos converer and a DCM boos converer, sharing one common swich. The circui design aspec has been sudied exensively by Sebasián e al. [1], [3]. Here, we focus on he applicaion of he proposed combined currenmode and D 2 T conrol for simulaneous regulaion of he wo oupus, and verify he conrol funcion wih an experimenal 25

4 (a) (b) (c) (d) Fig. 5. Experimenally measured seady-sae oupu volages versus load variaion in oupu 1, wih load a oupu 2 fixed a 60 Ω; (b) seady-sae oupu volages versus load variaion in oupu 2, wih load a oupu 1 fixed a 10 Ω; (c) ransien responses for sep changing loads wih o1 changing from 10 Ω o 35 Ω a =4ms, and back o 10 Ω a =14ms; (d) ransien responses for sep changing loads wih o2 changing from 50 Ω o 100 Ω a =4.2 ms, and back o 50 Ω a =14.2 ms. Verical scale: 10 V/div and 2 A/div. Horizonal scale: 2 ms/div. TABLE I COMPONENT AND PAAMETE VALUES OF EXPEIMENTAL POTOTYPE Componen/parameer Value Inducance 1 (CCM), L µh Inducance 2 (DCM), L 2 20 µh Oupu capaciance 1, C 1 47 µf Oupu capaciance 2, C µf Inpu volage, V in 12 o 16 V Load resisance 1, o1 10 o 40 Ω Load resisance 2, o2 40 o 147 Ω Power MOSFET IFP 360 Frequency 20 o 100 khz prooype. In he experimen, discree componens are used o consruc he he D 2 T conrol block, as shown in Fig. 1 (b). Componen and parameer values of he prooype are summarized in Table I. I is worh noing ha he inducor curren of he CCM converer is referenced a he urn-on insan, and hence is peak is generally unlimied (acually deermined by he D 2 T conrol). I is hus necessary o impose a peak limier o limi he maximum swiching period. This arrangemen has been incorporaed in our sudy. Several ess have been performed o verify he operaion of he proposed conrol. Firs of all, he seady-sae oupu characerisics characerisics have been obained by ploing he oupu volages agains load variaions. Measured resuls are Fig. 6. Oupu ransien responses for sep changing inpu volage. A = 6 ms, V in seps up from 12 V o 16 V, and a =14ms, i seps down back o 12 V. Verical scale: 10 V/div and 2 A/div. Horizonal scale: 2 ms/div. shown in Figs. 5 (a) and (b). Then, he ransien performance has been evaluaed, including inpu regulaion, load regulaion and cross regulaion. esuls are shown in Figs. 5 (c)(d) and 6. In Figs. 5 (c)(d), he ransien responses are recorded when he load resisances are sepped. Noe ha Figs. 5 (c)(d) reflec very saisfacory self load regulaion performance as well as cross regulaion performance, i.e., ransien of v o1 (or 26

5 v o2 ) when o2 (or o1 ) is sepped. In Fig. 6, he ransien responses are recorded when he inpu volage is sepped. V. CONCLUSION In a dual-oupu volage regulaor, where wo dc/dc converers (one operaing in CCM and he oher in DCM) are sharing one swich, generic curren-mode conrol can be achieved by applying convenional curren-mode conrol o he CCM converer and a D 2 T programming conrol o he DCM converer. In his paper we propose a sraegy for conrolling he D 2 T quaniy, whereby boh oupus can be ighly regulaed. Saisfacory cross regulaion is possible by virue of he CCM converer being insensiive o frequency changes and he DCM converer being direcly curren-programmed. Clearly, he concep of direc D 2 T conrol can also be applied o oher applicaions where one swich is required o achieve wo funcions, e.g., in single-swich single-sage PFC power supplies. ACKNOWLEDGMENT This work is suppored by Hong Kong Polyechnic Universiy under Gran G-T662. EFEENCES [1] J. Sebasián and J. Uceda, Double converer: a fully regulaed wo oupu dc-o-dc converer, IEEE PESC ec., pp , 1985 [2] J. Sebasián, J. Uceda, M. ico, M.A. Pérez and F. Aldana, A complee sudy of he double forward-flyback converer, IEEE PESC ec., pp , [3] T. Charanasomboon, M.J. Devaney and.g. Hof, Single swich dual oupu dc/dc converer performance, IEEE Trans. Power Elecron., vol. 5, no. 2, pp , [4] P.T. Tang and C.K. Tse, Design and implemenaion of a fully-regulaed four-oupu wo-swich one-conroller power module, IEEE PESC ec., pp , June [5]. edl and N.O. Sokal, Curren-mode conrol, five differen ypes, used wih he hree basic classes of power converers, IEEE PESC ec., pp , [6] P.. Severns and E.J. Bloom, Modern Swiching DC-o-DC Power Converer Circuis, New York: Van Nosrand,