Digital Variable Frequency Control for Zero Voltage Switching and Interleaving of Synchronous Buck Converters

Transcription

1 Dgal Varable Frequency Conrol for Zero Volage Swchng and Inerleavng of Synchronous Buck Converers Pål Andreassen, Guseppe Gud, Tore M. Undeland Norwegan Unversy of Scence and Technology, Trondhem, Norway Absrac When a synchronous buck converer s operaed wh zero volage swchng (ZVS) and fxed, he drecon of he curren n he oupu nducor s alernaed each swchng perod. Thus he oupu curren rpple of he converer s hgh o ensure zero volage swchng operaon a maxmum load. Wh nerleaved oupus hs leads o hgh crculang currens a low loads. In hs paper, o mnmze crculang currens a low loads, soluons for dgal conrol of ZVS wh varable swchng and nerleavng conrol for bdreconal power flow are presened. The mehods have been mplemened n a DSP and verfed by measuremens. I. RODUCTION When he synchronous buck converer s operaed wh zero volage swchng (ZVS), he drecon of he curren n he oupu nducor s alernaed each swchng perod. The energy sored n he oupu nducor s used o charge he dran source capacance of he ranssors, and he volage s naurally commuaed from one ranssor o he oher. Ths converer n ZVS operaon s also called quas square wave (QSW) converer due o he resonan ranson of he dran source volage, V ds. The advanages of he ZVS operaon are reduced urnon losses, mnmzed swch volage sress, naural commuaon of he body dode, and mnmzng he oupu nducor wh regard o sored energy. The dsadvanages are hgh npu and oupu curren rpple, whch leads o ncreased urnoff losses, ncreased conducon losses, ncreased nducor losses, and large flerng capacors [2]. By nerleavng mulple converers he npu and oupu curren rpple can be sgnfcanly reduced. The nerleaved quas square wave converer s documened o have hgh power densy, and fas ransen response [1]. I s mos common o mplemen he converer conrol wh a fxed swchng, as n [3] and [6]. Wh fxed, he oupu curren rpple of each of he paralleled converer oupus s fxed. The peak o peak oupu curren rpple of each converer mus be more han wce he maxmum average oupu curren n order o manan zero volage swchng for he whole operang regon. Ths leads o hgh crculang currens a low loads. Therefore, even f he effcency of he ZVS operaon s comparable o he hard swchng operaon a hgh loads, he effcency of ZVS operaon s much less a low loads +V ds2 V n + +V ds1 Fg. 1. Inerleaved ZVS synchronous buck converers due o he crculang currens beween he parallel oupus. The smlar crcu opology n ZVS operaon for DCAC converson s also known as he resonan pole nverer (RPI) [4]. For he resonan pole nverer, analog conrol mehods wh varable swchng have been presened. These conrol mehods mnmzes he curren rpple bu do no acvely conrol nerleavng of he paralleled oupus. Paralleled oupus cancel ou he curren rpple only sochascally, as dscussed n [5]. In hs paper, soluons for dgal conrol wh varable swchng and wh nerleavng conrol of synchronous buck converers are suggesed. The converer opology and he defnons of volage and curren polary are shown n Fg. 1. These mehods use he nducor curren value, whch could be ndrecly observed or drecly measured, n order o generae nerrups ha conrols he swchng. Boh parallel oupus are synchronzed o hs. The frs mehod presened, calculaes he upper ranssor onme (for posve reference curren) drecly based on measured npu and oupu volage, he nducance, and he reference curren value. The second mehod uses he synchronzed samplng of he peak curren o calculae a change n he upper ranssor onme (pos. ref. curren). The hrd mehod s smlar o he second mehod excep ha uses he nducance, he measuremen of he upper ranssor offme, and he oupu volage o esmae he peak curren. These mehods are presened for DCDC converers bu could be appled o nverer conrol, and allow bdreconal power flow. These mehods could be used for any number of paralleled oupus, bu o smplfy dscussons, wo nerleaved oupus are dscussed n hs ex. I 2 I I o oad V ou /06/$ IEEE 184 EPEPEMC 2006, Pororož, Slovena

2 +V n () peak,ref +V ds2 2 +V ou +V ds1 oad T on Dgal Conroller V n I 2 I Inerrup Ref.drecon V ou Comp. Selec + Fg. 2. Conrol sysem seup. +,negref II. THE ZVS CONTRO STRATEGIES AND PRINCIPE OF OPERATION Fg. 2 shows he seup of he nerleaved converers nerfaced o he dgal conrol sysem. The npu volage, he oupu volage, and he nducor currens are sampled. One of he converer oupus s used as a maser. For he maser oupu, wo comparaors are used on he measured nducor curren o generae nerrups ha conrols he swchng. Whch comparaor used o conrol he swchng depends on he drecon of he reference curren, avg,ref. When avg,ref s posve, he comparaor wh he npu s used o conrol he. When avg,ref s negave, he comparaor wh he,negref npu s used o conrol he. The value of and,negref s based on he mnmum curren requred n he nducor n order o ensure zero volage swchng. When avg,ref s posve, he nducor curren s dgally conrolled by conrollng he upper ranssor onme. When avg,ref s negave, he nducor curren s dgally conrolled by conrollng he lower ranssor onme. To smplfy furher dscussons, he more dealed descrpon of he conrol s descrbed for posve reference curren only.fg. 3 shows he waveforms of he maser oupu n ZVS operaon. When upper ranssor s off he nducor curren,, falls lnearly based on he oupu volage, v o, and he nducance, 1. The curren passes rough zero and reach he mnmum curren requred,, o ensure zero volage swchng. When s reached, he comparaor generaes an nerrup o he dgal conrol, whch conrols he urnoff of he lower ranssor. The nducor curren hen flows hrough he dransource capacance,, and commuaes he dransource volage, v ds, n a resonan ranson. Afer he ranson he upper ranssor s urned on and he nducor curren,, rses accordng o he, npu volage, v n, he oupu volage, v ou, and he nducance 1. V n T s,varable v ds1 () Fg. 3. Curren and volage waveforms A. Conrol of nducor curren by drec calculaon of onme The conrol objecve s o conrol he average nducor curren. Snce he curren a urn on,, s well defned he average curren can be calculaed. The peak curren, peak, afer a specfc upper ranssor onme, on, can be esmaed o be based on he basc dfferenal equaon of he nducor curren n (1), as shown n (3). The dgal conrol by drec calculaon se he onme drecly based on (6). The calculaon uses he varable reference npu, avg,ref, he varable sampled npu and oupu volage, v n and v ou, he he oupu nducance, 1, and he mnmum nducor curren for ZVS,. may also be sampled o compensae for delays n he nerrup generang crcu. The samplng of npu and oupu volage, v n and v ou, and he nducor currens are synchronzed wh he urnoff of he upper ranssor. Ths way, he peak curren wll be sampled. Wh he drec calculaon conrol sraegy, he sampled currens are used for error deecon only. d d 1 = v (1) peak on vn vou d = d (2) Imn 0 1 v v = + (3) n ou peak on Imn + I peak mn avg = (4) 2 = 2 I (5) peak, ref avg, ref mn on 2 ( avg, ref Imn) = ( v v ) n ou (6) 185

3 I peak,ref I peak,sample /(v n v ou ) V ds1 V ds2 I o T on1 T on2 Fg. 4. Conrol wh sampled peak curren feedback. T s,varable B. Conrol wh sampled peak curren feedback Wh he feedback of sampled currens, on does no need o be calculaed drecly. The change n he onme, on, s calculaed. Fg. 4 shows he prncple. Afer samplng he peak curren, he dfference beween reference curren, peak,ref and he measured curren, peak,sample, s used o calculae he change n onme. The equaon o calculae he change s shown n (7). The nex onme, on2, can hen be calculaed based on, he prevous onme, on1 and he calculaed change as shown n (8). The varable nerrup s wha sablzes he conrol loop. Wh he peak curren measuremen feedback s no necessary o use he sampled npu and oupu volage o calculae on2. Insead a consan may be used, as shown n (9). The consan wll be a fxed gan ha can be furher reduced o sablze he conrol loop. The conrol response wll no be as fas as he conrol wh drec calculaon, bu a no desrable dvson s avoded. Wh drec calculaon, he onme calculaed mus be used for all parallel oupus. Ths means ha he curren sharng wll be passvely governed. By usng he peak curren feedback mehod, he peak curren on all parallel oupus may be sampled o se a separae onme on each oupu. Wh he peak curren feedback mehod, acve curren sharng conrol beween all parallel oupus s possble. = ( ) 1 on peak, ref peak, sample ( vn vou ) on2 on1 on (7) = + (8) = ( ) (9) 1 on, prop peak, ref peak, sample Vn,max C. Conrol wh esmaed peak curren feedback The upper ranssor urnoff me, off, s calculaed a each nerrup based on on and s,varable as shown n (10). The peak curren can hen be esmaed based on off, v ou, and 1, as shown n (11). The esmaed peak curren, peak,es, s hen used o calculae he change, on,prop, nsead of he sampled peak curren, peak,sample n (9). = (10) off s,varable on peak, es v ou off = (11) 0.5*T s,varable Fg. 5. Varable nerleaved waveforms D. Generang nerleaved swchng waveforms Fg. 5 llusraes he waveforms of wo nerleaved parallel oupus. The nerrup generaed by he nducor curren n 1 do no only conrol he of he frs oupu. Also he 180 degrees shfed mer of he second oupu can be adjused dgally by hs nerrup. All he mers of any number of parallel oupus may be synchronzed o he same and phase shfed 360/n degrees o generae he nerleaved swchng waveforms. The slave phases are reloaded wh new mer perod value a s perod nerrup, and he mers are also adjused for correc phase shf a hs me. III. TEST SET UP AND MEASUREMENTS A conrol board wh a Texas Insrumens F2812 DSP s used o es he dgal conrol echnques. Two parallel synchronous buck converers are coupled ogeher. The npu volage, v n, he oupu volage, v ou, and he curren, x, n each of he wo nducors are sampled. The mehod mplemened was wh peak curren feed back. The curren sharng was no done by acve conrol, bu passvely governed by he ressance n he ranssors and nducances. Ths way, curren measuremen of he slave oupu was no needed. Fg. 6, Fg. 7 and Fg. 9 show he seady sae nducor curren waveforms, for wo dfferen load condons. The sum of he nducor currens s he brown waveform. The measuremens show how he changes for he dfferen load condons wh he same reference curren. The curren s he same for all hree load condons and he oupu load ressance s vared. I can be seen how he duy rao vares,.e. he oupu volage. I also shows how he curren rpple s reduced ou for dfferen duy raos. Fg. 9 shows he nducor curren and oupu volage ransen o a curren reference sep. The gan had o be reduced sgnfcanly lower han he heorecal lm n order for he conrol loop o be sable. The yellow curve, CH1, s he oupu volage. The green and purple curve, CH3 and CH4, are he nerleaved currens. Fg. 10, shows he nerrup mng. The blue curve, CH2, shows he nerrup mng. The frs lefmos pulse on CH2 s he exernally rggered nerrup, here he new me perod (.e. ) s se and a perod nerrup for he maser phase, CH3, s rggered. The second pulse s he maser phase perod nerrup. In hs nerrup he nex 186

4 Fg. 6. Seady sae waveforms 1 s load condon. 78kHz swchng Fg. 9. Reference sep ransen Fg. 7. Seady sae waveforms 2 nd load condon. 96kHz swchng Fg. 10. Real me conrol nerrups mng, CH2 I was suffcen o le he curren sharng be passvely governed by he oupu ressance due o a suffcenly hgh resoluon on he pulse wdh modulang oupu. The ess n he laboraory showed ha he gan n he feedback loop had o be sgnfcanly reduced below he heorecal calculaed lm n order o sablze he conrol. The reason for hs s no fully explored ye. ACKNOWEDGMENT The auhors would lke o exend her hanks o Wllam Gullvk a he company Acve DSP. The suppor on he real me conrol developmen ool DSPComm has been much apprecaed. Fg. 8. Seady sae waveforms 3 rd load condon. 82kHz swchng onme s calculaed. The hrd pulse s he slave phase, CH4, perod nerrup. In hs nerrup he nex perod and onme for he slave phase s loaded. Also he mer for he slave phase s adjused for he opmal phaseshf. IV. CONCUSION Ths paper has presened and dscussed mehods o mplemen a dgal conrol wh varable ha enables nerleavng and zero volage swchng of paralleled synchronous buck converers wh bdreconal power flow. The peak curren feedback mehod has been mplemened n a DSP and verfed by measuremens. REFERENCES [1] X. Zhou, P.. Wong, P. Xu, F.C. ee, A.Q. Huang, Invesgaon of Canddae VRM Topologes for Fuure Mcroprocessors, IEEE Transacons on Power Elecroncs, Volume: 15, No: 6, Nov [2] G. Hua, F.C. ee, SofSwchng Technques n PWM converers, Indusral Elecroncs, Conrol and Insrumenaon, Proceedngs of he IECON 93, p [3] X. Zhou, P. Xu, F.C. ee, A.Q. Huang, A Novel CurrenSharng Conrol Technque for owvolage HghCurren Volage Regulaor Module Applcaons, IEEE Transacons on Power Elecroncs, Volume: 15, No: 6, Nov [4] B. Acharya, D.M. Dvan, R.W. Gascogne, Acve power flers usng resonan pole nverers, IEEE Transacons on Indusry Applcaons, Volume 28, Issue 6, Nov.Dec Page(s): [5] D.J. Perreaul, J.G Kassakan, Analyss and conrol of a cellular converer sysem wh sochasc rpple cancellaon and mnmal 187

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