A Voltage Doubler Circuit to Extend the Soft-switching Range of Dual Active Bridge Converters Qin, Zian; Shen, Yanfeng; Wang, Huai; Blaabjerg, Frede

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1 Aalborg Universie A Volage Doubler Circui o Exend he Sof-swiching Range of Dual Acive Bridge Converers Qin, Zian; Shen, Yanfeng; Wang, Huai; Blaabjerg, Frede Published in: Proceedings of he 217 IEEE Applied Power Elecronics Conference and Exposiion (APEC) DOI (link o publicaion from Publisher): 1.119/APEC Publicaion dae: 217 Documen Version Acceped auhor manuscrip, peer reviewed version Link o publicaion from Aalborg Universiy Ciaion for published version (APA): Qin, Z., Shen, Y., Wang, H., & Blaabjerg, F. (217). A Volage Doubler Circui o Exend he Sof-swiching Range of Dual Acive Bridge Converers. In Proceedings of he 217 IEEE Applied Power Elecronics Conference and Exposiion (APEC) (pp. -6). IEEE Press. DOI: 1.119/APEC General righs Copyrigh and moral righs for he publicaions made accessible in he public poral are reained by he auhors and/or oher copyrigh owners and i is a condiion of accessing publicaions ha users recognise and abide by he legal requiremens associaed wih hese righs.? Users may download and prin one copy of any publicaion from he public poral for he purpose of privae sudy or research.? You may no furher disribue he maerial or use i for any profi-making aciviy or commercial gain? You may freely disribue he URL idenifying he publicaion in he public poral? Take down policy If you believe ha his documen breaches copyrigh please conac us a vbn@aub.aau.dk providing deails, and we will remove access o he work immediaely and invesigae your claim. Downloaded from vbn.aau.dk on: sepember 22, 218

2 A Volage Doubler Circui o Exend he Sof-swiching Range of Dual Acive Bridge Converers Zian Qin, IEEE Member, Yanfeng Shen, Huai Wang, IEEE Member, Frede Blaabjerg, IEEE Fellow Deparmen of Energy Technology, Aalborg Universiy Aalborg 922, Denmark zqi@e.aau.dk, yaf@e.aau.dk, hwa@e.aau.dk, fbl@e.aau.dk Absrac A volage doubler circui is realized o exend he sof-swiching range of Dual Acive Bridge (DAB) converers. No exra hardware is added o he DAB o form his circui, since i is composed of he dc blocking capacior and he low side full bridge converer, which already exis in DAB. Wih he volage doubler, he DAB converer can achieve sof swiching and high efficiency when he low side dc volage is close o 2 pu (1 pu is he high side dc volage divided by he ransformer urn raio), which can be realized only when he low side dc volage is close o 1 pu by using he convenional phase shif modulaion in DAB. Thus he sof swiching range is exended. The sof swiching boundary condiions are derived. A map o show he sof swiching or hard swiching in he full load and volage range is obained. The feasibiliy and effeciveness of he proposed mehod is finally verified by experimens. I. INTRODUCTION The DAB is a promising opology in applicaions like solid sae ransformers, DC grid, and elecrical vehicles, as i performs bidirecional power ransfer wih an efficien, compac, galvanic isolaed and simple circui [1 ]. The convenional Phase Shif Modulaion (PSM) o conrol he power delivered by he DAB is simple and effecive [4], where boh he wo full bridges generae square waveforms wih 5% duy raio, hen he power is conrolled by regulaing he phase shif angle beween he wo square waveforms. Neverheless, challenges sill exis in order o realize sof-swiching when he volage of he converer has a wide range. Various modulaion sraegies have been proposed in he lieraure o exend he sof-swiching range, e.g. he phase shif modulaion wih one duy raio (or hybrid phase shif modulaion) and he phase shif modulaion wih wo duy raios [5 7]. By inroducing duy raios, hese modulaions can realize riangular or rapezoidal ransformer curren. Alhough he wo modulaion sraegies are differen in erms of ransformer curren shape, heir essence o realize Zero Curren Swiching (ZCS) by running he converer in Disconinuous Curren Mode (DCM) is he same. However, he bybrid phase shif modulaion has wo conrol parameers and he phase shif modulaion wih wo duy raios has hree conrol parameers. The impac of hese parameers is deeply coupled wih each oher. Thus he complexiy of he conroller design is much increased. Acually, a look-up able is normally used insead of a linear conroller when hese complex modulain sraegies are applied. Therefore, he PSM deserves o be sudied more for an improved performance, due o is lower complexiy, before pursuing oher more complicaed modulaion sraegies. In his paper, a volage doubler circui is proposed in DAB o achieve a wide sof swiching range even when he oupu volage is doubled. Wihou exra hardware, he volage doubler circui is formed by he componens already exising in he DAB. The volage doubler circui is realized by sill using PSM wih a lile modificaion. The operaion principle of proposed mehod is demonsraed, he sof swiching area is obained by deriving he sof swiching boundary condiions, he analysis is finally verified by experimenal resuls. II. OPERATION PRINCIPLE A DAB converer consiss of wo full bridges (formed by Q 1 Q 8 ), a ransformer T (urn raio n :1), an inducor L s, wo DC blocking capaciors C bp and C bs, and dc bus capaciors C h and C l, as shown in Fig. 1. Besides, i p and are he ransformer currens a high and low side, respecively, v p and v s are he ransformer volages a high and low side, respecively, and are he volages generaed by he high and low side full bridge, respecively. I should be noed ha he dc blocking capaciors are used o preven he magneic sauraion of he ransformer. Acually, he sauraion issue of he ransformer is rarely sudied in he lieraure, where he soluions include peak curren conrol [9], magneic ear [1, 11], ec. Bu he peak curren signal can be easily disored by noise especially when he swiching frequency is high, while he magneic ear is composed of an auxiliary core and an exra circui. Compared wih hem, using DC blocking capaciors is an easier and more feasible mehod. As menioned in Secion I he PSM is used in he DAB. The gae signals, volage and curren waveforms are shown in Fig. 2, where φ is he phase shif angle beween he high and low side square waveforms and, and T s is he swiching cycle of he converer. The wo parameers and are criical for idenifying wheher sof swiching can be achieved. By solving he rigonomery shown in Fig. 2, hey can be obained as (1) and (2). φt s (V h + nv l ) 2πL s = (1) = + (π φ)t s (V h nv l ) 2πL s (2)

3 Q 1 Q Q 5 Q 7 Vh Ch A ip B L S Cbp vp T * * Cbs vs is C D Cl Vl Q 2 Q 4 n:1 Q 6 Q 8 Fig. 1. A dual acive bridge converer. deadime deadime deadime Q 1 & Q 4 Q 2 &Q Q2 &Q Q & Q 1 4 Q Q 7 8 & Q & Q nv S & n Q 5 & Q Q 6 & Q 7 Q 1 & Q 4 Q & Q & Q nv & Q 2 &Q Q2 &Q Q & Q 1 4 Q 5 &Q Q 8 S n Q 6 & Q 7 Q 1 & Q 4 & Q 5 8 Q 2 &Q Q2 &Q Q & Q 1 4 Q 5 &Q Q Q & Q nv & nv S CD Q 6 & Q 7 i p T s I 2 I1 i p I I 1 T 1 s T I ( ) T s 2 s ( ) Ts ( ) Ts (a) (b) (c) i p 2 Fig. 2. Gae signals, volages generaed by he full bridges, he volage and curren of he ransformer in he DAB using PSM when (a), (b), < (c) <,. Then and are derived as () and (4). derived as (7), where ω is defined as (8). = πnv l (π 2φ)V h 4πf s L s () = πv h (π 2φ)nV l 4πf s L s (4) where f s is he swiching frequency of he DAB. The equivalen circuis of he converer during he zero volage urn on ransien of he high and low side full bridges are shown in Fig. (a) ( = 2 ) and Fig. (b) ( = 1 ), respecively, where 1 and 2 are shown in Fig. 2, and C oss,qx is he parasiic capacior of he power swich Q x. In Fig. (a) when = 2, v coss,q2 = v coss,q = V h, hus he dynamic equaions of he parasiic capaciors and he series inducor can be obained as (5) and (6). L s di p () d C oss,q2 dv coss,q2 () d = v coss,q2 () nv l (5) = 1 2 i p() (6) The parasiic capacior volage in ime domain is hen v coss,q2 () =(V h nv l )cos(ω) ωl s sin(ω)+nv l (7) ω = 1 2Ls C oss,q2 (8) Because Zero Volage Swiching (ZVS) of Q 2 and Q can only happen if heir parasiic volages can ge o zero by resonance. The boundary condiion of he ZVS in high side full bridge is hen obained as (9) according o (7), and furher derived o (1). (V h nv l ) 2 + I2 2ω2 L 2 s nv l (9) 2C oss,q2 (2nV h V l V 2 h ) L s (1) n T dead 2 C oss,q6 V l (11) 2C oss,q6v l nt dead (12) In Fig. (b), he parasiic capacior C oss,q6 is charged by

4 he series inducor curren imes by he ransformer urn raio n. When = 1, he parasiic capacior volage of Q 5 is V l, as shown in Fig. 2. Thus, in order o realize ZVS in Q 5, he parasiic capacior volage of Q 5 mus be discharged o zero or C oss,q6 mus be charged from zero o V l. Because afer = 1, will have a relaively small di/d, as shown in Fig.2, n is hen considered o be consan during he shor swiching ransien. Therefore, wih he same principle, he boundary condiion o achieve ZVS in low side full bridge is obained in (11) and furher derived o (12). vcoss,q2 (V) =.2 A ZVS canno be =.5 = 1 A ZVS can be achieved ZVS can be achieved C oss,q1 C oss,q L S (s) x 1-7 V h ip nv l Fig. 4. The impac of on he parasiic capacior volage resonance of Q 2. C oss,q2 C oss,q4 if &&, Q = 2 + φts 2π nv l (1).5T s (a) if && <, Q = 2 I2 (π φ)ts 2π V h (14).5T s C oss,q5 if < &&, ni1 V l Q = 2 + (π φ)ts 2π nv l (15).5T s C oss,q6 φ 2 nv l 2π( + ), if && (b) Q = (π φ) 2 V h 2π( ), if && < (16) Fig.. Equivalen circuis of he converer during he zero volage urn on ransien of (a) he high side full bridge, = 2 (b) he low side full bridge = 1. (π φ) 2 nv l 2π( + ), if < && According o (7), he volage variaion of C oss,q2 saring from = 1 can be obained and i is shown in Fig. 4, where he values of he parameers are lised in Table I in Secion III. As seen, if is larger hen a cerain value, he parasiic capacior volage will decrease o zero by means of resonance and hen be clamped by he ani-parallel diode. Thus ZVS can be achieved. The dashed line illusraes he resonance of he parasiic capacior volage assuming he ani-parallel diode does no exis. Besides, anoher criical indicaor o evaluae he efficiency of he DAB is he circulaing power Q generaed during he commuaion of he ransformer curren. The circulaing power can be oained by solving he simple rigonomery in Fig. 2, and hey are shown in (1) (15) and furher derived o (16). The acive power delivered by he DAB using PSM is well known, and i is shown in (17). P = nv hv l 2π 2 f s L s φ(π φ) (17) Then, a map o show he sof swiching and hard swiching of he DAB as well as he ciculaing power raio (Q/P ) in differen load and low side volage condiions wih PSM is obained and i is shown in Fig. 5(a). As seen, he sof swiching range is wide when he low side dc volage is close o 1 pu (1 pu = V h /n), and he circulaing power is relaively low as well. The sof swiching range shrinks and he relaive circulaing power increases when he low side dc volage decreases. The performance degrades even worsely when low side dc volage increases. The sof swiching area

5 almos disappears and he relaive circulaing raio is larger han 1/1 in more han 8% of he load range when low side dc volage increases o 2 pu. Wih his awful performance, a 2 pu low side dc volage operaion condiion is no recommended. when V l is 1 pu, which is large and covers almos all he load range. The circulaing power raio is smaller han 1/1 in more han 8% of he load range. This much improved performance degrades slighly even V l decreases or increases from 2 pu. Q 1 & Q 4 deadime Q 2 &Q Q &Q Q & Q Q5 Q6 Q5 n Q 7 & Q 8 Q 6 V Cbs III. EXPERIMENTAL RESULTS Oscilloscope Thermo fiber sensor Volage sources Seup under es nv S Load Differenial volage probes Rogowski Coils Power analyzer i p T s Fig. 6. Gae signals, volages generaed by he full bridges, he volage and curren of he ransformer in he DAB when volage doubler circui is enabled In order o improve he performance of he DAB when low side volage is close o 2 pu, a volage doubler circui is proposed. Insead of adding componens ino he DAB converer, he idea is o rearrange he gae signal sequence of he low side full bridge o generae a bias volage on he low side dc blocking capacior C bs. Then his volage ogeher wih he volage of low side ranformer winding can generae a higher volage level. The modulaion sraegy of he volage doubler circui is shown in Fig. 6, where he only difference compared wih PSM shown in Fig. 2 is ha Q 7 is kep off and Q 8 is kep on insead of having PWM signals. Thus, he low side full bridge will generae a square wave form wih 5% duy raio and wo volage levels and V l insead of ±V l. The dc componen in his square wave form becomes V l /2, and i will drop on he dc blocking capacior C bs,as shown in Fig. 6 (V Cbs ). The low side volage of he ransformer hen urns ino ±V l /2 from ±V l. Since he volage doubler circui is recommended for 2 pu low side dc volage operaion condiion, he low side ransformer volage is hus close o ± 1 pu. Therefore, a similar sof swiching area wih Fig. 5(a), when V l is close o 1 pu, is expeced. The boundary condiions of sof swiching area in he volage doubler circui are acually he same wih PSM, which are (1) and (12), because he same equivalen circuis during ZVS as shown in Fig. are sill valid in he volage doubler circui. and can sill be calculaed as () and (4), bu V l should be changed o V l /2 since he low side ransformer volage changes from ±V l o ±V l /2. The same change should also be applied o he calculaions of he acive power P and ciculaing power Q in (16) and (17). Then by using he proposed volage doubler circui he map in Fig. 5(a) becomes Fig. 5(b). As seen, jus like expeced, he sof swiching area when V l is 2 pu is very similar o he scenario Fig. 7. A picure of he prooype and he es plaform. TABLE I. PARAMETERS USED FOR EXPERIMENTS. Parameers Values Nominal power 1 W High side volage V h 2 V Turn raio of he ransformer n :1.5:1 Low side volage V l 1pu= V h n =57V Series inducor L s 4 μh Swiching frequency f s 1 khz Deadime T dead 2 ns DC blocking capacior C bp 1 μf x8 DC blocking capacior C bs 1 μf x15 Oupu capaciors C oss,q1 C oss,q4 158 pf Oupu capaciors C oss,q5 C oss,q8 41 pf x 2 Experimenal resuls are obained from a es plaform shown in Fig. 7, where all he parameers are lised in Table I. Fig. 8 shows he square waveforms generaed by he high and low side full bridges, and he low side ransformer curren of he DAB when V l = 1.4 pu. As seen, he volage square waveform generaed by he low side full bridge jumps beween ±V l and has no dc componen when PSM is applied. While, he volage levels of become and V l when volage doubler circui is enabled, which inroduces a V l /2 dc componen in. These feaures maches wih he analysis in Secion II very well. Moreover, in he scenario shown in Fig. 55 W, is below zero, and i canno fulfill he sof swiching condiion of he high side full bridge in (1). Thus, he high side full bridge operaes in hard swiching condiion, which no only leads o a low efficiency bu also induces a high volage spike in he high side full bridge ha may desroy he power swiches by overvolage. The reason of he volage spike is ha in hard swiching condiion he parasiic capacior volage is larger hen zero when he power swich in parallel urns on. The shor circui of he parasiic capacior will hen creaes a very large di/d in he leg. Considering he sray inducance always exiss in he leg, a volage spike is hus generaed. The ampliude of he volage spike decreases when he load increases from 55 W o 85 W, hen disappears when

6 Yellow area: Q/P > 1/1 1.5 Po (kw) 1..5 maximum load line hard swiching sof swiching hard swiching boundary beween sof and hard swiching maximum load line hard swiching sof swiching maximum load line sof swiching boundary beween sof and hard swiching hard swiching 1 2 (pu) 1 2 (pu) V l V l volage doubler is enabled (a) (b) Fig. 5. A map o show he sof swiching and hard swiching of he DAB in differen load and low side dc volage condiions (a) w/o volage doubler (b) wih volage doubler. he load is 95 W, because increases as load increases, and he parasiic capacior is discharged more and hereby lower volage is lef for shor circui. This change in performance maches very well wih he analysis in Fig. 4. Then volage doubler circui is enabled, and he waveforms change from Fig. 8(a) o Fig. 8(b). Because is much larger, so he volage spikes in disappear and efficiency increases a lo, e.g. from 84.5% o 94.2% when load is 55 W. The efficiency of he DAB wih differen load and low side dc volages is measured and i is shown in Fig. 9. As seen in Fig. 9(a), by using PSM, he efficiency is around 96% when V l = 1 pu, hen i drops dramaically as V l increases. The es of he DAB by using PSM a V l = 2 pu is avoided for safy reasons, since he hard swiching creaes very high volage spikes on power swiches, which can break he power swiches. While he efficiency can be evaluaed according o he rend of he efficiency curves, and hould be lower han 9% or even lower. In Fig. 9(b), he volage doubler is enabled when V l is beyond 1.25 pu. The improvemen on efficiency is obvious. The efficiency sars o increase insead of dropping when V l increases beyond he hreshold, and i increases o 96% when V l is around 2 pu. IV. CONCLUSIONS The DAB converer by using he convenional phase shif modulaion can achieve sof swiching and high efficiency when low side dc volage is close o 1 pu, where 1 pu equals o he high side dc volage divided by he ransformer urn raio. When low side dc volage is close o 2 pu, he sof swiching is very difficul o be achieved. The hard swiching will no only reduce he efficiency of he converer bu also induce high volage spikes on he power swiches. In order o achieve sof swiching and high efficiency in DAB when low side dc Efficiency Efficiency Load (W) Volage doubler is enabled (b) V l (pu, 1 pu = V h /n) Fig. 9. The efficiency of he DAB wih (a) PSM (b) PSM when V l is close o 1 pu, volage doubler when V l is close o 2 pu. volage is close o 2 pu, a volage doubler circui is proposed. The circui is composed of he dc blocking capacior and he low side full bridge, hus no exra componens are added o he DAB. I is realized by rearranging he PWM signals based on he phase shif modulaion, which keeps he conrol complexiy (a)

7 W W W W W W W W W W (a) (b) Fig. 8. The square waveforms generaed by he high and low side full bridges, and he low side ransformer curren of he DAB using (a) PSM (b) volage doubler circui, when V l = 1.4 pu.

8 low. By using he volage doubler circui, he DAB converer can again achieve sof swiching as well as low circulaing power and hereby high efficiency even he low side dc volage becomes 2 pu. REFERENCES [1] G. Oriz, C. Gammeer, J. W. Kolar, and O. Apeldoorn, Mixed mosfe-igb bridge for high-efficien medium-frequency dualacive-bridge converer in solid sae ransformers, in Proc. of COMPEL 21, pp. 1 8, 21. [2] S. Anwar, W. Zhang, F. Wang, and D. J. Cosine, Inegraed dc-dc converer design for elecric vehicle powerrains, in Proc. of APEC 216, pp , 216. [] S. P. Engel, M. Sieneker, N. Solau, S. Rabiee, H. Sagge, and R. W. D. Doncker, Comparison of he modular mulilevel dc converer and he dual-acive bridge converer for power conversion in hvdc and mvdc grids, IEEE Trans. Power Elecron., vol., no. 1, pp , 215. [4] A. K. Jain and R. Ayyanar, Pwm conrol of dual acive bridge: Comprehensive analysis and experimenal verificaion, IEEE Trans. Power Elecron., vol. 26, no. 4, pp , 211. [5] P. A. M. Bezerra, F. Krismer, R. M. Burkar, and J. W. Kolar, Bidirecional isolaed non-resonan dab dc-dc converer for ulra-wide inpu volage range applicaions, in Proc. of PEAC 214, pp , 214. [6] F. Krismer, S. Round, and J. W. Kolar, Performance opimizaion of a high curren dual acive bridge wih a wide operaing volage range, in Proc. of PESC 26, pp. 1 7, 26. [7] Y. Wang, S. W. H. de Haan, and J. A. Ferreira, Opimal operaing ranges of hree modulaion mehods in dual acive bridge converers, in Proc. of IPEMC 29, pp , 29. [8] J. Hilunen, V. Vaisanen, R. Jununen, and P. Silvenoinen, Variable-frequency phase shif modulaion of a dual acive bridge converer, IEEE Trans. Power Elecron., vol., no. 12, pp , 215. [9] S. Han, I. Munuswamy, and D. Divan, Prevening ransformer sauraion in bi-direcional dual acive bridge buck-boos dc/dc converers, in Proc. of ECCE 21, pp , 21. [1] G. Oriz, L. Fssler, J. W. Kolar, and O. Apeldoorn, Applicaion of he magneic ear for flux balancing of a 16kw/2khz dc-dc converer ransformer, in Proc. of APEC 21, pp , 21. [11] G. Oriz, L. Fassler, J. W. Kolar, and O. Apeldoorn, Flux balancing of isolaion ransformers and applicaion of he magneic ear for closed-loop volsecond compensaion, IEEE Trans. Power Elecron., vol. 29, no. 8, pp , 21.