Analysis of the Output Ripple of the DC DC Boost Charger for Li-Ion Batteries

Transcription

1 Jurnal f Pwr Elctrnics, Vl. 4, N., pp. 35-4, January 4 35 JPE JPE ISSN(Print: / ISSN(nlin: Analysis f th utput ippl f th Bst hargr fr i-in Battris Van-Sang Nguyn *, Van-ng ran *, Wjin hi, and a-wk Kim ** * pt. f Elctrnic Eng., Sngsil Univrsity, Sul, Kra ** pt. f Ecnmics, Sngsil Univrsity, Sul, Kra Astract In th dsign f attry chargrs, limiting th utput rippl currnt accrding t th manufacturr s rcmmndatin is imprtant fr rlial srvic and xtndd attry lif. ippl cmpnnts can caus intrnal hating f th attry and thus rduc th srvic lif f th attry. ar must xrtd in th dsign f th switching cnvrtr fr th charg applicatin thrugh th accurat stimatin f th utput currnt rippl valu. his study prpss a mthd t rduc th utput currnt rippl f th cnvrtr and prsnts a dtaild analysis f th utput currnt rippl f th st cnvrtr t prvid a guidlin fr th dsign f th attry chargr. Kywrds: Battry hargr, i-in Battry, utput urrnt ippl, utput Inductr I. INUIN Undrstanding attry-charging fundamntals and systm rquirmnts allws dsignrs t slct a suital linar r switch-md charging tplgy and ptimiz attry prfrmanc in th applicatin. h attry-charging systm is ftn prvidd lw pririty, particularly in cst-snsitiv applicatins. Hwvr, th quality f th charging systm plays a ky rl in th lif and rliaility f th attry. signrs must familiar with th fundamntal rquirmnts fr charging lithium-in (i-in attris t dvlp an ptimizd charging systm fr such attris. h vltag rgulatin tlranc n th vltag applid t th cll shuld highr than ±% as rcmmndd y th attry manufacturr t maximiz th prfrmanc f i-in attris. h prfrrd charg algrithm fr i-in attris is a cnstant currnt-cnstant vltag (-V algrithm. In this mthd, charging is typically trminatd y a minimum-charg currnt. h minimum currnt apprach mnitrs th charg currnt during th cnstant-vltag stag and trminats th charg whn it diminishs in th rang f. t.7 [] [3]. his cnditin implis that th Manuscript rcivd Jul. 3, 3; rvisd Nv. 3, 3 cmmndd fr pulicatin y Assciat Editr hng Xu. rrspnding Authr: cwj777@ssu.ac.kr l: , Fax: , Sngsil Univrsity * pt. f Elctrical Enginring, Sngsil Univrsity, Kra ** pt. f Ecnmics, Sngsil Univrsity, Kra utput currnt rippl shuld nt xcd this valu. Althugh th magnitud f th rippl cmpnnt in th utput currnt is imprtant fr rlial prfrmanc and xtndd attry lif, this cmpnnt is ftn vrlkd in th chargr dsign. Mst f th chargrs dvlpd y far utiliz uck-typ cnvrtrs that can stp dwn th high A vltag frm th utility t a suital lvl f fr th lad. Givn that ths uck-typ cnvrtrs hav inductrs at thir utput, currnt cntrl can prfrmd asily and th utput rippl rquirmnts can satisfid withut any difficulty. Hwvr, in sm applicatins with a lw vltag surc (.g., ful clls, a st-typ chargr can prvid a ttr slutin caus th surc vltag can lwr than that f th attry. Althugh a uck-typ cnvrtr with a transfrmr can als an ptin fr this typ f applicatin, it is unlikly t slctd if cmpactnss and wight ar cnsidrd, particularly fr th chargr f a prtal lctrnic dvic. Figs. (a and ( shw svral typs f cnvntinal cnvrtrs, such as th nn-islatd st cnvrtr, fly-ack cnvrtr, uck-st cnvrtr, and SEPI cnvrtr, that hav similar utput structurs with a pur rsistiv lad and a attry, rspctivly. Givn that th tplgis hav a frnt-nd inductr and an utput did, th utput currnt is nt cntinuus; thus, th utput rippl currnt is significantly largr than that f th uck-typ tplgis that hav an utput inductr. Whn th lad is purly rsistiv, mst f th rippl currnt can filtrd y th utput capacitr as shwn in Fig. (a. Hwvr, this

2 36 Jurnal f Pwr Elctrnics, Vl. 4, N., January 4 Fig.. nvntinal cnvrtr tplgis that hav similar utput structurs. (a With pur rsistiv lad. ( With a attry mdl. Fig. 3. Nn-islatd st cnvrtr with an additinal inductr at th utput. Fig.. id currnt, utput currnt, and capacitr currnt f th cnvrtr shwn in Fig.. (a With pur rsistiv lad. ( With a attry mdl. cnditin is n lngr valid whn th lad is a attry caus mst f th currnt rippl may flw thrugh th attry du t th hug capacitanc in its quivalnt circuit as shwn in Fig. (. his situatin has t cnsidrd t guarant th rlial pratin f th attry. h currnt rippl flwing int th attry during charging can gnrat hat wing t th intractin with th intrnal rsistanc f th attry (I lsss. his hat adds t th intrnal hat gnratd insid th attry. hus, th xcssiv rippl currnt rducs th srvic lif f th attry. ar must xrtd in dsigning th cnvrtr fr th chargr t mt th rippl rquirmnts f th attry [5], [6]. A mthd t rduc th utput currnt rippl f th st cnvrtr is prpsd in this study and th utput currnt rippl f th cnvrtr is analysd. h dsign mthd and th ptimal slctin f th inductr and capacitr at th utput ar als discussd in dtail. II. ESIGN F HE UPU INU AN APAI F HE PPSE NN-ISAE BS NVEE h prpsd mthd fr th rductin f th utput rippl cmpnnt can simply implmntd y adding anthr inductr twn th utput capacitr and th attry as shwn in Fig. 3. In this apprach, an additinal inductr is placd twn th utput capacitr and th attry t rduc th utput rippl. Slcting th suital valu fr th inductr and th capacitr at th utput is ncssary t mt th rippl rquirmnts f th attry. As shwn in Fig. (, th capacitr currnt wavfrm f th nn-islatd st cnvrtr can drawn (s Fig. 4 Fig. 4. urrnt wavfrm f th utput capacitr in Fig. 3. whn th lad is a attry and can mathmatically xprssd y Equatin ( as a functin f tim und twn and. t - t i ( t i + I ( < t < ( n _ pak tff ( i I _ pak - I _ vally ( h vltag f th utput capacitr can taind y th intrgratin f i (t as in Equatin (3, and th vltag variatin at th capacitr can thn xprssd as Equatin (4. é t ù n - t v ò ê i + _ ú (3 I pak I êë tff úû h vltag n th utput inductr can thn xprssd as Equatin (5 (ΔI is th utput rippl currnt. t n - t d i + ç I _ pak tff i v ( - h utput currnt rippl can xprssd as Equatin (5 y using Equatins (3, (4, and KV. ( - i I - - v (4 ( (5 Equatin (5 shws th rlatinship twn th utput rippl and th ractiv cmpnnts at th utput. Hwvr, numrus pairs f th valus fr th utput inductr ( and th utput capacitr ( can satisfy th quatin. hus,

3 Analysis f th utput ippl f th Bst hargr fr i-in Battris 37 a z parasitic w ( Givn th slutin taind frm Equatin (8, th utput currnt may hav thr diffrnt frms dpnding n th valu f th damping factr. Fig. 5. Equivalnt circuit f th prpsd st cnvrtr whn th switch is clsd. anthr cnditin that rprsnts th rlatinship twn th inductr and th capacitr is ncssary t dcid th valus. In-dpth circuit analysis is rquird t driv th quatin fr th rlatinship twn th utput rippl valu and th ractiv cmpnnts at th utput. III. IUI ANAYSIS F HE PPSE NVEE EVAUAE HE MAGNIUE F HE UPU IPPE UEN Stady stat analysis is prfrmd whn th switch is clsd t analyz th utput rippl currnt f th prpsd st cnvrtr. Fr accurat circuit analysis, all th parasitic cmpnnts ar cnsidrd as shwn in Fig. 5. h vltag lp quatin in th rar-nd sucircuit whn th switch is clsd can xprssd as Equatin (6. di i i + i (6 ò ES ò Givn th hug capacitanc valu in th attry mdl (typically svral thusands f farad in cas f th i-in attry cmpard with th utput capacitr valu, th variatin in th attry vltag during th switch-ff prid can nglctd. hus, w tain di + + ( (7 ò i ( + + Q >> ES i iffrntiating th av quatin and dividing th sids y yild th scnd-rdr diffrntial quatin as Equatin (8 d i parasitic di + + i (8 + + parasitic h charactristic quatin f th scnd-rdr diffrntial quatin is s + a s + w (9 parasitic ( a and w h damping factr (ζ can xprssd as Equatin (. Whn th damping factr is lss than (ζ <, th circuit rspns is undr-dampd and th utput currnt can xprssd as Equatin (. d t i ( t A sin ( w t + j ( w w - a. Whn th damping factr is qual t (ζ, th circuit rspns is critically dampd and th utput currnt can xprssd as Equatin (. d i ( t + t t B t B ( Whn th damping factr is largr than (ζ >, th circuit rspns is vr-dampd and th utput currnt can xprssd as Equatin (3. - w ( z + z - t - w ( z - z - t 3 i ( t + ( Equatins (, (, and (3 must slvd undr th initial cnditins t driv th quatin fr th utput currnt at ach cas. uring th prid whn th switch is clsd, vltag lp Equatin (6 can rwrittn as Equatin (4. ( v v ES ( v v v v A. Undr-dampd as (4 As shwn av, th utput currnt can xprssd as Equatin ( whn th switch is clsd and can rwrittn as Equatin (5, including avrag utput currnt I. i t i ( t sin( w + j + I (5 At last tw cnditins ar rquird t dtrmin th magnitud f utput rippl currnt i and phas φ in Equatin (5. hus, th quatins fr th initial and final valus f th utput currnt must drivd whn th switch is clsd. Fig. 6 shws th quivalnt circuit f th prpsd st cnvrtr whn th switch is pn. By applying KV t this circuit, w tain dii di - VS + i + i (6 i i

4 38 Jurnal f Pwr Elctrnics, Vl. 4, N., January 4 Fig. 6. Equivalnt circuit f th prpsd st cnvrtr whn th switch is pn. Fig. 7. Frnt-nd inductr currnt wavfrm f th prpsd st cnvrtr. Fig. 7 shws th wavfrm f th frnt-nd inductr currnt f th st cnvrtr. h wavfrm can xprssd as Equatin (7. t - t i ( t i + I Q < t < (7 n i _ pak tff With Equatins (6 and (7, th first-rdr diffrntial quatin f th utput currnt can taind as Equatin (8. di ii tn - t + ( _ (8 i VS i i i ç i Ii pak t ff tff hus, th xprssin fr th utput currnt whn th switch is pn can taind as Equatin (9 y slving Equatin (8. i i ç t - + t ff ç + ( + ii VS + i - I _ - pak i tff - -( + t + (9 + _ pn hrfr, th utput currnt at t and t can xprssd as Equatins ( and (, rspctivly. i ( ç I pn t - + ( - ç + ( + ii VS + i - I _ - pak i ( - - -( ( i ( ç I pn t - + ( - ç + ( + ii VS + i - I _ - pak i ( - - -( V S I VS ç ii and Ii _ pak + i - i. ( h initial and final valus f th frnt-nd inductr currnt whn th switch is pn ar similar t its final and initial valus whn th switch is clsd as shwn in Fig. 7. hus, w tain I ( t I _ pn _ I ( t I ( _ pn _ By using this rlatinship and sustituting th initial cnditins f Equatin ( int (5, th magnitud f th utput rippl currnt in th undr-dampd cas can xprssd as Equatin (3. ì é I _ pn _ - I ùü ï ê úï í sin ( w ê úý i ï d ê-cs (3 ( w ( - úï î ë d I pn I ûþ ( I _ pn _ I + - B. ritically ampd as h utput currnt f th prpsd cnvrtr in th critically dampd cas can xprssd as Equatin ( whn th switch is clsd and can rwrittn as Equatin (4, including avrag utput currnt I. i ( t + t t B t B + I (4 Equatin (4 shws that th utput currnt rachs th maximum valu at t and th minimum valu at t. Hnc, th maximum and minimum valus f th utput rippl currnt can xprssd as Equatins (5 and (6, rspctivly. B parasitic i - ( + (5 i B (6 uring th switch-n tim, th variatin in th vltag n th utput capacitr and its ffctiv sris rsistanc (ES can calculatd as Equatins (7 and (8, rspctivly.

5 Analysis f th utput ippl f th Bst hargr fr i-in Battris 39 t t ìï é ( t - B ù ü ï v ò i íê - + ú ý B It a ïî ë a û ïþ ( - + B I B - + a a V i ES ES (7 (8 h vltag at th utput inductr can xprssd as Equatin (9. t t di d( Bt + B + I v (9 hus, th vltag variatin n th utput inductr twn t and t can drivd as Equatin (3. é parasitic -parasitic ê f s ç + ú f parasitic s ê ç ú ç v i (3 ê ç ú ë û h vltag variatin n th parasitic cmpnnts twn t and t can xprssd as Equatins (3, (3, and (33. v i v i ù (3 (3 v ( Q is hug (33 h vltag variatin n th capacitanc f th attry can nglctd givn th hug capacitanc f th attry. With Equatins (4 and (7 (33, th utput rippl currnt in th critically dampd cas can xprssd as Equatin (34. I i é ë( J - J + parasitic ù û (34 a é ù + J ê - ç - + a ú (35 ë û {( - + } é a + ù J ê ú - - ê a a ë ú û. vr-dampd as (36 h utput currnt f th prpsd cnvrtr in th vrdampd cas can xprssd as Equatin (37 whn th switch is clsd. his quatin shws that during such prid, th utput currnt rachs th maximum valu whn t and th minimum valu whn t. - w ( z + z - t -w ( z - z - t i ( t + + I (37 nsidring that th maximum and minimum valus f th utput rippl currnt can srvd at t and t, th cfficints in Equatin (37 can xprssd as Equatins (38 and ( w ( z - z w ( z + z - -w ( z - z w ( z + z w ( z + z - -w ( z - z - - i i (38 (39 uring th switch-n tim twn t and t, th variatin in th vltag n th utput capacitr and its ES can calculatd as Equatins (4 and (4, rspctivly. v ò - w ( z + z - K ç - + ç - w ( z + z - I ç i + (4 K -w ( z - z - - ç w ( z - z - i v i ES ES (4 h vltag at th utput inductr can xprssd as Equatin (4. ( ( ( ( - w z + z - t - w z - z - t di t d + + I v (4 hus, th vltag variatin n th utput inductr twn t and t can drivd as Equatin (43. - w ( z + z - - -w ( z - z - - é w ( z + z - ù ê ú v - (43 ê ú ê+ w ( z - z - ú ë û h vltag variatin n th parasitic cmpnnts twn t and t can xprssd as Equatins (44, (45, and (46. v v v i (44 (45 v ( Q is hug (46

6 4 Jurnal f Pwr Elctrnics, Vl. 4, N., January 4 With Equatins (4 and (4 (46, th utput rippl currnt in th critically dampd cas can xprssd as Equatin (47. i I Y - Y + ( parasitic - w ( z + z - - -w ( z - z w ( z + z - - w ( z z ç + - K Y - ç ç + Kw ( z - z - Y K ç ç - w ( z + z - ç K -w ( z - z ç w ( z z - - -w ( z - z - + K - - K - w ( z + z - -w ( z - z - - w ( z + z w ( z + z - -w ( z - z - (47 (48 (49 (5 (5 Using th quatins drivd av fr th magnitud f th utput rippl currnt in th thr diffrnt cass rsults in Equatins (3, (34, and (47. ABE I SYSEM PAAMEES Input vltag V i 6 9 V utput vltag V.6 V utput pwr P 5 W Frquncy f s 6 khz Input inductr i 3 µh apacitr 3 µf 4 µf utput inductr / µh Spcificatin f th attry i-in Battry Pack (3SP Nminal currnt I nminal 5. A ( harging currnt I 4 A (.8 harging vltag V.6 V Battry initial vltag V.8 V utput rippl currnt Equivalnt capacitanc f th attry Equivalnt sris rsistanc f th attry I utput_rippl.6 A ( F.3 Ω IV. SIMUAIN ESUS AN PIMA VAUES F HE EAIVE MPNENS h prpsd st chargr is dsignd with th spcificatins in al I and simulatd with PSIM t vrify th circuit analysis and drivatins. Svral pairs f th inductanc and capacitanc valus ar slctd t rprsnt ach cas accrding t th valu f ζ. h prpsd st chargr chargs th i-in attry pack at /V charg md with 4 A (.8 charg currnt and.6 V charg vltag until th charg currnt dcrass y.5 A (., in which th attry is assumd t fully chargd. As shwn in Equatins (3, (34, and (47, th utput currnt rippl is dirctly prprtinal t th charg currnt, which mans that th utput currnt rippl has th largst valu in th charg md. h filtr is thn dsignd with th charg currnt rat cnditin t satisfy th utput currnt rippl during th ntir charging prid. As shwn in Fig. 8, th utput currnts shw thr diffrnt wavfrms fr ach cas. h utput currnt varis frm its maximum valu t th minimum valu during th switch-n tim in th critically dampd and vr-dampd cass as xpctd. h valus f th utput currnt rippl fr ach cas calculatd with Equatins (3, (34, and (47 ar 86 ma, 43 ma, and 9 ma, rspctivly; th valus ar cls t ths in th PSIM simulatin rsults. Fig. 8. Wavfrms f th utput currnt rippl fr th thr diffrnt cass. As prviusly mntind, numrus pairs f th inductanc and capacitanc valus can satisfy th rippl rquirmnts. ptimal sizing f th ractiv cmpnnts in trms f cst and siz is thus ncssary in th dsign f th cnvrtr. An filtr with a varial rang fr th capacitr and inductr is dsignd t vrify th ffct f th filtr n th utput currnt rippl in th thr cass. Fig. 9 shws th utput currnt rippl valus fr th 8 diffrnt pairs f th inductanc and capacitanc valus f th prpsd st chargr at ach cas asd n th calculatd and PSIM simulatin rsults, which ar wll matchd. As shwn in th figur, whn th ractiv cmpnnts ar slctd t satisfy th undr-dampd cas, th siz f th ractiv cmpnnts may rlativly small, whras th utput currnt rippl is much largr than that in th critically dampd cas. Whn th ractiv cmpnnts ar slctd t satisfy th vr-dampd

7 Analysis f th utput ippl f th Bst hargr fr i-in Battris 4 Fig.. utput currnt wavfrm and PWM signal fr th undr-dampd cas with µh, 5 µf, ζ.9. Fig. 9. utput currnt rippl simulatin rsults y using PSIM and rsults f th calculatins using Equatins (3, (34, and (47 fr th 8 pairs f and. cas, th utput currnt rippl is nt smallr than that in th critically dampd cas; hwvr, th ractiv cmpnnts ar much largr in siz. hus, asd n ths rsults, ptimal slctin can mad with th pair f ractiv cmpnnts in th critically dampd cas caus this pair ffrs ttr attnuatin fr th utput currnt rippl than that in th undr-dampd cas and is ttr in trms f siz and cst than that in th vr-dampd cas. Fig. 9 als shws th ffct f th inductanc and capacitanc valus n th utput currnt rippl. h utput currnt rippl is significantly changd y a small variatin valu f th utput inductr, whras th valu f capacitanc chang nly slightly affcts th utput currnt rippl. In th dsign f th prpsd st cnvrtr, a small utput inductr can significantly rduc th utput currnt rippl and in turn, minimiz th siz f th utput capacitr. Givn that th critically dampd cas prvids ptimal rsults, th rlatinship twn th inductanc and capacitanc valus can taind with Equatin ( and ζ. By using this rlatinship and Equatin (5, th uput inductr and utput capacitr valus can taind with Equatins (5 and (53, rspctivly. i Iparasitic ( - ç v + - v ç ( - i parasitic 4 parasitic parasitic (5 i Iparasitic ( - ç v + - v ç ( - (53 i hus, if th currnt rippl limitatin f th attry and th cnvrtr spcificatins, such as switching frquncy, duty cycl, and th parasitic cmpnnts valus in th circuit, ar knwn, thn th ractiv cmpnnt valus f th cnvrtr can calculatd. V. EXPEIMENA ESUS A 5 W st chargr asd n th spcificatins in al I is prpsd t validat th analysis f th utput rippl discussd in th prvius sctin. h maximum allwal currnt rippl is 5% f th ratd charg currnt as spcifid in al I. h wavfrm f th utput currnt rippl and its magnitud ar vrifid with a 3SP i-in attry pack fr a laptp cmputr. uring charging, th attry paramtrs chang slightly caus f th limitd rang f th attry tmpratur, that is, frm 3 t 4. Figs.,, and shw th utput currnt wavfrms f th prpsd st chargr fr th thr diffrnt cass. h slctd ractiv cmpnnts ar similar in ach cas as shwn in Fig. 8. h magnitud f th utput currnt rippl in th thr cass is cls t that in th calculatin and simulatin valus. All th rrrs ar lss than %; hwvr, th rrr is slightly largr than % in th undr-dampd cas caus f th rrr valus f th xprimntal capacitr, inductr, and parasitic rsistr. h rrr in th capacitanc valu significantly affcts th rippl in th undr-dampd cas as shwn in Fig. 9. h simulatin and xprimntal utput currnt wavfrms in th critically dampd and vr-dampd cass match wll as shwn in Figs. 8,, and. Hwvr, a slight diffrnc xists twn th simulatin and xprimntal wavfrms in th undr-dampd cas as shwn in Figs. 8 and. h diffrnc is causd y th small variatin in th attry paramtrs during charging, which is assciatd with th larg currnt rippl in this cas. VI. NUSINS In this study a tplgical mthd was dvlpd t rduc th utput currnt rippl in th st chargr and th utput currnt rippl was analysd. Givn that th utput

8 4 Jurnal f Pwr Elctrnics, Vl. 4, N., January 4 [4]. Shimizu, Y. Jin, and G. Kimura, rippl currnt rductin n a singl-phas PWM vltag-surc rctifir, IEEE rans. Ind. Appl., Vl. 36, N. 5, pp , Sp./ct.. [5]. ujic, M. Jns, and E. vi, Analysis f utput currnt rippl MS in multiphas drivs using spac vctr apprach, IEEE rans. Pwr Elctrn., Vl. 4, N. 8, pp , Aug. 9. [6] V. S. Nguyn and W-J. hi, A nn-islatd st chargr fr th i-in attry suital fr ful cll pwrd laptp cmputr, 7th Intrnatinal Pwr Elctrnics and Mtin ntrl nfrnc (IPEM, Vl., pp ,. Fig.. utput currnt wavfrm and PWM signal fr th critically-dampd cas with µh, 5 µf, ζ.6. Van-Sang Nguyn was rn in Hani, Vitnam, in 985. H rcivd his B.S. dgr in lctrical nginring frm Hani Univrsity f Scinc and chnlgy, Hani, Vitnam, in 8. H is currntly pursuing his M.S. dgr at Sngsil Univrsity, Sul, pulic f Kra. His rsarch intrsts includ attry chargrs and cnvrtrs. Fig.. utput currnt wavfrm and PWM signal fr th vr-dampd cas with µh, µf, ζ.84. rippl f th chargr is an xtrmly snsitiv factr, car was xrtd in th dsign f th chargr. h quatins fr th ptimal slctin f th utput inductr and utput capacitr valus wr drivd fr th suital dsign f th st chargr. h prpsd mthd and analysis can utilizd t imprv th dsign f th st cnvrtr fr attry charg applicatins, thry guaranting th srvic lif f th attry. AKNWEGEMENS his wrk was supprtd y th Sngsil Univrsity sarch Fund (. EFEENES [] V. Vrpérian, A rippl thrm fr PWM -t- cnvrtrs prating in cntinuus cnductin md, IEEE 35th Annual Pwr Elctrnics Spcialists nfrnc, pp. 8-35, 4. []. iu, J-S. ai, w frquncy currnt rippl rductin tchniqu with activ cntrl in a ful cll pwr systm with invrtr lad, IEEE rans. Pwr Elctrn., Vl., N. 4, pp , Jul. 7. [3] J-M. Kwn, E-H. Kim, B-H. Kwn, K-H. Nam, High-fficincy ful cll pwr cnditining systm with input currnt rippl rductin, IEEE rans. Ind. Elctrn., Vl. 56, N. 3, pp , Mar. 9. Van-ng ran was rn in Haiphng, Vitnam, in 988. H rcivd his B.S. dgr in lctrical nginring frm Hani Univrsity f Scinc and chnlgy, Hani, Vitnam, in. H is currntly pursuing his M.S. dgr at Sngsil Univrsity, Sul, pulic f Kra. His rsarch intrsts includ attry chargrs and cnvrtrs. Wjin hi was rn in Sul, pulic f Kra, in 967. H rcivd his B.S. and M.S. dgrs in lctrical nginring frm Sngsil Univrsity, pulic f Kra, in 99 and 995, rspctivly. H rcivd his Ph.. dgr in lctrical nginring frm xas A&M Univrsity, USA, in 4. H wrkd with aw Havy Industris as a rsarch nginr frm 995 t 998. In 5, h jind th Schl f Elctrical Enginring, Sngsil Univrsity. His rsarch intrsts includ mdling and cntrl f lctrchmical nrgy surcs such as ful clls, attris, and suprcapacitrs, pwr cnditining tchnlgis in rnwal nrgy systms, and cnvrtrs fr ful clls and hyrid lctric vhicls. a-wk Kim was rn in Sul, pulic f Kra, in 973. H rcivd his B.A. dgr in Ecnmics frm Ynsi Univrsity, pulic f Kra, in 999. H rcivd his Ph.. dgr in cnmics frm th Univrsity f alifrnia at avis, USA, in 4. H wrkd fr Kra Institut fr Industrial Ecnmics and rad as a rsarch fllw frm 4 t 7. In 7, h jind th partmnt f Ecnmics, Sngsil Univrsity. His currnt rsarch intrsts includ nrgy cnmics, particularly markt structur and cmptitin in nrgy industris.