A Novel Soft-Switching Two-Switch Flyback Converter with a Wide Operating Range and Regenerative Clamping

Transcription

1 77 Journal of ower Electroncs, ol 9, No 5, September 009 JE 9-5- A Novel Soft-Swtchng Two-Swtch Flybac Converter wth a Wde Operatng Range and Regeneratve Clampng Marn-Go Km and Young-Seo Jung * Dvson of Electrcal and Control Eng, uyong Natonal Unversty, Busan, Korea * Dvson of Mechancal Eng, uyong Natonal Unversty, Busan, Korea ABSTRACT A novel soft-swtchng two-swtch flybac converter s proposed n ths paper Ths converter s composed of two actve power swtches, a flybac transformer, a blocng dode, and two passve regeneratve clampng crcuts The proposed converter has the advantages of a low cost crcut confguraton, a smple control scheme, a hgh effcency, and a wde operatng range The crcut topology, analyss, desgn consderatons, and expermental results of the new flybac converter are presented Keywords: Two-swtch flybac converter, Soft swtchng, Regeneratve clampng, ower factor correcton ntroducton Flybac converters are popular topologes wdely used n solated DC-DC power converters [-4] These topologes are favored by desgners for ther smplcty, ablty to handle multple solated outputs, and the ease of optmzng ther duty cycle by selectng the transformer turns rato The smplcty s partally based on the fact that conventonal flybac converters employ a sngle MOSFET swtch, whch s prmary ground referenced for convenent gate drve mplementaton However, the drawbac to ths sngle swtch approach s Manuscrpt receved Jan 3, 009; revsed July 9, 009 Correspondng Author: mgm@pnuacr Tel: , Fax: , KNU Dvson of Electrcal and Control Eng, uyong Nat l Unv Busan, Korea * Dvson of Mechancal Eng, uyong Nat l Unv Busan, Korea that the voltage stress on the swtch s the sum of the nput voltage, the reflected transformer voltage and the turn-off voltage spe caused by leaage nductance A common problem wth flybac converters s the leaage nductance of the power transformer, whch causes hgh turn-off voltage spes Ths problem s partcularly serous at hgh lne voltage or at lght loads [5] Addng a second MOSFET swtch on the hgh sde results n a two-swtch flybac topology, where the voltage stress on each MOSFET s clamped to the nput voltage The leaage nductance energy s also clamped and recycled bac to the nput to mprove effcency The dsspatve snubber crcut that s often requred n a sngle swtch approach s no longer requred MOSFET swtches wth a rated voltage slghtly hgher than the nput voltage can be employed n the two-swtch topology, whle a ratng of greater than twce the nput voltage s requred for a sngle-swtch topology For many applcatons the added complexty and ncreased part count of two-swtch

2 A Novel Soft-Swtchng Two-Swtch Flybac Converter wth 773 flybac converters s a small prce to pay for the benefts receved However, the duty rato of a conventonal two-swtch topology cannot be more than 50%, whch s the same dsadvantage as a conventonal two-swtch forward one [6,7] The reset voltage of the transformer can t be more than the nput voltage because there are two dodes D 3 and D 4 as shown n Fg, whch clamp the transformer prmary voltage to the nput voltage When the reflected output voltage to the transformer s prmary sde s hgher than the nput voltage, most of the magnetzng nductor energy as well as the leaage nductor energy s returned to the nput source n addton, the converter s operated under a hard swtchng condton Because of these dsadvantages, t cannot be used for power factor correcton (FC) applcatons [8,9] n order to overcome these problems, a novel soft-swtchng two-swtch flybac converter wth a wde operatng range and regeneratve clampng s proposed n ths paper Due to ts smple crcut confguraton, consstng of a mnmum number of components and snce only passve components are utlzed n the clampng crcut to acheve zero voltage soft swtchng, the proposed converter can be controlled by a sngle WM sgnal, establshng a low-cost crcut confguraton and a smple control scheme The duty rato of the proposed converter can be more than 50%, and magnetzng nductor energy transfer to the transformer s secondary sde s possble even f the reflected output voltage s hgher than the nput voltage n Secton the lmtatons of the operaton range n a conventonal two-swtch flybac converter wll be dscussed n Secton 3 a detaled analyss of the proposed topology wll be carred out n Secton 4 the expermental results wll be gven Lmtatons of Operaton Range n Conventonal Two-swtch Flybac Converter n a contnuous conducton mode(ccm) of operaton, the output voltage of Fg s gven by: D n o D where n s the transformer turns rato and D s the duty rato f the effcency of the flybac converter s, the output power n dscontnuous conducton mode(dcm) can be expressed as: () L p f o o s () R o p DT D (3) L s f s L where L prmary magnetzng nductance, p pea current of L, f s swtchng frequency, R o load resstance, D duty rato and T s swtchng perod From () and (3), the output voltage n DCM s gven by: D o (4) f s L / R o n Fg, both swtches are turned on and off smultaneously, as n the two-swtch forward converter The operaton of the flybac transformer s best descrbed as a two-wndng coupled nductor Energy s suppled to Fg Conventonal two-swtch flybac converter Fg Energy returnng mode of Fg ( n o > )

3 774 Journal of ower Electroncs, ol 9, No 5, September 009 the nductor n the prmary crcut when the prmary swtches S and S are actve, then the energy s released to the secondary when the prmary swtches are turned off f the reflected transformer prmary voltage n o s lower than the nput voltage However, ths energy s returned to the nput source f n o s hgher than as shown n Fg n the steady-state, n o needs to be lower than n order to transfer the magnetzng energy to the secondary output n a conventonal two-swtch topology Therefore, from () and (4), the lmtatons n operaton range of Fg can be calculated as: Fg 3 roposed two-swtch flybac converter D < 05 for CCM and D < f s L /( n Ro ) for DCM (5) v c o 3 roposed Two-swtch Flybac Converter 3 Crcut Descrpton The crcut of the proposed two-swtch flybac converter s llustrated n Fg 3 Ths converter s composed of two actve power swtches ( S, S ), a flybac transformer T, a blocng dode D b, and two passve regeneratve clampng crcuts that consst of two dodes ( D 3, D 4 ), two nductors ( L s, L ), two capactors ( C s, C ) and a dode D The dode D s used to trap leaage nductor energy nto C s and C when the two swtches are turned off, and then regenerate the energy to the nput The dode D b s nserted to suppress resonance between the transformer nductor and the capactors around t n addton, nsertng D b provdes zero voltage swtchng for S and S when the two swtches are turned off Thus, the reduced swtchng losses of S and S cancel out the conducton loss of D b - mode mode D 3 D4 S L S L S S C S C S S / - D L / - S D b n : DO Ll o mode 3 mode 4 S S Dp v cs v c C oss n : C os Do o C s C oss D S Dp S v c v cs v c L l C oss n : C os n o mode 5 Equvalent crcut of mode 5 / Do o / n : / n : / l o o 3 Crcut Operaton Fg 4 shows the operatng modes of the proposed mode 6 mode 7 Fg 4 Operatng modes of Fg 3

4 A Novel Soft-Swtchng Two-Swtch Flybac Converter wth 775 converter under steady-state, where the followng assumptons are made: ) All swtchng components are deal ) The nductors L s and L are dentcal, L s L L s 3) The capactors C s and C are dentcal, C s C C s 4) The swtch output capactors are dentcal, C oss C os C oss 5) The output voltage rpple can be neglected 6) The converter s operatng n DCM Typcal voltage and current operatng waveforms for the proposed crcut topology are llustrated n Fg 5 The steady-state operaton of ths crcut can be descrbed as follows: Mode ( t 0 t ): At tme t 0, accordng to the duty factor D of the power converter treated here, S and S are turned on smultaneously under a zero current condton The prmary sde of the transformer s clamped by nput voltage The prmary current crculates through S and S and ncreases lnearly artal resonance based on Ls and C s begns Assumng that v cs ( 0) /, Ls (0) 0, v S, vs v, cs vc ( ) / p p / / the equatons for the resonant nductor current and capactor voltage durng ths operaton mode can be expressed as: Ls vcs where sn w s ( t t ) Z 0 s cos w s ( t t ) (6) 0 w s / L s Cs s the resonant angular frequency and Z s L s /Cs s the characterstc mpedance On the other hand, the current through S and S can be derved from the flybac transformer s prmary sde current and resonant nductor current Therefore, the actve power swtch current can be represented as: s ( t t ) sn w ( t t ) 0 s (7) L L 0 l Z s Mode ( t t ): ntally, at t, vcs equals / and Ls equals sn w s ( t t ) The dode D Z 0 s turned on s to provde a path for the regeneraton current For energy recovery, current Ls flows through the current regeneraton loop composed of D 3, D 4, D, L s, L and The regeneraton current can be represented as: p, s Do n p ( ) sn ( 0 ) Ls t w s t t ( t t ) (8) Z s L s p Dp L p The flybac transformer s prmary sde current flows through the actve power swtches The capactor voltage, Ls L t t 0 t t 3 t4 t t 5 6 t 7 t vcs s clamped at / and the regeneraton current Ls s decreased towards zero DT S T S Fg 5 Key theoretcal waveforms of proposed converter Mode 3 ( t t 3 ): n ths mode, the magnetzng

5 776 Journal of ower Electroncs, ol 9, No 5, September 009 nductance L and the leaage nductance L l are charged up lnearly by the nput voltage source At t 3, the pea transformer s prmary sde current can be gven by: D ( 3 0 ) DT t t s (9) fsl L L L l where T s / f s Mode 4 ( t 3 t 4 ): Accordng to the duty factor D, at t 3, S and S are turned off smultaneously under zero voltage soft swtchng The dode D begns to conduct The output capactor of the actve power swtches Coss s charged and the voltage across the swtches ncreases lnearly wth a certan slope The voltage across Cs s lnearly changed from / to n o / The voltage across the actve swtches and the voltage across Cs are gven by: vs vcs C s C oss ( t t 3 ) ( t t 3 ) (0) C s C oss Mode 5 ( t 4 t 5 ): At t 4, vcs s equal to no and the output dode D o begns to conduct The output current starts to ncrease and the leaage nductor current begns to decrease From the equvalent crcut of mode 5, the equatons for the leaage nductor current and the capactor voltage can be expressed as: l cos w ( t t 4 ) v c v cs Z sn w ( t t 4 ) n o () where w / L l ( C s Coss ) / and Z L l /( C s Coss ) At t 5, l drops to zero and the pea capactor voltage s gven by: Z n o () Substtutng (4) and (9) nto () gves: Z n ( ) D (3) f s L f s L / R o The maxmum off-state voltage across the actve swtches S and S s gven by: s,max,max ( ) / (4) Mode 6 ( t 5 t 6 ): At t 5, l drops to zero All the remanng magnetzng current flows nto the output The voltage across the prmary sde of the transformer s Mode 7 ( t 6 t 7 n o ): The magnetzng current n the secondary wndng reduces to zero at t 6 The output capactor s dscharged through the load resstance Swtches S and S are turned on at t 7 to start the next swtchng cycle 4 Expermental Results 4 Desgn Consderatons ractcal condtons must be met n order to select optmum parameters To acheve zero voltage swtchng when S and S are turned off, should be hgher than Usng (3), ths s gven by: ( Z n ) D > f s L f s L / R o (5) The mnmum on-tme of actve swtches S and S s to

6 A Novel Soft-Swtchng Two-Swtch Flybac Converter wth 777 be longer than the full dschargng nterval of capactors C s and C : π L s C s < D mn T s (6) Ls L Dp From (0), the maxmum dv / dt actve power swtches should be from (7), the maxmum power swtches should be d / dt L s durng turn-off of Fnally, C s C oss durng turn-on of actve L L l Fg 6 mplemented crcut of Ls and L 4 Expermental Evaluatons To determne the feasblty of the proposed soft-swtchng two-swtch flybac converter, a prototype of the crcut shown n Fg 3 was bult accordng to the followng specfcatons: nput voltage: 00 Output voltage: 80 The followng are the crcut parameters used n the experment: fs 35 Hz, n 4, L 33 mh, L l 38 μh, C s C 4 4 nf, L s L 00 μη For S and S, an RF 840 s used For D b, D 3, and D 4, a DSE -06A s used And a DSE 30-0A s used for Do and D p Fg 6 shows the coupled nductor used to mplement L s and L The prmary and secondary nductances of the coupled nductor are 00 uh, respectvely, to effectvely acheve the expermental two 00 uh nductors The control crcut s shown n Fg 7 Gate sgnals g, g are generated n order to control the two transstors S, S The expermental voltage and current waveforms of the actve power swtch S are llustrated n Fg 8 for D04 From equatons (9) and (), the pea transformer s prmary sde current and the pea capactor voltage are 76 A and 43, respectvely From (4), Fg 7 Control Crcut and related waveforms the maxmum off-state voltage across swtch S s 35 From the expermental waveforms, the actve swtch turns on under a zero current condton and turns off under a zero voltage condton The expermental waveforms show excellent agreement wth the theoretcal results The expermental voltage across C and current through Dp are llustrated n Fg 9 From Fg 5, the theoretcal pea postve and negatve values of c are 5( / ) and -00 ( / ), respectvely From the Dp waveform, we can see the regeneraton current after c reaches -00 Fg 0 shows the expermental waveforms of c and L From (6), Zs s 3 Ω and the pea value of the resonant current s 099 A Fg shows the expermental voltage waveforms across S, S, C s, and C From the experment, t can be verfed that s and cs are dentcal to

7 778 Journal of ower Electroncs, ol 9, No 5, September 009 and c, respectvely Fg shows the voltage across S and the current through output dode D o These expermental results confrm the theoretcal waveforms llustrated n Fg 5 n Fg 3, the measured effcency of the proposed converter as a functon of output power s represented The maxmum effcency obtaned from ths flybac converter s 95 % Fg 4 shows the expermental lne voltage and current waveforms The measured nput power factor s 098 v S v S v c (a) v S v cs Fg 8 Expermental waveforms : v (00 /dv) and ( A/dv), tme: 5 μs /dv (b) Fg Expermental waveforms : (a) v s and v (00 /dv) (b) vcs and v c (00 /dv), tme: 5 μs /dv v c v S Do Dp Fg 9 Expermental waveforms : v c (00 /dv) and Dp ( A/dv), tme: 5 μs /dv Fg Expermental waveforms : v (00 /dv) and Do ( A/dv), tme: 5 μs /dv v c L Fg 0 Expermental waveforms : v c (00 /dv) and ( A/dv), tme: 5 μs /dv L % Effcency, Output ower, W Fg 3 Measured effcency as a functon of the output power when 00 and o 80

8 A Novel Soft-Swtchng Two-Swtch Flybac Converter wth 779 v 5 mh 0 n v 00 n Fg 4 Lne voltage and current waveforms Outer trace: lne voltage at 00 /dv; nner trace: lne current at 0 A/dv Tme base s 5 ms /dv 5 Conclusons A novel soft-swtchng two-swtch flybac converter wth a wde operatng range and regeneratve clampng s proposed The voltage stress on each man swtch s clamped to a lmted level The leaage nductance energy s also clamped and recycled bac to the nput to mprove effcency Due to ts smple crcut confguraton, consstng of a mnmum number of components and snce only passve components are utlzed n the clampng crcut to acheve zero voltage soft swtchng, the proposed converter can be controlled by a sngle WM sgnal, establshng a low-cost crcut confguraton and a smple control scheme The duty rato of the proposed converter can be more than 50%, and the magnetzng nductor energy transfer to the transformer s secondary sde s possble even f the reflected output voltage s hgher than the nput voltage A detaled analyss and the desgn and mplementaton of the crcut have also been dscussed Expermental results have demonstrated that the proposed converter can functon very effcently as a FC crcut References [] T Nnoma, T Tanaa, and K Harada, Analyss and optmzaton of a nondsspatve LC turn-off snubber, EEE Trans ower Electron, ol 3, no, pp 47-56, Apr 988 [] F Tsa, Marows, and E Whtcomb, Off-lne flybac converter wth nput harmonc current correcton, n roc EEE-NTELEC 96, pp 0-4, 996 [3] B Sngh and GD Chaturved, Analyss, Desgn and Development of a Sngle Swtch Flybac Buc-Boost AC-DC Converter for Low ower Battery Chargng Applcatons, Journal of ower Electroncs, ol 7, no 4, pp38-37, 007 [4] BH Lee, CE Km, KB ar, and GW Moon, A New Sngle-Stage FC AC/DC Converter wth Low Ln-Capactor oltage, Journal of ower Electroncs, ol 7, no 4, pp , 007 [5] Q Zhao, FC Lee, and F Tsa, oltage and current stress reducton n sngle-stage power factor correcton AC/DC converters wth bul capactor voltage feedbac, EEE Trans ower Electron, ol 7, no 4, pp , July 00 [6] Y Gu, X Gu, L Hang, Z Lu, and Z Qan, mproved wde range dual swtch flybac dc/dc converters, n roc EEE-AEC 04, pp , 004 [7] Y We, X Wu, Y Gu, and H Ma, Wde range dual swtch forward-flybac converter wth symmetrcal RCD clamp, n roc EEE-ESC 05, pp 40-44, 005 [8] KW Su and YS Lee, A novel hgh-effcency flybac power-factor-correcton crcut wth regeneratve clampng and soft swtchng, EEE Trans Crcuts Syst, ol 47, pp , Mar 000 [9] J Zhao and F Da, Soft-swtchng two-swtch flybac converter, n roc EEE-CEA 08, pp 50-54, 008 Marn-Go Km receved a B S degree n electrcal engneerng from Kyungpoo Natonal Unversty n 986, and hs M S and hd n electrcal engneerng from Korea Advanced nsttute of Scence and Technology n 988 and 99, respectvely From 99 to 994, he was wth the Korea Telecom Research Center, where he wored on the research of telecom power systems such as unnterruptble power supples, DC/DC converters and dstrbuted power systems Snce 995, he has been wth the Department of Control and Automaton Engneerng, uyong Natonal Unversty, where he s now a professor Hs research nterests nclude the modelng, analyss, and control of resonant converters, power semconductor crcuts, and soft swtchng converters Dr Km s a member of KE and KEE

9 780 Journal of ower Electroncs, ol 9, No 5, September 009 Young-Seo Jung receved hs BS, MS and hd n Electrcal Engneerng from the Korea Advanced nsttute of Scence and Technology (KAST), Daejon, Korea, n 99, 994, and 999, respectvely He wored for the Hyunda Autonet, owertran Team, from 999 to 00 Snce 00, he has been wth the Dvson of Mechancal Engneerng at uyong Natonal Unversty, Korea Hs research nterests are n the areas of power converters and varable speed motor drves Dr Jung s a member of the Korean nsttute of ower Electroncs (KE)