A NEW SOFT SWTCHNG FLYBACK-FORWARD PWM DC-DC CONERTER 1 MAJD DELSHAD, 2 M TAHERPOOR 1,2 Electricl Engineering Deprtent, sfhn (Khorsgn) Brnch, slic Azd University, rn E-il: delshd@khuisf.c.ir Astrct- This pper presents new soft switching flyck-forwrd PWM converter. The proposed circuit eploys n uxiliry circuit to chieve soft switching condition for switches nd lso sors the voltge spikes cused y the lekge inductnce of trnsforers in the converter. n the proposed converter in switch opertes t ZS nd the uxiliry switch opertes t ZCS. The proposed converter hs soe dvntges such s the low output current ripple, shring power etween the gnetic eleents nd work t high power levels, soft switching condition in the oth turn on nd off switch. Soft switching condition is stisfied for oth step-up (oost) nd step-down (uck) in the proposed converter. Furtherore this converter hs high efficiency nd low voltge stress. The converter is controlled y PWM. Keywords- Zero Current Switching (ZCS), Zero ltge Switching (ZS), Soft Switching, nd PWM.. NTRODUCTON solted DC-DC converters re widely eployed in the couniction power syste nd industril pplictions, fuel cell systes, etc. soltion trnsforer links priry switching circuitry such s flyck, forwrd, push pull, hlf ridge or full ridge topologies with secondry rectifiers. The well- known clssicl forwrd converter needs three windings. Two re used to trnsfor the energy nd the third one is used to rest the core. So fr ny of the flyck nd forwrd trnsforers hve een introduced. The new coined forwrd-flyck converter doesn t need reset windings. The trnsforer is esier to nufcture, is sller, hs lower weight nd is therefore cheper. n the isolted interleved ZS flyck-forwrd oost type converter, the ctive clp circuit is dded in the interleved two phses of the converter to recycle the lekge energy. There re severl wys to control the converters. n the PWM converter, switching frequency is fixed; nd output converter controlled y controlling the duty cycle. Therefore PWM soft switching converters ore thn resonnt nd qusi-resonnt converters re considered. n these converters, n uxiliry circuit is dded to the switches of converter, so tht converter control reins PWM. This pper descries new soft switching flyck-forwrd PWM converter, which sored voltge spikes due to the lekge inductnce of trnsforers. The in switch opertes t ZS nd the uxiliry switch opertes t ZCS. Soft switching condition is stisfied for oth step-up (oost) nd step- down (uck) in the converter. n the oth turn on nd off switch, power is trnsferred to the output therefore this converter work t high power levels. Furtherore this converter is high efficiency, low voltge stress nd the low output current ripple.. PROPOSED CONERTER AND OPERATONAL PRNCPLE A. Proposed converter The uxiliry circuit is shown in Fig1. The uxiliry circuit connected in prllel to the in switch. The uxiliry circuit consists of uxiliry switch S, series diode D, resonnt eleents L nd C. This circuit when switched properly ensures lossless switching. The proposed flyck-forwrd converter is illustrted in Fig2; where S 1 is the in switch, C S is dopted to ipleent the ZS soft switching perfornce, which includes the prsitic cpcitnce of the in switch. C is the lock cpcitor, C O1 nd C O2 re the output cpcitors, in nd O re the input nd output voltges, D 1, D 2 nd D 3 re the proposed converter diodes, nd R O is the lod. There re two trnsforers in the proposed converter, which re ned y flyck (T 1 ) with 1: n turns rtio nd forwrd (T 2 ) with 1: turns rtio nd n inductnce L T which is coupled with forwrd trnsforer y 1: turns rtio. L 1 nd L 2 re the gnetizing inductors; L K1 nd L K2 re the lekge inductnces of trnsforers. Fig. 1. uxiliry circuit. Proceedings of TheER nterntionl Conference on Bngkok, Thilnd, 28 th Deceer 2014, SBN: 978-93-84209-79-7 45
A New Soft Switching Flyck-Forwrd PWM DC-DC Converter Fig. 2. Proposed flyck-forwrd PWM converter. B. Opertion of proposed converter To siplify the nlysis of the converter in stedy stte, these ssuptions re considered: The prsite eleents re ignored. The output cpcitors (C O1, C O2 ) re lrge enough, thus the output voltge ( O ) in one switching cycle is constnt. The input voltge ( in ) in one switching cycle is constnt. Before t 0, the in switch S nd the uxiliry switch S re in the off-stte. The resonnt cpcitor C is chrged to c y the current of the gnetizing inductors L 1 nd L 2.(Eqution(1)). The secondry side diodes D 1 nd D 2 re oth reverse-ised nd other seiconductor devices re not conducts (see fig 4()). Eqution (2) nd (3) re gives the voltge cross the priry of flyck trnsforer nd forwrd trnsforer. Eqution (4) gives the voltge cross the coupled inductor (L T ). o C c in 2 2n (1) L 1 2 n (2) L 2 (3) L T (4) Mode1:This intervl egins when the uxiliry switch S is turned-on with ZCS t t=t 0. The equivlent circuit is shown fig4 () with 0 0 s the initil current L of resonnt inductor L. The current in the uxiliry switch S increse linerly s in (6). Fig. 3. Key wvefors of proposed flyck-forwrd PWM converter. S t 0 (5) L 2 o C in 2n S t 0 (6) L At t=t 1, current s (t) reches. During this condition is equivlent to: L t o (7) 2 o in 2n Mode2: When the current in the uxiliry switch S reches, the diode D 1 is turn off with ZCS t t=t 1. The resonnt eleents L nd C re resonting during this intervl. The current in the uxiliry switch t S will rises sinusoidl s shown in 4(d). Eqution (8) nd (9) gives the current through the resonnt inductor nd the voltge cross the resonnt cpcitor respectively. This intervl ends, when the voltge cross the resonnt cpcitor t decrese to zero. This forwrd ises the ody diode of the in switch S t tie t=t 2. Body diode of the in switch s is turnson nd it cn e now turned-on with zero voltge Proceedings of TheER nterntionl Conference on Bngkok, Thilnd, 28 th Deceer 2014, SBN: 978-93-84209-79-7 46
switching (ZS). The current of the in switch nd the voltge of C in resonnt stte re shown in elow. S sin wo t 1 (8) z C COS w A New Soft Switching Flyck-Forwrd PWM DC-DC Converter c o t 1 (9) L T t t 4 3 B 4 (20) B (21) The equivlent circuit condition is shown in fig4 (e). wo 1 (10) LC Mode5: At t=t 4 i.e., t the end of DTS, the in switch S is switched-off. The turn off of the in switch is t zero voltge (ZS), ecuse of snuer cpcitor cross the in switch. L C Z (11) During this condition is equivlent: t2 t1 2 L C (12) Mode3: Body diode of the in switch (S) turns on, therefore in switch (S) cn e turn on with zero voltge switching (ZS). n this intervl, the resonnt stop nd the current of the in switch (S) rise linerly to zero whit slope of C. L The iportnt equtions introduce s following: L (13) 1 2 n L L (14) 2 2 S S (15) T 2 o C in 2 2n 2 C t (16) 2 (17) L 2 (18) L C t t Mode4: n this intervl, the current of the in switch S is zero, the current of inductor L T trnsfer fro the ody diode S to the in switch S. Therfore diode D 3 turns on y ZS nd diode D 2 turns off y ZCS. During in this intervl, the current of uxiliry switch S, decrese to zero with the pervious slope. Thus the uxiliry switch S is turned off on ZCS. According to KCL eqution (17), fro the oent, the current of in switch S will e otined. S S (19) The voltge cross the in switch rises slowly therey the turn off trnsition losses reduced. n this condition the diodes D 1 nd D 2 turn on under ZCS. The voltge cross the priry of flyck nd forwrd trnsforers for this sitution is s follows: L (22) 1 2 n L 2 (23) C T (24) This eqution circuit is shown in fig4 (f). t is oserved tht turn on nd turn off of in switch nd uxiliry switch y the uxiliry circuit re loss-less.. DESGN GUDELNES n this section, guidelines for the design of the proposed flyck-forwrd PWM converter re presented. Selection of resonnt inductor L r nd resonnt cpcitor C r. The slope of current in the uxiliry circuit in T 1 depends on L r. The vlue of L r nd C r together decides the resonnt intervl T 2.The resonnt intervl should e iniu. This intervl iniizes the conduction losses in the uxiliry switch; xiizes the effective duty cycle s well. The length of the resonnt intervl is pproxitely qurter of the resonnt period (Eqution (25)). 2 L C r r 5to10% oft x on (25) Lrger vlues of C r increse the pek of uxiliry circuit current. Lower vlues of C r increses voltge slope of in switch. lue of C r is so Proceedings of TheER nterntionl Conference on Bngkok, Thilnd, 28 th Deceer 2014, SBN: 978-93-84209-79-7 47
A New Soft Switching Flyck-Forwrd PWM DC-DC Converter Fig. 4. Opertion processes of proposed converter:():[t<t0],():[t0 t1],(c):[t1 t2],(d):[t2 t3], (e):[t3 t4],(f):[t4 t5]. selected tht voltge cross the in switch does not exceed the specified liit. The C r vlue is otined elow. t f Cr (26) 2 is the on-stte current, is the off-stte voltge, t f is the fll tie of the in switch. A. Selection of uxiliry switch S, Diodes D nd D The device used for the uxiliry switch should hs low output cpcitnce. The diodes D nd D re fst recovery diodes. The gting signls to S nd S re s shown in fig.3 to ensure ZS nd ZCS trnsitions of the in nd uxiliry switch, respectively. The turn-on of uxiliry switch is t=t 1 +T 2 +T 3.This ensures tht the turn-off trnsition of the uxiliry switch S is t zero current. The iniu dely T d for the turn-on of the in switch S is t=t 1 +T 2. n severl ZS schees, the dely is function of lod nd other operting conditions.. SMULATON RESULTS The flyck-forwrd PWM converter is siulted shown in fig.5 Fig.6 nd Fig.7. The circuit preters nd specifictions of proposed converter re surized in Tle.1. As you cn see in Fig. 5 the current in turns on instnt is negtive therefore ody diode is on nd ZS condition is provided. Also the voltge t turn off instnt rises slowly due to snuer cpcitor therefore ZS condition exists. n Fig. 6 the current of uxiliry switch is zero t turn on nd turn off instnt therefore ZCS conditions re provided. n Fig. 7 Proceedings of TheER nterntionl Conference on Bngkok, Thilnd, 28 th Deceer 2014, SBN: 978-93-84209-79-7 48
the voltge nd current wvefors of D1 is presented. As you cn see ZCS condition exists for turn on nd off instnt. A New Soft Switching Flyck-Forwrd PWM DC-DC Converter TABLE 1 Utilized coponents nd preters in the proposed flyck-forwrd PWM converter D Duty cycle 0.52us in nput voltge 24v out Output voltge 48v P o Output power 50w f s Switching 100KHZ C s frequency Snuer cpcitor 4.7nf C Resonnt 1nF C cpcitor Block cpcitor 10µf C o1,c o2 Output cpcitors 47µF L Resonnt inductor 4 µh LT Coupled 30µH L1 inductnce gnetizing 200 µh L2 inductor of fly-ck gnetizing 100 µh inductors of N,M turns rtio 1 Fig.5. siulted nd experientl results ZS perfornces of the in switch (S). Fig.6. shows the siulted nd experientl results ZCS perfornces of the uxiliry switch (S ). Fig.7. shows the siulted nd experientl results the current wve for of (D 1) CONCLUSON This pper presents new soft switching flyck-forwrd PWM converter. The in switch opertes under ZS condition nd the uxiliry switch opertes under ZCS condition (turn on nd off). Soft switching condition is stisfied for oth step-up (oost) nd step-down (uck) in the converter. Siultion results re presented for 50 w, 100KHZ flyck-forwrd converter. REFERENCES [1] L. Yn nd B. Lehn, An integrted gnetic isolted two-inductor oost converter: Anlysis, design nd experienttion, EEE Trns. Power Electron, l. 20, No. 2, pp. 332 342, Mr. 2005. [2] L. Zhu, K. R. Wng, F. C. Lee, nd J. S. Li, New strt-up schees for isolted full-ridge oost converters, EEE Trns. Power Electron, l. 18, No. 4, pp. 946 951, Jul. 2003. [3] L. Zhu, A novel soft-coutting isolted oost full-ridge ZS-PWM dc dc converter for idirectionl high power pplictions, EEE Trns. Power Electron, l. 21, No. 2, pp. 422 429, Mr. 2006. [4] E. S. Prk, J. C. Sung, J. M. Lee, nd B. H. Cho, A soft-switching ctive clp schee for isolted full-ridge oost converter, EEE APEC, l. 2, pp. 1067 1070, 2004. [5] H. Mo, O. Adel-Rhn, B. Higgins nd G. Potter, Soft-Switched Asyetric Hlf-Bridge Flyck-Forwrd DC-DC Converter, Applied Power Electronics Conference nd Exposition, pp.528-532, Mrch. 2006. [6] N. Mohn, T. Undelnd, nd W. P. Roins, Power Electronics, New York: John Wiley & Sons, 2003. [7] R. W. Erickson, D. Mksiović, Fundentls of Power Electronics. 2nd edition, New York: Springer Science+Business Medi, nc., 2001. [8] B. Bose, Power Electronics nd Motor Drives, Elsevier Acdeic Press, 2006. [9] M. P. Kzierkowski, Control in Power Electronics. Selected Proles, Acdeic Press Series in Engineering, 2002. [10] S.K. Chngchien, T.J. Ling, K.C. Tseng, J.F. Chen nd R.L. Lin, Adegnetiztion Circuit for single-ended forwrd converter, EEE ECON, pp. 1390-1395, 2007 [11] T. Jin, K. Zhng, A. Azzolini, K.M. Sedley, A new interleved forwrd converter with inherent degnetizing feture, EEE AS, l. 1, pp.625-630, 2005. [12] C. Frcs, D. Petreus, E. Siion, N. Plghit, Z. Jihos, A novel topology sed on forwrd converter with pssive power fctor correction, SSE, pp. 268-272, 2006. Proceedings of TheER nterntionl Conference on Bngkok, Thilnd, 28 th Deceer 2014, SBN: 978-93-84209-79-7 49
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