Long-Y Chang, Kue-Hsang Chao, Tsang-Chh Chang Applcaton of Hgh oltage Rato and Low Rpple nterleaved DC-DC Converter for a Fuel Cell LONG-Y CHANG, KUE-HSANG CHAO and TSANG-CHH CHANG Department of Electrcal Engneerng, Natonal Chn-Y Unversty of Technology No.57, Sec., Zhong-Shan Road, Tapng Dstrct, Tachung TAWAN, R.O.C. chaokh@ncut.edu.tw Abstract: - Ths paper proposes a hgh voltage rato and low rpple nterleaved boost DC-DC converter, whch can be used to reduce the output voltage rpple. Ths converter transfers the low DC voltage of fuel cell to hgh DC voltage n DC lnk. The structure of the converter s parallel wth two voltage-doublers boost converters by nterleavng ther output voltages to reduce the voltage rpple rato. Besdes, t can lower the current stress for the swtches and nductors n the system. Fnally, some expermental results are made to verfy the feasblty of the proposed converter. Key-Words: Fuel Cell, Hgh oltage Rato DC-DC Converter, Low Rpple, nterleaved Converter. 1 ntroducton Owng to worldwde energy crss and awareness of envronmental protecton n recent years, to seek for substtute energy has become an mportant ssue. Among many substtute energes, solar energy, wnd energy, hydroelectrc power, bomass energy and fuel cells are green energes wth potental development. As for fuel cells, there tend to have been more and more researches and applcatons recently. The fuel cell s a clean energy wthout polluton. ts energy, derved from reversed reacton of electrolyzed water, produces dynamc power. Only water s produced after the reacton; hence, there s hardly any envronmental polluton. Fuel cells as a source of power are usually appled to electrc hybrd automobles, dstrbuted electrc generaton system, portable and statonary power. Among them proton exchange membrane fuel cells are the most commonly used because of the followng merts: (1) lower temperature durng operaton, accordngly leadng to rapd turnng on and off and rapd reacton to the load change () lower operaton pressure, thus wth hgher safety (3) easly set n mode system (4) lower Emsson Rato and hgher converson rato[1-4]. Although the proton exchange membrane fuel cell has the advantages mentoned above, due to ts own actvaton loss, ohmc loss and concentraton loss, the output voltage s lowered as a result of load ncrease. Namely the fuel cell lowers the output voltage but rases the output current gradually as the output power rses under added load. Thus t s a low voltage hgh current output equpment. f we can transfer the low voltage produced by the fuel cell to hgh voltage, sendng t to DC Lnk, there wll be a wder range of applcaton[5-13]. n order to upgrade the fuel cell voltage output to the necessary electrcty level and avod the unsteady voltage caused by load change, t s necessary to adjust the fuel cell energy by means of power electronc technque, thus keepng steady the output voltage. Based on ths, presented n ths paper s a hgh voltage rato nterleaved DC-DC converter parallelly connected and further nterleaved by means of two sets of voltage-doubler boost converters. So besdes the advantages of hgh voltage rato converter, also because of the effect of parallel connecton, the current s dspersed nto four routes, thus lowerng the current stress of the swtch and nductance. n ths way t can wthstand the hgh current output whle there s hgh load. Through the parallel connecton of two sets of converters and controllng ther nterleaved voltage, t s possble to lower the output voltage rpple rato. Fg. 1 s the structure of the hgh voltage nterleaved DC-DC converter presented n ths paper. The fuel cell provdes electrcty for the dual nterleaved voltage-doubler of hgh voltage rato converter. Electronc load s used to test the amount of load (lght or heavy load);also mcrocontroller PC 18F870 manufactured by Mcrochp company s used for closed loop control. Fuel Cell There are a great varety of fuel cells; also there are dfferent ways to classfy them. The common approach s to classfy them accordng to the varous qualtes of the electrolyte. Thus they can be dvded nto the followng sx knds: 1. Proton Exchange Membrane Fuel Cell, PEM.. Alkalne Fuel Cell, A. E-SSN: 4-66X 6 ssue 1, olume 1, January 013
Long-Y Chang, Kue-Hsang Chao, Tsang-Chh Chang Fg. 1 The system of the presented dual nterleaved voltage-doubler of hgh voltage rato converter 3. Phosphorc Acd Fuel Cell, PA. 4. Molten Carbonate Fuel Cell, MC. 5. Sold Oxde Fuel Cell, SO. 6. Drect Methanol Fuel Cell, DM. Among them, the proton exchange membrane fuel cell s the best choce when we choose fuel cells for the source of the appled power because of the followng reasons: (1) lower operaton temperature, thus t can be rapdly turned on and off; () lower operaton pressure, hence greater safety; (3) t can be easly set nto mode system; (4) lower emsson rato and hgher converson rato..1 Mold-buldng of Fuel Cells As for fuel cells, ths paper adopts the NEXA TM proton exchange membrane fuel cell produced by Ballard Co. The specfcatons of ths proton exchange fuel cell n shown n Table 1 [14]. n buldng up the proton exchange membrane fuel cell math model, currently there are many smple precse model parameters and calculaton formulae beng presented and developed[15, 16]. n ths paper we refer to the electrochemstry formulae already presented to buld up the math model of the proton exchange membrane fuel cell, also wthn the range of the load current operaton smulate the characterstc curve of the output voltage and power rate of the fuel cell [15, 16]. The math model of the proton exchange membrane fuel cell s shown n (1) and () stack = N (1) = ENernst act ohmc con () Theren, stack :Stack output voltage. N:Number of cells formng the stack. :The output voltage of the fuel cell. Table 1 Specfcatons of the Ballard NEXA TM proton exchange membrane fuel cell [14] Power Emssons Physcal Fuel Rated Power Operatng voltage range oltage at Rated Power Current at Rated Power Start-up Tme Nose Water Dmensons Mass Purty Pressure Consumpton 100W to 50 DC 6 46A mnutes 7 dba 870 ml/hr 56 x 5 x 33cm 13 kg 99.99 % H (vol) 0.7 to 17. bar <18.5 SLPM E Nernst :Output voltage produced by every pece of fuel cell n thermodynamcs. act :Actvaton loss. ohmc :Ohmc loss. con :Concentraton loss. And the thermodynamc output voltage of every pece of fuel cell can be shown as follows. E Nernst = 1.9 0.85 10 + 4.31 10 5 3 T ln( P ( T 98.15) H 1 ) + ln( P O ) (3) Theren, T:Cell temperature (n Kelvn). P H :Partal pressures of hydrogen. P O :Partal pressures of oxygen. As for actvaton loss voltage, t can be shown ths way: act = [ ξ1 + ξ T + ξ3 T ln( C + ξ T ln( )] 4 O ) (4) Theren, ξ 1 ξ ξ 3 ξ 4 :The parametrc coeffcent for each cell model. C O : The concentraton degree of oxygen n the catalytc nterface of the cathode. :Fuel cell current. And the respectve coeffcents of the actvaton loss are: ξ = 0.0086 + 0.000 ln( A) 5 + 4.3 10 ln( C H ) 6 ( 498 / T ) C = P /[5.08 10 e ] O O (5) (6) E-SSN: 4-66X 7 ssue 1, olume 1, January 013
Long-Y Chang, Kue-Hsang Chao, Tsang-Chh Chang Theren, A:Cell actve area. C H :Lqud phase concentraton of hydrogen. As for ohmc loss voltage, t can be shown as follows: ohmc = ( RM + RC ) (7) Theren, R M :Resstance coeffcent of the membrane. R C :Resstance coeffcent constant to protons transfer through the membrane. The resstance coeffcent of the membrane theren s: R Theren, ρ M = ρ L A (8) M M / :Specfc resstvty of the membrane to the electron flow. L:Thckness of the membrane. The resstance coeffcent of the membrane can be shown to be: ρ M = {181.6 [1 + 0.03 ( / A).5 + 0.06 ( T /303) ( / A) ]} (9) [4.18 ( T 303) / T ] /{[ λ 0.634 3 ( / A) e ]} Theren, λ : Adjustment parameter, the range of whch s between 14 and 3. Concentraton loss formula s shown to be: con = B ln( 1 j / jmax ) (10) Theren, B:Constant varable dependng on the cell type and ts workng status. j:current densty of the cell. j max :Maxmum current densty. Theren the current densty of the cell s: j = / A (11) Therefore the equvalent crcut of the fuel cell can be worked up as n Fg.. f we take the dynamc response of the fuel cell nto consderaton, when two dfferent substances come nto contact, or the load current flows from one end to the other, accumulaton of charge s produced on the contact area. n the fuel cell, the layer of change between the electrode and electrolyte (or compact contact face) wll accumulate electrc charge and energy, whose acton s smlar to capactance. So when the load current changes, these wll be charge and dscharge phenomena happenng on the charge layer. Meanwhle actvaton loss voltage and concentraton loss voltage wll be under the nfluence of transent response, causng delay. But ohmc loss voltage wll not be nfluenced or delayed. We can take ths nto consderaton to let frst order lag exst n actvaton loss voltage and concentraton loss voltage. Thus ts dynamc response equaton can be shown to be[15, 16]: d dt c = E (1) Nernst ohmc c c = (13) C τ τ = C R a (14) Theren, τ:tme constant. C:The equvalent capactance of the system. c :Dynamc voltage of the fuel cell. R a :Equvalent resstance. Fg. The equvalent crcut of the fuel cell The analyss shown above can be used to buld up the mathematc model of the proton exchange membrane fuel cell so as to carry on the smulaton analyss of the system.. The Smulaton of the Fuel Cell n ths paper PSM smulaton software s used to buld up the smulated model of the proton exchange membrane fuel cell. ts composton module s shown n Fg. 3, n whch the upper rght ncreased k value s 4, representng the stack amount of the sngle cell n the cell stack. The smulated crcut of the equvalent capactance dynamc acton s shown n Fg. 4. Fg. 3 The fuel cell model bult up by means of PSM software E-SSN: 4-66X 8 ssue 1, olume 1, January 013
Long-Y Chang, Kue-Hsang Chao, Tsang-Chh Chang Fg. 4 The smulated crcut of capactance equvalent dynamc acton bult up by means of PSM software The DLL n Fg. 3 s the dynamc lnk lbrary of PSM smulaton software. Through software Mcrosoft sual C++ 6.0, the necessary DLL fle for lnkng can be set up. By means of Mcrosoft sual C++ 6.0, we can make use of programs to wrte the mathematc formulae n them, savng the trouble of buldng up numerous nner crcut fgures. After buldng up fuel cell module, we have ts load current operated wthn fxed rate and value. The hydrogen and oxygen pressures are respectvely set up at 1 bar. The characterstc curve of the smulated fuel cell output voltage and power rate s shown n Fg. 5. The upper part of Fg. 5 s the curve of the current and voltage of the fuel cell, whle the lower part s the power rate curve. Compared wth Fg. 7, the actual measurng output curve of Ballard Co. NEXA TM fuel cell, we can fnd both of the curves of the output characterstcs are closely smlar. Only because the curve of Fg. 6 s formed by connectng from pont to pont, t follows that there s slght dfference between them. Fg. 5 The curve of the fuel cell output by means of PSM software smulaton Fg. 6 The curve of the actual measurng output of Ballard Co. NEXA TM fuel cell [14] 3 Sngle Set of oltage-doubler Boost Converter Shown n Fg. 7 s the crcut structure of sngle set voltage-doubler boost converter[17, 18]. t s made up of nterleaved boost converters wth a clamp capactor C 1. The crcut structure s smple and t can reach the same hgh voltage rato wth lower duty cycle. Therefore, t can reduce the conducton loss of the swtch, to further upgrade the effcency of the whole converter. The work theorem of the whole crcut can be dvded nto four operaton modes, of whch the equvalent crcuts are respectvely shown n Fg. 8(a)-(d). Fg. 7 Crcut structure of voltage-doubler boost converter The equvalent crcuts of mode 1 and mode 3 are exhbted n Fg. 8(a) and (c). n ths stuaton, swtches S 1 and S are turned on. nput voltage stays between nductance L 1 and L, makng the nductance current ncrease lnearly and begns to depost energy, and the load current s provded by capactor C o. The change of the nductance current and L can be shown n (15) dl 1 dl = = L (15) dt dt Fg. 8(b) s the equvalent crcut n mode, n whch swtch S 1 s turned off whle S s turned on. The nductance current n forward drecton conducts dode D 1. n the meantme nductance L 1 voltage releases energy to clamp capactor C 1, chargng capactor C 1 whle nductance L goes on depostng energy. The change of the nductance current can be shown n (16). dl 1 C1 = (16) dt The equvalent crcut of mode 4 s exhbted n Fg. 8(d), n whch swtch S 1 s turned on and swtch S s turned off. The nductance current n forward drecton conducts Dode D. Then nductance L and clamp capactor C 1 smultaneously release energy to output capactor C o and load. The change of nductance current L can be shown n (17). dl + C1 = (17) dt L Through the analyss of the four modes mentoned E-SSN: 4-66X 9 ssue 1, olume 1, January 013
Long-Y Chang, Kue-Hsang Chao, Tsang-Chh Chang above, only C1 capactor voltage s an unknown varable. Accordng to crcut structure and KL theorem, nductance L 1, L and the voltage of dode D 1 plus clamp capactor voltage C1 should be zero, and n steady state the average voltage of nductance L 1 and L s zero. Therefore t s known that the average voltage of D 1 s dentcal wth clamp capactor voltage C1. The waveform of D 1 voltage s exhbted n Fg. 9, so the clamp capactor voltage C1 can be shown n (18). C1 = D1, avg = (18) C1 (1 D) T + DT = 0 (19) ( / ) (1 D) T + DT = 0 (0) = (1) 1 D From (1) t s known that voltage-doubler boost converter can reach the same hgh voltage rato wth shorter duty cycle. Moreover on account of the added clamp capactor, the voltage of the swtch can be reduced to only half of the output voltage. Ths can be known from the swtch voltage of () and (3) whle operatng under mode and mode 4. ds1,max = C1 = () ds,max = C1 = (3) D1 C1 o Mode 1 3 4 t Fg. 9 oltage waveform of dode D 1 under each mode Fg. 8 The four swtch modes of voltage doubler boost converter n the duty cycle:(a) model 1 (b) model (c) model 3 (d) model 4 After gettng the clamp capactor voltage, we work out (15) (17) accordng to volt-second balance theorem and get (19). Then carry n (18) to work out (0). Therefore we can nfer that the voltage ncrease of the converter s shown n (1), n whch T s the swtchng cycle, D s the duty cycle and f s the swtchng frequency. The output and nput power can be shown respectvely n (4) and (5). PO = (4) R P = = ( + L) (5) From (5), assumng L=L 1 =L, t follows: P = = L (6) f there s no power loss of the converter, then P o =P wth the followng result. ( ) D 4 L = = 1 = (7) R R (1 D) R L = ; { 1 } L L, L (8) (1 D) R The waveform of nductance current s exhbted n Fg. 10, n whch though and L waveforms are n complementary relaton, ts maxmum and mnmum nductance current are the same. Hence based on, the related formulae of the maxmum and mnmum nductance current are respectvely shown n (9) and (30). ΔL 1 DT L 1,max = + = + (9) (1 D) R L 1 E-SSN: 4-66X 30 ssue 1, olume 1, January 013
Long-Y Chang, Kue-Hsang Chao, Tsang-Chh Chang Δ DT,mn = = (30) (1 D) R Fg. 10 The waveform of the change of nductance current The condton on whch the converter can be operated n contnuous current mode s that,mn and L,mn should at least be greater than zero. So the boundary condton of contnuous and dscontnuous nductance current s: DT,mn = 0 = (31) (1 D) R So we get D(1 D) R,mn = (3) 4 f Because the maxmum and the mnmum nducton current of nductance L 1 and L are the same, the mnmum nducton rates derved from L 1 and L are dentcal. Hence f the converter s to be operated n the contnuous current mode, nductance L 1 and L must at least be greater than or equal to L 1,mn. From the mathematc functon D(1-D) of (3), t can be observed f D value s at 1/3, the mathematc functon D(1-D) wll have the greatest value, whch also means the greatest D value created by (3) s 1/3. Hence n desgnng nductance, when D as 1/3 s substtuted nto (3), and let the nductance value derved from calculaton be multpled by surplus value 1.5, t can be assured that the nductance current can really work n the contnuous current mode. The so-called lght load and heavy load n ths paper, ther load mpedances are respectvely,00 Ω and 450 Ω. So at swtchng frequency 15 khz, heavy load duty cycle about 0.85 when t s substtuted nto (3), the result s that n order to let the current contnue under lght load, the least nductance should be 6.3 mh, whle under heavy load t should be 179 μh. n ths paper 60 μh s the opton to make t possble to be n contnuous current conducton mode under heavy load. The change of output capactor current s shown n the Co of Fg. 11. From Fg. 11 we know the amount of capactor electrc charge change as DT Δ Q = = COΔ (33) RO Then ts voltage rpple rato may be expressed as follows. So the result s Δ O = DT ROCO (34) D CO = R f Δ / ) (35) O ( O O Therefore n the converter, we can decde the sze of the capactor accordng to the amount of voltage rpple rato. From (35) t s observed that the output capacty and duty cycle are n lnear relaton. t means the desgned output capacty must be greater than the requred capacty wth the greatest duty cycle. n ths paper voltage rpple rato s set at 5%. When t s substtuted nto (35), the output capacty s.5 μf. So 150 μf s selected to make the voltage rpple rato lower than 5%. By means of the above-descrbed workng mode of the converter, the swtch control sgnal n the crcut, nductance and capacty current waveform can be exhbted n Fg. 11, and ts nput voltage rpple and current rpple can be shown n (36) and (37) / Δ = ( )(1 D) T Ln Ln (36) 4 = (1 D) T; Ln { L 1, L } Ln Δ Co = ODT (37) From (36) t s known that voltage-doubler boost converter has the advantage of lower nput current but the amount of ts output voltage rpple s the same as the tradtonal hgh voltage converter. Hence n ths paper we set forth an amelorated nterleaved voltage-doubler boost converter. By means of the orgnal voltage-doubler boost converter parallelly connected, makng output voltage nterleaved, so as to reduce output voltage rpple, the flaw of greater output voltage s further amelorated. Fg. 11 The swtch sgnal, nductance and capacty waveform under each operaton mode E-SSN: 4-66X 31 ssue 1, olume 1, January 013
Long-Y Chang, Kue-Hsang Chao, Tsang-Chh Chang 4 The Presented Dual nterleaved oltage-doubler of Hgh oltage Rato Converter The crcut structure of the dual nterleaved voltage-doubler of hgh voltage rato converter presented n ths paper s shown n Fg. 1. By means of parallelly connected orgnal voltage-doubler boost converter to have the two sets of upper and lower voltage mutually nterleaved, we can lower ts output voltage rpple by controllng one set of ther swtch control sgnals to make ts output voltage rpple offset that of the other set. n controllng both the upper and the lower sets of swtches S 1 S and S 3 S 4 to make S 1 S and S 3 S 4 swtch control phase dscrepancy 180 o lead to voltage rpple phase dsplacement, the functon of lowerng voltage rpple s thus acheved. And because the nterleaved swtches of these two sets of voltage-doubler boost converters make the nput current crcut dvde nto four routes, thus further lowerng the current stress of the nductance and swtch, t s possble to wthstand the hgh current of the output of the fuel cell under heavy load. Also t s controlled by mcrocontroller PC18F870. n ths way the output voltage can be kept steady at a fxed value. D 1 D dfferent loads. The fuel cell produces output voltage about 6 to 43, to be upgraded to 300, and the electronc load s respectvely adjusted at,00 Ω (about output power 43 W) and 450 Ω (about output power 00W) under test. L 1 L C o1 C o R o S 1 S C 1 D 3 D 4 L 3 L 4 o S 3 S 4 C Fg. 1 The crcut structure of dual nterleaved voltage-doubler of hgh voltage rato converter Fg. 13 shows the control sgnal, nductance current, and output voltage rpple waveforms n the crcut. From Fg. 13 are observed the output voltage rpples of the two converters O1 and O. Through the phase dsplacement of the swtch control sgnal, the phase dsplacement of two sets of voltage rpples s brought about, thus resultng n the effect of lowerng the output voltage rpple. 5 Expermental Results n order to prove the feasblty of the dual nterleaved voltage-doubler of hgh voltage rato converter set forth n ths paper, a test wll be carred on under two Fg. 13 The rpple waveforms of swtch control sgnal nductance current and output voltage under each operaton mode Fg. 14 s the waveforms of the swtch sgnal control n dual nterleaved voltage-doubler of hgh voltage rato converter. Swtches S 1 S 3 and S S 4 have respectve control phase dscrepancy 180 o. Fgs. 15 and 16 show the waveforms of swtch sgnal and the waveforms of fuel cell output voltage and swtch output voltage respectvely under output power 43 W and 00 W. From the fgures t s observed that under dfferent loads, by controllng the duty cycle of the swtch sgnal, the output voltage can be kept steady at 300. E-SSN: 4-66X 3 ssue 1, olume 1, January 013
Long-Y Chang, Kue-Hsang Chao, Tsang-Chh Chang Fg. 14 The swtch sgnal waveforms of dual nterleaved voltage-doubler of hgh voltage rato converter Fg. 17 The swtch sgnal, and L nductance current waveform under output power 43W Fg. 18 The swtch sgnal, and L nductance current waveform under output power 00W Fg. 15 The swtch sgnal and nput/output voltage waveforms under output power 43 W Fg. 19 and Fg. 0 are the respectve output voltage rpple waveforms of sngle set voltage-doubler boost converter and the presented dual nterleaved voltage-doubler of hgh voltage rato converter. From Fg. 19 and Fg. 0 t s observed that through comparson we fnd there s mprovement n output voltage rpple waveform. n Fg. 19 the peak to peak voltage of the sngle set voltage-doubler boost converter s about 15.8, whle that of the presented dual nterleaved voltage-doubler of hgh voltage rato converter n Fg. 0 s about 9.5. Ther respectve voltage rpple ratos are 5.7% and 3.17%. Fg. 16 The swtch sgnal and nput/output voltage waveforms under output power 00 W Fg. 17 and Fg. 18 are the waveforms of swtch sgnal, nductance current and L under respectve output power 43 W and 00 W. From the fgures t s observed that wth the gradual ncrease of loads, the nductance currents and L are also on the ncrease to enable t to work n contnuous current mode under hgher output power. Fg. 19 The output voltage rpple waveform of sngle voltage-doubler boost converter under output power 43 W E-SSN: 4-66X 33 ssue 1, olume 1, January 013
Long-Y Chang, Kue-Hsang Chao, Tsang-Chh Chang Fg. 0 The output voltage rpple waveform of the presented dual nterleaved voltage-doubler of hgh voltage rato converter under output power 43 W Fg. 1 and Fg. are the respectve output voltage rpple waveforms of sngle set voltage-doubler boost converter and the presented dual nterleaved voltage-doubler of hgh voltage rato converter. From Fg. 1 and Fg. t s observed that through comparson we fnd there s mprovement n output voltage rpple waveform. n Fg. 1 the peak to peak voltage of the sngle set voltage-doubler boost converter s about 36, whle that of the presented dual nterleaved voltage-doubler of hgh voltage rato converter n Fg. s about 6.5. Ther respectve voltage rpple ratos are 1% and 8.75%. Thus t s proved that the dual nterleaved voltage-doubler of hgh voltage rato converter can mprove the flaw of hgher voltage rpple rato of the orgnal sngle set voltage-doubler boost converter. Fg. 1 The output voltage rpple waveform of sngle voltage-doubler boost converter under output power 00 W Fg. The output voltage rpple waveform of the presented dual nterleaved voltage-doubler of hgh voltage rato converter under output power 00 W 6 Concluson Ths paper sets forth an amelorated dual nterleaved voltage-doubler of hgh voltage rato converter to mprove the problem of output rpple voltage of sngle set voltage-doubler boost converter. Wth two parallelly connected voltage-doubler boost converters to nterleave the output voltage rpple, we further lower the output voltage rpple. Not only does t mantan the advantages of voltage doubler boost converter, but also, owng to the nterleaved sngle set converter wth two separate current routes and the two sets of swtches of the double voltage-booster once agan n parallel connecton leadng to four separate current routes, t s thus possble to further lower the current stress of the swtch and nductance. Through test and experment, ths paper proves and confrms the feasblty of the presented dual nterleaved converter. 7 Acknowledgement Ths work was supported by the Natonal Scence Councl, Tawan, R.O.C., under the Grant of #NSC99-63-E-167-001-ET. References [1] J. C. Amphlett, R. F. Mann, B. A. Peppey, P. R. Roberge, and A. Rodrgues, A Model Predctng Transent Response of Proton Exchange Membrane Fuel Cells, Journal of Power Source, ol. 61, No. 1-, 1996, pp.183-188. E-SSN: 4-66X 34 ssue 1, olume 1, January 013
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