Soft-Swtched CCM Boost Converter wth Hgh oltage Gan for Hgh Power Applcatons Sungsk Park and Sewan Cho, IEEE Senor Member Seoul Natonal Unversty of Technology Dept. of Control and Instrumentaton Eng. 17 Kongneung-Dong, Nowon-Ku, Seoul 139-743, Korea scho@snut.ac.kr Abstract -- Ths paper proposes a new soft-swtched CCM boost converter sutable for hgh power applcatons such as power factor correcton, hybrd electrc vehcles and fuel cell power converson systems. The proposed converter acheves ZS turn-on of actve swtches n contnuous conducton mode and ZCS turn-off of dodes leadng to neglgble reverse recovery loss. The components voltage ratngs and energy volumes of passve components of the proposed converter are greatly reduced compared to the conventonal ZT converter. oltage converson rato s almost doubled compared to the conventonal boost converter. Extenson of the proposed concept to realze mult-phase DC-DC converters wll be dscussed. Expermental results from a 1.5kW prototype are provded to valdate the proposed concept. Index Terms-- soft-swtched, non-solated, hgh voltage gan, hgh power applcatons, CCM I. INTRODUCTION Contnuous conducton mode(ccm) boost converters have wdely been used as the front end converter for actve nput current shapng[6]. In recent years, CCM boost converters are ncreasngly needed n hgh power applcatons such as hybrd electrc vehcles and fuel cell power converson systems. Hgh power densty and hgh effcency are major concerns n hgh power CCM boost converters[1,7]. The hard-swtched CCM boost converter suffers from severe dode reverse recovery problem n hgh current hgh power applcatons. That s, when the man swtch s turned on, a shoot-through of the output capactor to ground due to the dode reverse recovery causes a large current spke through the dode and man swtch. Ths not only ncurs sgnfcant turn-off loss of the dode and turn-on loss of the man swtch, but also causes severe EMI emsson. The effect of the reverse recovery related problems become more sgnfcant for hgh swtchng frequency at hgh power level. Therefore, the hard-swtched CCM boost converter s not capable to acheve hgh effcency and hgh power densty at hgh power level. Many soft-swtchng technques on CCM boost converters have been proposed[,3,4,6,8,9,10]. The zero-voltageswtched (ZS) quas-resonant converter (QRC) acheves soft swtchng of the man swtch wth ZS and the dode wth ZCS, but both man swtch and dode suffer from an excessve voltage stress due to resonant operaton. The ZS quas-square-wave converter (QSW) technque offers ZS turn on for both man swtch and dode wthout ncreasng ther voltage stresses. However, both man swtch and dode suffer from a hgh current stress resultng n sgnfcant conducton losses. Furthermore, turn off loss of the man swtch s consderable. Snce both ZS-QRC and ZS-QSW technques acheve soft swtchng only at the expense of ncreased conducton losses due to voltage or current stresses of the components, they are not sutable for hgh power applcatons[10]. The zero-voltage-transton (ZT) PWM converter [10] acheves soft swtchng of the man swtch and dode wthout ncreasng ther voltage or current stresses snce ZS s acheved by partal resonance of the shunt branch across the man swtch. Furthermore, the reverse recovery related problem s allevated by controllng dode current decrease rate d/dt durng ts turn off. However, severe undesred resonance may occur n the shunt branch. Addng a rectfer and saturable nductor can mtgate the resonance, but ths causes crcut complexty and addtonal cost[5]. Also, the auxlary swtch n the shunt branch s hard swtched, and the duty rato of the auxlary swtch lmts the effectve duty rato of the man swtch leadng to decreased voltage converson rato of the converter. Ths paper proposes a new soft-swtched CCM boost converter sutable for hgh power applcatons such as power factor correcton, hybrd electrc vehcles and fuel cell power converson systems. The proposed converter has the followng advantages: ZS turn-on of the man swtches n contnuous conducton mode Neglgble dode reverse recovery due to ZCS turn-off of the dode oltage converson rato s almost doubled compared to the conventonal boost converter Greatly reduced components voltage ratngs & energy volumes of most passve components The operatng prncples and features of the proposed 978-1-444-893-9/09/$5.00 009 IEEE 1999
S D 1 I L1 L1 C1 I L L C L 1 C 1 L R o o D S 1 C 3 Fg. 1. Proposed soft-swtched CCM boost converter converter are descrbed. Expermental results from a 1.5kW prototype are also provded to valdate the proposed concept. II. PROPOSED SOFT-SWITCHED BOOST CONERTER Fg. 1 shows the crcut dagram of the proposed CCM boost converter, and Fg. shows key waveforms llustratng the operatng prncple of the proposed converter. Upper swtch S n the proposed converter replaces the rectfer dode n the conventonal boost converter. Lower swtch S 1 and upper swtch S are operated wth asymmetrcal complementary swtchng to regulate the output voltage as shown n Fg.. An auxlary crcut that conssts of a capactor C 1, an nductor L, two dodes D 1 and D, and a capactor C s connected on top of the output capactor C 3 to form the output voltage of the converter. The auxlary crcut not only ncreases the output voltage but also helps ZS turnon of actve swtches S 1 and S n CCM. A. Operatng prncple As shown n Fg., the operaton of the proposed converter can be dvded nto the fve modes. The equvalent crcuts for each mode are shown n Fg. 3. Mode I : Ths mode begns when L decreases to zero and D s turned on as shown n Fg.. Durng ths mode, the lower swtch S 1 mantans on-state. Both nput nductor current L1 and auxlary nductor current L flows through lower swtch S 1. The slope of these currents are gven by, d L 1 = (1) dt L 1 d L ( ) dt C1 C3 = () L Mode II : Ths mode begns when S 1 s turned off and the body dode of S s turned on. The gatng sgnal for S s appled durng ths mode, and S s turned on under ZS condtons. Both L1 and L are decreasng wth the slope determned by the followng equatons: d L ( ) dt 1 C3 = (3) L1 d L C dt Fg.. Key waveforms of the proposed converter 1 = (4) L At the end of ths mode, nductor current L changes ts drecton of flow and D 1 starts to conduct. It should be noted that D s turned off under ZCS. Mode III : Durng ths mode L1 keeps decreasng wth the slope determned n Mode II, and L ncreases wth slope determned by the followng equaton: d L ( ) dt C1 C = (5) L 000
and decrease, respectvely, wth the slope determned by the followng equatons: d L 1 = dt L1 (6) d L C1 C C3 = dt L (7) Ths state ends when the decreasng current L reaches to 0. Ths s the end of one complete cycle. Note that dode D 1 s also turned off under ZCS B. oltage converson rato To obtan the voltage gan of the proposed converter, t s assumed that the voltage across C 1, C, and C 3 are constant durng the swtchng perod of T S. The output voltage s gven by, or = + (8) o C C3 o = (9) 1 D eff where the effectve duty D eff s defned by, D = D+ M M (10) eff 1 The output voltage can also be expressed as, o = Δ (11) 1 D where Δ s the voltage drop caused by the duty loss (M - M 1 ). Note that the duty loss of the proposed converter s much less than that of the phase-shfted full-brdge converter snce there s no crculatng current n the proposed converter. From eqns. (9), (10), and (11) the voltage drop Δ can be obtaned by, Fg. 3. Operaton modes of the proposed converter At the end of ths mode swtch current S reverses ts drecton of flow and conducts the man channel of S. Mode I : Durng ths mode L1 and L keep flowng wth the same slope determned n Mode III. Mode : Ths mode begns when S s turned off and the body dode of S 1 s turned on. The gatng sgnal for S 1 s appled durng ths mode, and S 1 could be turned on under ZS condtons. Inductor currents L1 and L start to ncrease ( M1 M) Δ = (1 D)(1 D M + M ) 1 (1) Accordng to volt-sec balance prncple on L, capactor voltage C1 can be obtaned by, C1 C( D M1 M) + C3( D 1) = D + 1 where C and C3 can be expressed as, C3 (13) 1 = (14) 1 D 001
1 C = Δ 1 D (15) In the mean tme, snce the output load current equals the average current of D 1 and D the followng equatons can be derved by, 1 I = = (1 D+ M M ) T I (16) o D1, av 1 S L, + pk Ro 1 I = = ( D M + M ) T I (17) o D, av 1 S L, pk Ro where I L,+pk and I L,-pk are postve and negatve peak values of nductor current I L, respectvely and gven by(see Fg. ), I I ( ) M T C1 C C3 1 S L, + pk = (18) L M T C1 S L, pk = (19) L Usng equatons (11) to (19) the effectve voltage gan of the proposed converter s plotted as shown n Fg. 4. Even though there s a slght drop off the deal voltage gan whch s caused by duty loss (M -M 1 ), the effectve voltage gan of the proposed converter s almost twce compared to that of the conventonal boost converter. Ths s a very desrable feature n hgh voltage gan applcaton snce reduced duty rato leads to reduced current stresses on the components resultng n ncreased effcency. Duty loss (M M 1 ) can be reduced by choosng smaller nductance L, but ths reduces ZS range of man swtch S 1. Therefore, nductance L should be properly chosen consderng a trade-off of 10 9 8 7 6 5 4 3 1 Ideal voltage gan Effectve voltage gan 0 0 0.1 0. 0.3 0.4 0.5 0.6 0.7 0.8 Fg. 4. oltage gan as a functon of duty rato ( =80, L =7uH, f S=70kHz, P o=1.5kw) swtchng loss and voltage gan. C. ZS characterstc for man swtch ZS of the upper and lower swtches depends on the dfference of the flter nductor current L1 and auxlary nductor current L, as shown n Fg.. The ZS current for lower swtch, I S1,ZS, s the postve peak of L1 - L the when the upper swtch s turned off and can be expressed as, IS1, ZS = IL, + pk IL 1,mn ( C1 C C3) M1 T s o D = + L R o L1fs (0) The ZS current for upper swtch, I S,ZS, s the negatve peak of L1 - L when the lower swtch s turned off and can be expressed as, I = I + I S, ZS L, pk L1,max C1 M T s o D = + + L R o L1fs (1) To ensure the ZS turn on of upper swtch S the followng condton should be satsfed, 1 1, ( 1 ) LIS ZS > Cos + Cos 1 D eff () where C os1 and C os are the output capactances of lower swtch S 1 and upper swtch S, respectvely. In fact, the condton of equaton () can easly be satsfed, and ZS of upper swtch S can be acheved over the whole load range. To ensure the ZS turn on of lower swtch S 1 the followng condton should be satsfed, 1 1 1, ( 1 ) LIS ZS > Cos + Cos 1 D eff (3) The equaton (3) may not be satsfed under the condtons of small auxlary nductance, large nput flter nductance, and/or lght load. Increasng auxlary nductance to enlarge the ZS regon makes the duty loss (M -M 1 ) large. Alternatvely, n order to enlarge the ZS regon, the nput nductance can be decreased so that I S1,ZS can be ncreased. However, decreasng the nput flter nductance ncreases the current ratng of the power devces, and therefore the nput flter nductance should be properly chosen consderng a trade-off between the ZS regon and the devce current ratngs. Therefore, ZS for lower swtch S 1 can be acheved more easly wth smaller value of L 1 and/or larger value of L at the cost of the large current rpple. A trade-off of swtchng loss and conducton loss should be consdered. Usng equatons (0), (1), () and (3), the ZS currents and ZS ranges of lower and upper swtches as the functon 00
of voltage gan and output power are plotted, respectvely, as shown n Fg. 5. As shown n Fg. 5(a), the ZS current of the lower swtch tends to ncrease as the output power ncreases and decrease as the voltage gan ncreases. Ths means that the ZS turn-on of the lower swtch can be more easly acheved under the condton of hgher output power and lower voltage gan. It s noted that the ZS range of the lower swtch becomes broader for smaller total output capactance C os,tot =C os1 +C os of MOSFETs. For example, f MOSFETs wth total output capactance C os,tot of 1.8nF are selected n ths example, the ZS turn-on of the lower swtch can be acheved wth output power whch s greater than 500W at voltage gan of 5(See Fg. 5(a)). The ZS current of the upper swtch also tends to ncrease as the output power and voltage gan ncrease. It should be noted from Fg. 5(b) that the ZS turn-on of the upper swtch can be acheved n the overall nput voltage and output power ranges. D. Comparson of component ratngs In order to perform a comparson of the proposed converter to the conventonal ZT converter[10] n terms of the component ratng, the converters have been desgned accordng to the followng specfcatons: P o = 13kW = 50 ~ 450 o = 600 ΔI = 10% Δ o = 3% f s = 15 khz (a) The component ratngs of the proposed converter and the ZT converter calculated accordng to the desgn specfcaton are lsted n Table I. Due to the proposed connecton of the auxlary crcut, the voltage ratngs of all components of the proposed converter are much smaller compared to those of the ZT converter whch are the same as the output voltage. Therefore, the swtch and dode utlzatons of the proposed converter are greatly mproved. Energy volume of the output capactor of the proposed converter s slghtly ncreased compared to the ZT converter snce two capactors are requred at the output. However, energy volumes of the other passve components are sgnfcantly reduced n the proposed converter. Input nductance of the proposed converter s smaller because the voltage across the nductor s smaller. Also, the current ratng of the auxlary nductor s much smaller compared to that of the ZT converter snce the proposed converter does not utlze resonance whle the ZT converter utlzes partal resonance for soft swtchng. (b) Fg. 5. ZS currents and ZS ranges of lower and upper swtches as the functon of the voltage gan and output power (a) lower swtch S 1 (b) upper swtch S ( o = 400, P o : 100W~1.5kW, L =7uH, f s=70khz) III. EXTENSION OF THE PROPOSED CONCEPT Usng the converter shown n Fg. 1 as a basc cell the proposed concept can be extended to realze mult-phase DC- DC converters for hgh voltage and hgh power applcatons. Fg. 6 shows the generalzed crcut of the proposed multphase DC-DC converter. The generalzed converter has N groups of converters, where each group of swtch legs s connected n parallel at the low voltage hgh current sde whle output capactors n each group s connected n seres at the hgh voltage low current sde. Each of N groups also has P parallel connected swtch legs to ncrease the output power, where P s the number of swtch or dode legs connected to the same output capactor. That s to say, N could be ncreased to get hgher output voltage, and P could be ncreased to get hgher output power. Fg. 7 shows an example crcut confguraton (P=1) 003
TABLE I. A COMPARISON OF COMPONENT RATINGS OF THE PROPOSED CONERTER AND THE ZT CONERTER Components Desgn tem ZT Converter[10] Proposed Converter pk 61 387 Actve Swtches I pk 653 A 156 A P o / ( pk I pk q) 0.0 0.11 pk 61 3 Dodes I pk 5 A, 650A 101 A P o / ( pk I pk q) 0.03 0.3 Fg. 6. Extenson of the proposed concept for mult-phase DC-DC converter (N s the number of seres-connected output capactors on top of C 1, and P s the number of dode legs connected to the same output capactors) Output Capactor Capactance 50 uf 10 uf EA pk 61 3, 387 SN1 S1 S11 C1,N1 L,N1 DN1 D N1 C,N C (PU) 1 1.3 L1,N1 C1,1 L,1 D1 C, out Inductance 1400 uh 100 uh L1,1 D 1 Input Inductor I rms 5 A 5 A LI (PU) 1 0.86 n L1,11 S N1 S 1 S 11 C1,11 L,11 D11 D 11 C,1 C1 Auxlary Capactor Capactance uf 30 uf I rms 98 A 50 A pk 61 84 Fg. 7. An example crcut confguraton of the proposed mult-phase converter to ncrease the output voltage (P=1) C (PU) 1 3. Auxlary Inductor Inductance uh 5 uh I rms 77 A 50 A LI (PU) 1 0.4 Fg. 8. An example crcut confguraton of the proposed mult-phase converter to ncrease the output power (N=1) whch llustrates how to ncrease output voltage by ncreasng N. Fg. 8 shows an example crcut confguraton (N=1) whch llustrates how to ncrease power level by ncreasng P. In both cases the nterleavng technque can be appled to reduce the sze of nput flter nductors and nput and output capactors. Therefore, N and P could properly be chosen accordng to gven output voltage and power level. Ths could gve flexblty n devce selecton resultng n optmzed desgn even under harsh desgn specfcatons. I. EXPERIMENTAL RESULT Usng the prevously desgned parameters, a 1.5kW softswtched CCM boost converter prototype of Fg. 1 has been bult n the laboratory. P o = 1.5kW = 80 o = 400 ΔI = 30% Δ o = 3% f s = 70 khz Both lower and upper swtches are mplemented wth IXYS IXFN48N50(500, 48A, 1m ohm) MOSFET. Fast 004
(a) recovery dodes of DAWIN Electroncs DWMF10N030(300, 3A) are used for dodes D 1 and D. Input flter nductor L 1 and auxlary nductor L are 50uH and 7uH, respectvely. The desgned values of the auxlary capactor C 1 s 50uF 00. An off-the-shelf electrolytc capactor of 0uF 400 was used for C 1 snce rms value of rpple current s 6A at 1.5kW. Electrolytc capactors of 0uF 400 was also used for output capactors C and C 3. The expermental waveforms of the proposed scheme are shown n Fg. 9. Fg. 9 (a) shows voltage and current waveforms of auxlary nductor L. Fg. 9 (c) and (d) llustrate that both swtch S 1 and swtch S are beng turned on wth ZS. Fg. 9 (d) and (e) show that dode D 1 and D are beng turned off wth ZCS. The measured effcency s shown n Fg. 10. The effcency of the proposed converter mantans over 90% n most of the load range. The maxmum effcency of 97.4 % was measured at 100W load. Fg. 11 shows the photograph of the prototype. (b) 100 98 96 94 9 90 88 86 (c) 84 8 100 300 500 700 900 1100 1300 1500 Fg. 10. Measured effcency of the proposed converter (d) (e) Fg. 9. Expermental waveforms (a) voltage and current waveforms of auxlary nductor L (b) voltage and current waveforms of lower swtch S 1 (c) voltage and current waveforms of upper swtch S (d) voltage and current waveforms of upper dode D 1 (e) voltage and current waveforms of lower dode D 1 Fg. 11. Photograph of 1.5kW soft-swtched CCM boost converter prototype 005
. CONCLUSION In ths paper a new soft-swtched CCM boost converter sutable for hgh voltage and hgh power applcaton has been proposed. The proposed converter has the followng advantages: ZS turn-on of the actve swtches n CCM Neglgble dode reverse recovery due to ZCS turn-off of the dodes oltage converson rato s almost doubled compared to the conventonal boost converter Greatly reduced components voltage ratngs & energy volumes of most passve components Extenson of the proposed concept to realze mult-phase DC-DC converters for hgher voltage and hgher power applcatons has been explored. Expermental waveforms from a 1.5kW prototype have been provded and peak effcency of 97.4% was measured at 100W. REFERENCES [1] C. Wang, M.H. Nehrr, H. Gao, Control of PEM fuel cell dstrbuted generaton systems, IEEE Trans. Energy Converson, ol. 1, No., pp. 586-595, June 006. [] K. Lu, R. Orugant, F.C. Lee, Resonant swtched-topologes and characterstcs, IEEE Trans. Power Electron. ol. PE-, pp. 6-74, Jan. 1987. [3]. orpe ran, Quas square-wave converters: Topologes and analyss, IEEE Trans. Power Electron. ol. 3, pp. 183-191, March 1998. [4] G. Hua, E.X. Yang, Y. Jang, F.C. Lee, Novel zero-current-transton PWM converters, IEEE Trans. Power Electron. ol. 9, pp. 601-606, Nov. 1994. [5] Q. Zhao, Peng Xu, F.C. Lee, A smple and effectve method to allevate the rectfer reverse-recovery problem n contnuous-currentmode boost converters, IEEE Trans. Power Electron. ol. 16, No. 5, pp. 649-658, Sep. 001. [6] Amr Ostad, Xng Gao, Gerry Moschopoulos, Crcut propertes od Zero-oltage-Transton PWM converters, Journal of power electroncs. ol. 8, No. 1, pp. 35-50, Jan. 008. [7] Byung duk Mn, Jong pl Lee, Eu ho Song, A novel grd-connected P PCS wth new hgh effcency converter, Journal of power electroncs. ol. 8, No. 4, pp. 309-316, Oct. 008. [8] Mchael S. Elmore, Input current rpple cancellaton n synchronzed, parallel connected crtcally contnuous boost converters, n Proc. IEEE APEC, pp. 15-158, 1996. [9] B. Eckardt, A. Hofmann, S. Zeltner, Automotve powertran DC/DC converter wth 5kW.dm3 by usng SC dodes, n Proc. ECPE CIPS, March 008. [10] T. Mzoguch, T. Ohga, T. Nnomya, A famly of sngle-swtch ZS- C DC DC converters, n Proc. IEEE APEC, ol., pp. 139-1398, 1994. [11] E. Ismal, A. Sebzal, A new class of quas-square wave resonant converters wth ZCS, n Proc. IEEE APEC, pp. 1381-1387, 1997. [1] G. Hua, C. Leu, F.C. Lee, Novel zero-voltage-transton PWM converters, n Proc. IEEE PESC, pp. 55-61, 199. 006