zvs fuel cel

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AbstractI n this paper a new bidirectional dc-dc converterfor a fuel cell system is proposed that has a current triplerrectifier in its low voltage side. The converter is based on a ZVShalf bridge topology with current tripler rectifier at thesecondary side of transformer. The asymmetrical PWM is usedin the converter to achieve ZVS of power switches and toregulate the output voltage at the desired value. The proposedconverter has the advantages of high efficiency, high powerdensity and simple circuit configuration. The operationprinciple and system analysis are described and discussed.Finally, the simulation results for the proposed converter areprovided to verify the theoretical analysis.

Index TermsBidirectional dc-dc converter, current triplerrectification, fuel cell, zero voltage switching.

I. INTRODUCTIONFuel cell is one of the very promising renewable energy

sources for power generation in distributed generationtechnologies. It also can be used in high power and lowpower applications such as electric vehicle, communicationsystems. The main disadvantage of fuel cell is slow dynamic

response due to the natural electrochemical reactions andhence requires Power Conditioning System (PCS) tointerface the load. PCS plays an important role to deliver thepower from the fuel cell to various loads. All PCS should bedesigned and operated with high efficiency, highperformance and high reliability. There are four types of PCSconfigurations for fuel cell systems that are proposed in [1].The best one type of these configurations is ones that have abidirectional dc-dc converter between the low voltage batteryand high voltage bus. High voltage bus may be directlyconnected to the fuel cell or may be connected to it via anisolated high frequency dc-dc converter. But the important

thing in all case is that we need a bidirectional dc-dcconverter. Among the proposed topologies forbidirectional dc-dc converter, full bridge and half bridgeconverters are good candidate for medium and high powerapplications. In some paper bidirectional dc-dc converterwith full bridge topology in both sides is presented [2]-[4].The advantages of these converters are symmetricconfiguration and ZVS due to PWM and they have no extrareactive components. But in such converters the outputvoltage is proportional to load however they can transmitmore power in comparison with half bridge topology. Insome literatures converters are consist of half bridge in both

Manuscript received July 20, 2012; revised August 21, 2012.A. Salami is with the Arak University of Technology, Iran (e-mail:

[email protected], [email protected], [email protected]).

side [5]-[8]. These converters can be controlled easily andthey have not neglecting current that is major problem in dualfull bridge dc-dc converters. Another bidirectional converterhave half bridge or full bridge in high voltage side and centertapped rectifier in low voltage side [9]-[11].Theseconfigurations have advantages of simple circuit and easyimplementation but they have a low current output duehaving only one self in their output. A dc-dc converter with anovel current tripler rectifier is presented in [12]. But, onlypower flow in one direction is inspected. In this paper a new

bidirectional dc-dc converter based on a half bridge converterwith asymmetrical PWM scheme in high voltage and currenttripler rectification in low voltage is presented to achievebidirectional power flow. The proposed converter hasadvantage of high current output capability with smallerreactive components in comparison with converters that useother rectifier topologies in their low voltage side. Theasymmetrical PWM scheme can help the power switches athigh voltage side to turn on at ZVS. To safety requirement,the isolated transformer is used between the low voltagebattery and high voltage bus to achieve voltage step up orstep down. The operation principle and system analysis of

proposed bidirectional converter are presented herein. Inorder to demonstrate the effectiveness based on thecharacteristic performances, simulations are carried out indifferent positions.

II.CIRCUIT ANALYSISIn order to combine advantages of half bridge with

ZVS-PWM control and current tripler rectifier that hashigher power flow than other rectifier topologies, a novelbidirectional dc-dc converter is proposed.

Fig. 1 shows the circuit configuration of the proposed

isolated bidirectional DC-DC converter. The half bridgeconverter is adapted in high voltage side and a current triplerrectifier is used in low voltage side for increasing power flowof proposed converter. When power flow from high voltageside to low voltage side, the converter operates in buck modefor charging battery from fuel cell. When power flows fromlow voltage side battery to high voltage side, the converteroperates in boost mode for draw the energy from battery. S1and S2in half bridge side and SR1and SR2in battery sideoperate complementary each other.

A. Buck Mode OperationWhen power flows from high voltage side to low voltage

side, the converter operates in buck mode for chargingbattery from fuel cell. In the proposed converter Lm is thetransformer magnetizing inductance. Lr is the resonantinductance for ZVS.

A Novel ZVS Bidirectional DC-DC Converter forFuel Cell Applications

A. Salami and E. Rezaei

DOI: 10.7763/IJIEE.2013.V3.273

International Journal of Information and Electronics Engineering, Vol. 3, No. 1, January 2013

91


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Fig. 1. Topology of proposed converter.

Cr1andCr2are the output capacitance of S1and S2. Also Cr1,Cr2andLrare resonant to achieve ZVS turn on ofS1andS2. Inthe secondary sideSR1and SR2are synchronous switches toachieve full wave rectification. The output inductors are used

for reduce the output current ripple. The turn on conductionloss in secondary side is very low due to use synchronousswitches. Before the system analysis, we should make someassumption.

-Clamp capacitanceC is large enough so that the capacitorvoltageVc is a constant value and it is much larger than the

output capacitance of two power switches (C>>Cr1,Cr2).-Leakage inductance Lr is smaller than the magnetizing

inductanceLm.There are eight operating mode during one switching cycle

according to the conduction state of all switches. SwitchS1,synchronous switchSR2and diodeD4are conducting before

starting time.

B.Boost Mode OperationThe proposed converter can draw power from battery

voltageVbat, when the high voltageVbusis lower than desiredvalue, tripler synchronous rectifier is operated in anasymmetrical PWM with an overlapping to regulate the highvoltage Vbus. During one switching cycle, there are eightoperating modes in converter. Switch SR1, SR2 andsynchronous switch S1are conducting before starting time.

III. SIMULATION RESULTSThe time intervals in some modes are very short in the

proposed converter at buck and boost operations. Thus, onlythose modes have long times are used to drive the voltagetransfer ratio of the proposed converter. In mode 1, S1andSR2 are in on state and the voltage across Lr and Lm isVbus/2-Vc.

In mode 5, S2and SR1are in on state and the voltage acrossLr and Lmis Vbus/2-Vc. Based on the voltage second balanceacrossLrand Lm, in mode 1 and 5, one can obtain the clampcapacitor voltage.

( ) .50.0 busc VDV = (1)

where D is the duty ratio of the switch S1. We can obtain theoutput voltage equation.

( ).

12

n

DDVbat

= (2)

Based on the voltage second balance acrossL3, in modes 1

and 5, we can obtain the voltage transfer ratio in buckoperation.

( ).

12

n

DDAv

= (3)

The voltage transfer ratio shows that D=0.5 is the optimalvalue for duty cycle. When the input voltage is minimumVbus, the duty cycle D is close to maximum value (D=0.5), todeliver power from high voltage side to low voltage side. Inthis case, the higher current will flow through power switchS1and S2. To maintain the clamp voltage VC, the averagecurrent flows through capacitor C must be zero. If the

allowed voltage ripple in clamp capacitorcV is given, the

required capacitance of the clamp capacitor C can bedetermined.

( ).

12

nV

TDDIC

c

L

= (4)

If we consider that, the current ripple of each outputinductor is constant; we can determine all of output inductors.

( ).

1

2,1

21

L

bat

i

TVDLL

== (5)

( ).

21

3

3

L

bat

i

TVDL

= (6)

To simulation, the power of converter is selected 420 wattsand is operated in 100 kHz. The nominal voltage at lowvoltage side is 42v that may be varies between 30 to 45 volts.The nominal voltage at high voltage side is 350v that may bevaries between 250 to 400 volts. If we select the duty cycleD=0.5, then the circuit will be symmetric but we shouldselect the duty cycle between 0.3 and 0.48 for desired voltage.

Based on these duty cycles we can calculate transformerratio:

( ) .66.612min,

max,

maxmax ==

bat

bus

V

VDDn


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Since we consider the power of converter 420w, the outputcurrent, equals 10A and consequently if the allowed ripplevoltage on capacitor C is 10V, then the capacitance of this

capacitor can be determined:( )

.75.012 maxmax F

nV

TDDIC

c

L=

=

If the allowed current on inductor L3 is considered %25,we can determine it:

( ).36

2

21

3

max,min

3 Hi

TVDL

L

bat=

=

If we consider inductorsL1 andL2 to be equal L3, then theripple current of these inductor will be %75. The selectedpower MOSFETs for simulation are IRF460 in half bridgeside that can withstand voltage up to 500V and current up to20A and IRF260 as power MOSFETs switch at low voltage

side that can withstand voltage up to 250V and current up to38A. To ensure ZVS, selecting of Cr and Lr are veryimportant so that resonant frequency will be bigger thanswitching frequency and Resonant amplitude will be enoughbig. For this we can useLr=8H and Cr=650 PF.

The key waveforms of this converter are shown in Fig. 2and Fig. 3 for both modes buck and boost operationrespectively. In the proposed converter, the measuredefficiency is about 88% for 420 watt in buck mode operationand this efficiency is about 91% in boost mode operation.From Fig. 4 we can see that the efficiency of proposedconverter is higher than efficiency of the converter with push

pull rectifier for both buck and boost operations, becauseoutput inductors in the proposed converter in comparisonwith one output inductor in the converter using current centerrectifier in its low voltage side ensure more current at outputso we can get more power at output, however these threeinductors are smaller than one inductor in converter withpush pull rectifier. Consequently, the proposed converter hasmore power output than converter with push pull rectifier inbuck and boost operations. We can discover the advantagesof proposed converter in compare with converter that pushpull rectifier are used at its secondary side of transformer.

These advantages are consisting of same voltage output

with smaller inductor and more power output in samecondition.

1.94 1.95 1.96 1.97 1.98 1.99 2

x 10-3

35.5545

35.555

35.5555

Vo(volt)

1.94 1.95 1.96 1.97 1.98 1.99 2

x 10-3

-5

0

5

iL1(am

p)

1.94 1.95 1.96 1.97 1.98 1.99 2-3

1

1.2

1.4

iL3(am

p)

Fig. 2. Waveforms of proposed converter in buck operation.

1.94 1.95 1.96 1.97 1.98 1.99 2

x 10-3

425.5

426

426.5l

1.94 1.95 1.96 1.97 1.98 1.99 2

x 10-3

0

200

400

Vc1(volt)

1.94 1.95 1.96 1.97 1.98 1.99 2

x 10-3

0

200

400

Vc2(volt)

1.94 1.95 1.96 1.97 1.98 1.99 2

x 10-3

-20

0

20

iLr(am

p)

Fig. 3. Waveforms of proposed converter in boost operation

100 200 300 400 500 600 700 800 900 100040

60

80

100

Load Power (w)

Efficiency(%)

(a)

100 200 300 400 500 600 700 800 900 100050

60

70

80

90

100

Load P owerr(w)

Effici

ency(%

)

(b)

:converter with current tripll rectifier

:converter with push pul rectifier

:converter with current tripl rectifier

:converter with push pull rectifier

Fig. 4. Measured converter efficiency with different load conditions,

(a) Buck operation and (b) boost operation.

IV. CONCLUSION

The operating principle and mathematical analysis of abidirectional DC-DC converter with tripler synchronousrectifier were presented in this paper and we saw that currenttripler rectifier can be used as converter in buck and boostmode operations. The asymmetrical pulse width modulationis used in the circuit to regulate the output voltage. With theaid of capacitance of switch at high voltage side and withleakage inductance, the power switches at high voltage sideare operated at ZVS turn on. A simulation example at 420watt in this paper is provided and we saw, this converterwhich using tripler current rectifier at its low voltage side incomparison with converters that use another rectifiers such aspush pull, has an extra power density and consequently withthis topology we can have more power density and moreefficiency.

REFERENCES

[1] D. K. Choi, B. K. Lee, S. W. Choi, C. Y . Won, and D. W. Yoo, Anovel power conversion circuit for cost-effective battery-fuel cell


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hybrid systems, J ournal of Power Sources, vol. 152, 2005, pp.245255.

[2] C. Mi1, H. Bai1, C. Wang, and S. Gargies, Design and control of dualH-bridge-based isolated bidirectional DCDC converter, IET PowerElectron., 2008, vol. 1, no. 4. pp. 507517.

[3] S. Han, Dual Active Bridge Buck-Boost Converter, IEEETransactions on Power Electronics, July 2009, vol. 24, no. 7,pp.18261838.

[4] L. Zhu, A Novel Soft-Commutating Isolated Boost Full-BridgeZVS-PWM DCDC Converter for Bidirectional High Power

Applications, IEEE Transactions on Power Electronics,March 2006,vol. 21, no. 2, pp.422429.

[5] M. Gang, Q. Wenlong, and L. Yuanyuan, A Novel Soft SwitchingBidirectional DC/DC Converter and Design Consideration, PowerElectronics and Applications, 2009. EPE '09. 13th EuropeanConference on Issue Date, pp. 8-10, Sept. 2009, pp.110.

[1] Z. Zhang and C. Ole, A Novel PPFHB Bidirectional DC-DCConverter for Supercapacitor Application, Clean Electrical Power,2009 International Conference on Issue Date, pp. 9-11, June 2009, pp.350354.

[6] F. Z. Peng, H. Li, G. J . Su, and J. S. Lawler, 'A New ZVSBidirectional DCDC Converter for Fuel Cell and BatteryApplication, IEEE Transactions on Power Electronics, january2004 ,vol.19, no. 1, pp.5465.

[7] T. M. E. Hiraki, T. Tanaka, and M. Nakaoka, A New Soft-SwitchedBidirectional DC-DC Converter Topology for Automotive HighVoltage DC Bus Architectures, Vehicle Power and PropulsionConference, 2006. VPPC '06. IEEE, 2006, pp.16.

[8] F. G. Capponi, P. Santoro, and E. Crescenzi, A Bidirectional DC/DCConverter For Optimal Power Flow Regulation in SupercapacitorApplications, Industry Applications Conference, 2007. 42nd IASAnnual Meeting. Conference Record of the 2007 IEEE, pp. 2009-2015.

[9] S. Pattnaik, A. K. Panda, K. K. Mahapatra, S. Pattnaik, A. K. Panda,and K. K. Mahapatra, A Novel Improved Soft Switching PWM

DC-DC Converter, India Conference, 2008. INDICON 2008. AnnualIEEEIssue Date, pp. 11-13 Dec. 2008, vol. 1, pp. 9297.

[10] B. R. L in, C. L. Huang, and Y. E. Lee, Asymmetrical pulse-widthmodulation bidirectional DCDC converter, IET Power Electron.,2008, vol. 1, no. 3. pp. 336347.

[11] L. Yao, H. Mao, and I. E. Batarseh, A Rectification Topology for highcurrent isolated DC-DC Converter, IEEE Transactions on PowerElectronics, J uly 2007, vol. 22, no. 4, pp.15221530.

A. Salami was born in Islamic Republic of Iran on 1974. He received the B.S.degree from Isfahan University Technology, M.S., and Ph.D. degree fromIran University of Science and Technology in electrical engineering in 1996,1999 and 2005, respectively. He has been working as an Assistant Professorat the Department of electrical Engineering, Arak University Technology. Hehas published more than 20 paper on power system modeling and simulation,artificial methods and planning issues.


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zvs fuel cel

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