A Multiple-Input DC/DC Converter for Renewable Energy Systems

Huang-Jen Chiu, Member, IEEE, Hsiu-Ming Huang, Li-Wei Lin, Ming-Hsiang TsengDept. of Electrical Engineering, Chung-Yuan Christian Univ., Taiwan

Abstract This paper presents a multiple-input DC/DCconverter for renewable energy systems. The proposedDC/DC converter can be used to obtain well-regulatedoutput voltage from several power sources, such as windturbines, photovoltaic arrays, fuel cells, etc. The energyprovided by these power sources can be simultaneouslytransferred into the load. The proposed multiple-inputDC/DC converter has the advantages of simpleconfiguration, fewer components, lower cost and highefficiency. The operating principle, theoretical analysis,and design criteria are provided in this paper. Alaboratory prototype with two different power sourceswas successfully implemented and tested. Thesimulation and experimental results are given to verifythe feasibility of the proposed scheme.Keywords- Multiple-Input DC/DC Converter,Renewable Energy System, Current-Fed

I. IntroductionRecently, clean energy resources such as wind

turbines, photovoltaic arrays or fuel cells have beenexploited for developing renewable electric powergeneration systems [1-3]. The multiple-input DC/ DCconverter shown in Figure l(a), is useful for combiningseveral energy sources whose power capacity and/ orvoltage levels are different to obtain well-regulatedoutput voltage [4-7]. The input-stage circuits fordifferent energy sources can be put in parallel using acoupled transformer. Only one power source is allowedto transfer energy (El-EN) into the load at a time. Theseinput-stage circuits are designed in an "interleavingoperation mode" as shown in Fig. 1(b) to prevent powercoupling effects. The current ratings for theseinput-stage circuits are higher with more complicatedinterleaving control circuits. A multiple-input powersupply system based on the current-fed full-bridgeconverter, shown in Fig. 2(a), is considered a betterchoice for such applications [8-10]. By using aphase-shift PWM control scheme, the energy from thedifferent power sources can be simultaneouslytransferred into the load through a multi-windingtransformer as shown in Fig. 2(b). However, a largenumber of power switches and a complicated gatingdriver and controller are needed, thus requiring high costand a large size for this conventional converter. Asshown in Fug. 3, a novel multiple-input DC/DCconverter is presented for renewable energy systems.The proposed converter has the advantages of simpleconfiguration, fewer components, lower cost and highefficiency. The design and operating principles will bediscussed in detail in the following sections, IsSpicesimulations are given to confirm the theoretical analysis.A two-input laboratory prototype was implemented andtested to show the performance of the proposedconverter.

ElWind Turbie

E3Piotovoltaic AiTa F

Fuel Ce 3

ENColmercial AC Lhie

Mtle-Iiput A LoadDCDC Couererle

(a)

I tj

IE30E7t*-

14 SSwtchlLi Peijod. T P-_N(b)

Fig. 1 The Multiple-Input DC/DC Converter for Renewable EnergySystem

(a)

*ElI F -I |-E 2

I I I IIP*Ex4 ~ SwitciL Period, T 1

(b)Fig. 2 The Conventional Multiple-Input DC/DC Converter

JI

L _I

+ iJ EIrpt-Stage CDircuit 7* I

InpugStage Circit MtNFig. 3 The Proposed Multiple-Input DC/DC Converter

0-7803-9484-4/05/$20.00 ©2005 IEEE 1304

II.Operating Principle of the Proposed ConverterFor operating analysis convenience, a simplified

circuit diagram of the proposed DC/DC converter withtwo-input is shown and discussed in Fig. 4. It iscomposed of two current-fed input-stage circuits, acoupled transformer, and a secondary bridge-rectifier.Each input-stage circuit consists of two power switchesand two choke inductors. The duty ratio 61 for the powerswitches Ql and Q2 in the first input-stage circuit andthe duty ratio 62 for the power switches Q3 and Q4 in thesecond input-stage circuit are regulated independently bytwo separated control loops. Both 61 and 62 exceed 0.5 toimplement the overlapping operation. The proposedmultiple-input DC/DC converter is suitable forhigh-voltage output applications due to the current-fedcircuit topology that requires no output indictor.

I IL2

Fig. 4 The Circuit Diagram of the Proposed DC/DC Converter withTwo-Input

Based on the symbols and signal polaritiesintroduced in Fig. 4, the theoretical waveforms areshown in Figs. 5, where the overlapping-times +j and +2can be expressed as follows.

jI = (261 -1)Ts 2, (1)

+2 = (262 -])T/ 2, (2).i~~~~~~~~.

Fi.5 h ThoetclT Waeom fo_n wthnyl

TIn p in of the pr s Dr Q2

convrte cabheTexpainedWavfrs follOnSws. higCyl

Mode I (QI,Q2, Q3, and Q4 on): During this interval,all power switches Q1VQ4 are on. The inductor currents,ILi and IL2 flow back to the first voltage source, Vi

through QI and Q2. The inductor currents, IL3 and IL4flow back to the second voltage source, V2 through Q3and Q4. The surviving energy in the leakage inductanceof the second primary winding is also delivered to theload through the transformer. When the surviving energyis released completely, the transformer secondarywinding current will collapse to zero. All the rectifyingdiodes are off and the load power is provided alone bythe output capacitor CO.Mode II (Qi, Q2 and Q3 on; Q4 off): During thisinterval, the power switches Ql and Q2 are on. Both ofthe inductor currents, ILi and IL2 flow back to the firstvoltage source, VI through Ql and Q2. The fallingcurrent in Q4 occurs at zero voltage and there is nodissipation caused by Q4 turning off. The inductorcurrent, IL3 flows through Q3 back to the second voltagesource, V2. Only the inductor current, IL4 flows into thetransformer primary winding. The energy from thesecond power source is transferred to the load throughthe rectifying diodes DI and D4. The transformersecondary current, Is satisfies Equation (3).

IS N IL4' (3)

Mode III (Qi and Q3 on; Q2 and Q4 off): The fallingcurrent in Q2 occurs at zero voltage and there is nodissipation caused by Q2 turning off. The inductorcurrent, ILI flows through QI back to the first voltagesource, VI and the inductor current, IL3 flows throughQ3 back to the second voltage source, V2. The inductorcurrent, IL2 flows into the first transformer primarywinding and the inductor current, IL4 flows into thesecond transformer primary winding. Thus, thetransformer secondary current, Is satisfies Equation (4).The energy from the two power sources is transferred tothe load through the rectifying diodes DI and D4.

IS= N IL2 + N IL4 1 (4)Mode IV (Qi, Q2, Q3, and Q4 on): During thisinterval, all power switches Q1VQ4 are on. The inductorcurrents, IL, and IL2 flow back to the first voltage source,VI through QI and Q2. The inductor currents, IL3 and IL4flow back to the second voltage source, V2 through Q3and Q4. The rising currents in Q2 and Q4 occur at zerovoltage and there is no dissipation caused by Q2 and Q4turning on. The surviving energy in the leakageinductance of the second primary winding is alsodelivered to the load through the transformer. When thesurviving energy is released completely, the transformersecondary winding current will collapse to zero. All therectifying diodes are off and the load power is providedalone by the output capacitor CO.Mode V (Qi, Q2 and Q4 on; Q3 off): During thisinterval, the power switch Q3 is off. The falling currentin Q3 occurs at zero voltage and there is no dissipationcaused by Q3 turning off. The inductor current, IL3 flowsthrough the transformer primary winding. Both of theinductor currents, IL1 and'L2 still flow back to the firstvoltage source, VI through Qi and Q2. The inductorcurrent, IL4 flows through Q4 back to the second voltage

1305

source, V2. The energy transferred to the load throughthe rectifying diodes D2 and D3 is from the secondpower source. The transformer secondary current, Issatisfies Equation (5).

(5)N I3

Mode VI (Q2 and Q4 on; Qi and Q3 off): The fallingcurrent in QI occurs at zero voltage and there is nodissipation caused by Ql turning off. The inductorcurrent, IL2 flows through Q2 back to the first voltagesource, VI and the inductor current, IL4 flows throughQ4 back to the second voltage source, V2. The inductorcurrent, ILI flows through the first transformer primarywinding and the inductor current, IL3 flows through thesecond transformer primary winding. Thus, thetransformer secondary current, Is satisfies Equation (6).The energy from the two power sources is transferred tothe load through the rectifying diodes D2 and D3.

Npl Np2IS =-( LI + IL3)^ (6)

Ns NsThe circuit will then proceed back to Mode I after

completing one operating cycle Ts. The rising currents inQI and Q3 occur at zero voltage and there is nodissipation caused by Ql and Q3 turning on. Theleakage inductances of the first and second primarywindings release their surviving energy to the loadthrough the transformer. When the surviving energy isreleased completely, the transformer secondary windingcurrent will collapse to zero.

Mode I Mode II(Q1, Q2, Q3, and Q4 on) (Q1, Q2 and Q3 on; Q4 off)

k~~~~~~~jI L q | L X~~~~J- Q2 _N

,'E`7v _~_,LQS- P

Mode III Mode IV(QI and Q3 on; Q2 and Q4 off) (Q1, Q2, Q3, and Q4 on)

V~~+I2i Q20~~I1

L._^D2 AjjW AD4IIs~~~~~~~~~ IMode V Mode VI

(Q1, Q2 and Q4 on; Q3 off) (Q2 and Q4 on; QI and Q3 off)

\ + 1 < |~~iD In3 Q' XD1 In

|D i-q1, Q jI 11

Q ~ ~~ ~ ~ ~ ~ ~ ~ ~~ Q

frFig. 6 Equivalent Circuits under Different Switching Modes

III. Steady-State AnalysisAssuming the output capacitor CO is large enough so

that the voltage ripple on it is negligible. The voltagetransfer ratio VTR of the proposed multiple-inputDC/DC converter can be derived from the volt-secondbalance condition across the choke inductors andexpressed by Equations (7) and (8).

VTR VO = NS 1 (7)V1 Npl 1-81

VO Ns 1VTR2 = S

V2 Np2 1-52(8)

where 61 is the duty ratio of the power switches Ql, Q2in the first input-circuit and 62 is the duty ratio of thepower switches Q3, Q4 in the second input-circuit. Thevoltage and current stresses of the power switches can befound as follows:

N-P VIVQJ,Q2 N .O

_2 V2VQ3,Q4 NV 1-8

IQi,Q2 =ILI + IL2 =Ii

IQ3,Q4 = IL3 + IL4 = I2

(9)

(10)

(11)(12)

From Equations (11) and (13), it is obvious that thepower switches subject only one source current and nottwice as do the conventional two-switch circuittopologies such as push-pull converter or half-bridgeconverter. For the low-voltage energy source such asfuel cells, this is a very important merit. As discussedabove, all switches have negligible dissipation as theyhave the transformer leakage inductance in series andhence turn on at zero voltage. Thezero-voltage-switching condition of the power switchescan be derived as follows.

Ll 2 2Votr(max) NP1 (13)

I1 NS2VOtr(max) Np2

t12- AT(14)

12 IVswhere LI, and L12 are respectively the leakageinductances of the first and the second primary windingsand t, is the maximum rise time of the switch current.

The inductances of LI-L4 can be determined by therequirements of the inductor current ripples as follows:

LI = L2 2 26V217A,L 0/0)IIfs

(15)

L3 = L4 2 21V2 (16)AIL (0/0)I2fs

where fs is the switching frequency of the powerswitches, AIL(Jo%) is the percentage ripple current on thechoke inductors Lr-L4. The equivalent circuit of theinput-stage circuit is shown in Fig. 7(a). Vs and Isrepresent respectively the source voltage (V1, V2) andsource current (I1, 12). IL,i is the upper inductor current(IL2, IL4) and ILj is the lower inductor current (IL1, 1L3)

1306

Assuming the transformer turn ratio is n, the voltages Vi'and Vj' may be 0 or (VJ/n) for different switching modes.From the theoretical waveforms shown in Fig. 7(b), theripple cancellation on source current can be observed.This produces relatively ripple-free source current that isdesirable for the low-voltage renewable energy sourcessuch as fuel cells, photovoltaic arrays etc. From theenergy conservation relationship, the output power PO isthe summation of the two source powers P1 and P2. Thus,the following Equations must be satisfied.VoIo = VjII + V2I2, (17)

(18)

A voltage feedback loop is necessary to regulate theoutput voltage and two current control loops are alsoneeded to control the source current and power by eachpower source.

(a) (b)Fig. 7(a) The Equivalent Circuit and (b) the Theoretical Waveforms ofthe Input-Stage Circuit

IV.Design Example of the Proposed ConverterTo verify the feasibility of the proposed scheme, a

laboratory prototype with following specifications wasdesigned and tested. The design considerations for thekey components will be discussed in this section.* Source voltage: V1=12VDC, V2=24VDC* Source current: I1=4A, I2=6A* Output voltage: VO=192VDC* Output current: Io=1A* Switching frequency: fs=IOOkHzA. Power Switches

The voltage and current ratings for the powerswitches can be calculated using Equations (9) to (12) asfollows:

VQQ2 =-1VI 12= 30V,VQQ -1-1 1-0.6

V2 24VQ3,Q4 - - ~ 60V,

1- '2 1- 0.6

IQJ,Q2 = I-= 4A,

(19)

(20)

(21)

IQ3,Q4 = I2 = 6A, (22)B. Transformer Design

From the output voltage specification, thetransformer turn ratios can be determined usingEquations (7) and (8).

(18)V 192n _=0s 1 )-='(1-0.6) -6.4, (23)

Npl ~VI 12

Ns V 192n2 =N = 1- 2) 0= (1 - 0. 6) 2 3.2, (24)

Np2 ~ V2 24

Equations (13) and (14) must be satisfied suchthat the zero-voltage-switching of the power switches

can be achieved. Thus, the leakage inductances of thetransformer first and second primary windings can bedetermined as follows.

L 21. 0r(max) Np13l H (5Lll 2 ° lg = 3plH, (25)I ] N s

L 2Votr(max) Np2 4pH (26)12 '-2NI2s

C. Choke InductorsLet the peak-to-peak current ripples be 30 percent of

the inductor currents under full power. The inductance ofthe choke inductors LI-L4 can be determined usingEquations (15) and (16) as follows.

L = L2 . 2 IV7 2x0.6x12 1.2mH (27)IL (lo)IIfs 0.3 x 4 x100k

L3 L4 231V2 2x0.6x24 -1.6mH,(28)AIL(O)I2fs 0.3x6x100k

Because of the ripple cancellation on the sourcecurrent, a larger ripple current in choke inductors can beallowed in practical applications. Thus, the inductanceand the size of these choke inductors might be smaller.D. Output Capacitor

The output capacitor must be large enough toprovide the load power during Mode III and Mode VIwithout the voltage across it decreasing too much. Theoutput capacitance can be then determined usingEquation (27).

CO 2 o,max x Ts l xlO,u 52uF, (29)IAVO O.OOlx 192

Select C0=200&F.E. Controller Design

A block diagram of the control circuit for theproposed multiple-input DC/DC converter is shown inFig. 8. VO is the output voltage, Vref is the referencevoltage, 11ref and 12 ref are the reference currents for thetwo source currents, h1 and I2. 'e1 and Ie2 are the currenterror signals generated by the corresponding currenterror amplifiers (CEAI and CEA2). V, is the voltageerror signal generated by the voltage error-amplifiers,VEA. The overlapping-time control signal f,, is thesummation of V, and 1e1, where kc2 is the summation ofVe and Ie2. The overlapping-times +, and +2 for thepower switches in each input-stage circuit can becontrolled by the control signals f,, and ,c2. Byadjusting the overlapping-times +, and k2, the outputvoltage regulation and proper power sharing betweendifferent energy sources can be achieved.

Gating Signals Gatfg Sigalsfor Q1 ad Q2 for Q3 d Q4

I FI Gate Driv-

O1eppi,g Si || hpp Signl1|Ge-lert or Cicuit |Geeat .rCircit

CEAI Zi2 ZBAZ2 CEA2 Zi4

I r ;fZile i I

Fig. 8 Block Diagram of the Control Circuit for the ProposedMultiple-Input DC/DC Converter

1307

Npl Np2I, = (I -51 ) I, +(I 152) I21Ns Ns

V. Simulation and Experimental Verification

To verify the theoretical operating principles, a

design example was simulated using IsSpice. Figure 9

shows the simulated results for the proposed

multiple-input DC/DC converter. There is a good

agreement between the simulation results and theoretical

analysis. In this research, a laboratory prototype was

implemented and tested to evaluate the performance of

the proposed DC/DC converter. Figures 10(a) and 10(b)show the measured waveforms for the laboratory

prototype. Figure 10(a) shows the measured waveforms

for the first and second inductor currents, ILi and'L2. The

ripple cancellation can be observed and it is desirable for

low-voltage power sources. In Figure 10(b), the

zero-voltage-switching details of the power switches are

shown. Figure shows the efficiency comparisons with

a conventional converter presented in [8] under the load

variations. Because of the device count reduction, higher

efficiency can be achieved using the proposed scheme in

this paper.

QIQ2

Q3Q4

-~~~~~~~~~~~~~I -

Fig. 9 The Essential Simulated Waveforms in the Proposed

Multiple-Input DC/DC Converter

LIl(2AIdiv 2.5uS/div) IL2 (2A/div 2.5uS/div)

________________(a)__________

Vdl2v/div 400OnS/div)

(b)

Fig. 10 The Measured Waveforms for the Laboratory Prototype

Efficiency. %7

Fig. 11I

40 60 RO u0 E2' }4 160 1W0 200

Load Powver. W

The Efficiency Comparisons with a Conventional Converter

Presented in [8]

VI. Conclusion

A multiple-input DC/DC converter for renewable

energy systems was proposed in this paper. This novel

type of DC/DC converter has advantages such as simple

configuration, fewer components, lower cost and high

efficiency. The operating principles and designconsiderations were analyzed and described in detail.

Simulation results from the proposed circuit were givento verify the theoretical analysis. A laboratory prototype

was implemented and tested to show its performance.The experimental results were satisfactory.

AcknowledgmentThe authors would like to acknowledge the financial

support of the National Science Council of Taiwan, R. 0.

C. through grant number NSC93-2213-E033-009.

References

[1] M. Park, M. Michihira, and K. Matsuura, "A Novel MPPT

Control Method using Optimal Voltage of PV with SecondaryPhase-Shift PWM Control DC-AC Converter," International

Conference on Electric Power Engineering, Aug. 1999, pp.

269.

[2] F. Z. Peng, H. Li, G. J. Su, and J. S. Lawler, "A New ZVS

Bidirectional DC-DC Converter for Fuel Cell and Battery

Application," IEEE Transactions on Power Electronics, Vol.

19, Jan 2004, pp. 54-65.

[3] K. Wang, C. Y Lin, L. Zhu, D. Qu, F. C. Lee, and J. S. Lai,

"Bi-directional DC to DC Converters for Fuel Cell Systems,"

PET'98, pp. 47-5 1.

[4] G. J. Su, F. Z. Peng, and D. J. Adams, "ExperimentalEvaluation of a Soft-Switching DC-DC Converter for Fuel

Cell Applications," PET'02, pp. 39-44.

[5] H. Matsuo, Wenzhong Lin; F. Kurokawa, T. Shigemizu, and N.

Watanabe, "Characteristics of the Multiple-Input DC-DC

Converter," IEEE Transactions on Industrial Electronics, Vol.

51, June 2004, pp. 625-63 1.

[6] B. G. Dobbs, and P. L. Chapman, "A Multiple-Input DC-DC

Converter Topology" IEEE Power Electronics Letters, Vol.

1, March 2003, pp. 6-9.

[7] H. Matsuo, K. Kobayashi, Y Sekine, M. Asano, and Lin

Wenzhong, "Novel Solar Cell Power Supply System using the

Multiple-Input DC-DC Converter," IEEE INTELEC, Oct.

1998, pp. 797-802.

[8] Y M. Chen, Y C. Liu, and F. Y Wu, "Multi-Input DC/DC

Converter Based on the Multiwinding Transformer for

Renewable Energy Applications," IEEE Transactions on

Industry Applications, Vol. 38, July-Aug. 2002, pp.

1096-1104.

[9] Y M. Chen, Y C. Liu, and F. Y Wu, "Multi-Input DC/DC

Converter with Ripple-Free Input Currents," IEEE PESC, Vol.

2, June 2002, pp. 796-802.

[10] Y M. Chen, Y C. Liu, F. Y Wu, and T. F. Wu, "Multi-InputDC/DC Converter Based on the Flux Additivity" IEEE IAS,Vol. 3, Oct. 2001, pp. 1866-1873.

1308

-4- Con vei tiolial

---Proposedt

VgslI(l OV/div 400OnS/d)

I

iI...

r...L. L.j