1538 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2006

Double-Input PWM DC/DC Converter forHigh-/Low-Voltage Sources

Yaow-Ming Chen, Senior Member, IEEE, Yuan-Chuan Liu, and Sheng-Hsien Lin

Abstract—A novel double-input pulsewidth-modulation (PWM)dc/dc converter for high-/low-voltage sources is proposed in thispaper. With a PWM control scheme, the proposed double-inputdc/dc converter can draw power from two different voltage sourcessimultaneously or individually. The operation modes and thesteady-state analysis of the proposed double-input dc/dc converterare introduced in detail. The PWM control scheme for the powerflow balancing is also presented. By using a single passive losslesssoft-switching cell, switching losses of all power switches canbe reduced significantly. Finally, experimental measurements aredemonstrated to verify the performance of the proposed converter.

Index Terms—Double-input converter, pulsewidth-modulation(PWM) control.

I. INTRODUCTION

THE RENEWABLE energy such as photovoltaic (PV) en-ergy and wind energy has created various electric energy

sources with different electrical characteristics for the modernpower system [1]–[6]. In order to gain higher energy conversionefficiency, the solar cell array for PV energy prefers a parallel-connected structure to form a low-voltage source, while thewind turbine for wind energy is designed to have a high outputvoltage with less armature current losses [7]–[9]. Usually, thesolar cell array and the wind turbine in the power system can beconsidered as a low-voltage source and a high one, respectively.

Traditionally, two dc voltage sources are connected to twoindependent dc/dc power converters to obtain two stable andequivalent output voltages, which are then connected to thedc bus, to provide the electric energy demanded by the load.Fig. 1 shows the block diagram of the two voltage-sourcepower system with two individual dc/dc converters. In order tosimplify the power system and reduce the cost, a double-inputdc/dc converter, as shown in Fig. 2, can be used. As shownin Fig. 2, two dc voltage sources can be connected either inparallel or in series to form an input voltage source for the dc/dcconverter to transfer the desired power to the load.

By using a coupled transformer, two dc sources can be put inparallel to implement the double-input pulsewidth-modulation(PWM) dc/dc converter, and the regulated output voltage can

Manuscript received April 27, 2004; revised April 5, 2006. Abstract pub-lished on the Internet July 14, 2006. This paper was presented at IEEEINTELEC’03, Yokohama, Japan, October 19–23, 2003.

Y.-M. Chen and Y.-C. Liu are with the Elegant Power Application ResearchCenter, Department of Electrical Engineering, National Chung Cheng Univer-sity, Chia Yi 621, Taiwan, R.O.C. (e-mail: ieeymc@ccu.edu.tw).

S.-H. Lin was with the Elegant Power Application Research Center, Depart-ment of Electrical Engineering, National Chung Cheng University, Chia Yi 621,Taiwan, R.O.C. He is now with Cyntec Company, Hsinchu 300, Taiwan, R.O.C.

Digital Object Identifier 10.1109/TIE.2006.882001

Fig. 1. Power system with two individual dc/dc converters.

Fig. 2. Power system with one double-input dc/dc converter.

be achieved [10], [11]. Because of the voltage amplitude dif-ferences between two dc sources, only one of them can beconnected to the input terminal of the dc/dc converter at atime. The control scheme is based on the time-sharing conceptbecause of the clamped voltage on the winding of the coupledtransformer. Consequently, power from difference dc sourcescannot be transferred to the load simultaneously. In order toconquer the limitation of the time-sharing control concept, theauthors have proposed a multi-input dc/dc converter based onthe multiwinding transformer with phase-shifted PWM controlwhich can successfully transfer power from two different volt-age sources to the load individually or simultaneously [12].However, high component counts and the relatively compli-cated circuit structure are the main drawbacks of the previouslyproposed multi-input converter.

Another approach for the double-input dc/dc converter is toput two dc sources in series to form a single voltage sourcewhere traditional dc/dc power converters can be used to transfer

0278-0046/$20.00 © 2006 IEEE

CHEN et al.: DOUBLE-INPUT PWM DC/DC CONVERTER FOR HIGH-/LOW-VOLTAGE SOURCES 1539

Fig. 3. Proposed double-input PWM dc/dc converter.

power to the load [13]. In order to transfer power individually,each dc voltage source needs a controllable switch to provide abypass short circuit for the input current of the other dc voltagesource to deliver electric energy continuously. Also, if one ofthe dc sources is diminished, it will be very difficult to obtainthe regulated voltage output since the input voltage variation issignificant.

The objective of this paper is to propose an innova-tive double-input PWM dc/dc converter for high-/low-voltagesources. The proposed circuit topology has the following ad-vantages: The dc sources can deliver power to the load indi-vidually or simultaneously; the multiwinding transformer is notneeded; the extra bypass short circuit is not required; the soft-switching technology is accessible; and the magnitude of theinput dc voltage can be higher or lower than the one with aregulated output. The proposed double-input dc/dc converter isgood for renewable-energy applications.

II. OPERATION PRINCIPLE OF THE PROPOSED CONVERTER

The schematic circuit diagram of the proposed double-inputPWM dc/dc converter for high-/low-voltage sources is shown inFig. 3. It is consist of two input voltage sources VHI and VLO,and an output voltage VO, where VHI > VO > VLO. Powerswitches SHI and SLO are connected to the high-voltage sourceVHI and the low-voltage source VLO, respectively. When thepower switches are turned OFF, power diodes DHI and DLO

will provide the bypass path for the inductor current to flowcontinuously. By applying the PWM control scheme to thepower switches SHI and SLO, the proposed double-input dc/dcconverter can draw power from two voltage sources individu-ally or simultaneously.

According to the status of the power switches, there are fourdifferent operation modes which can be explained as follows.

Mode I (SHI: ON/SLO: OFF): The equivalent circuit ofmode I is shown in Fig. 4(a). In mode I, the power switch SHI

is turned ON and SLO is turned OFF. Because of the conductionof SHI, power diode DHI is reverse biased and can be treatedas an open circuit. On the other hand, power switch SLO forthe low-voltage source VLO is turned OFF and the power diodeDLO will provide a bypass path for inductor current iL. In thismode, the high-voltage source will charge the energy storage

components, inductor L, and capacitor C, as well as providethe electric energy for the load.

Mode II (SHI: OFF/SLO: ON): Fig. 4(b) shows the equivalentcircuit for mode II. In mode II, the power switch SHI is turnedOFF, and SLO is turned ON. Also, the power diode DHI isforward biased as a short circuit, and DLO is reverse biased asan open circuit. In this operation mode, the low-voltage sourceVLO will charge the inductor L, while the demanded load isprovided by the output capacitor C.

Mode III (SHI: OFF/SLO: OFF): The equivalent circuit formode III is shown in Fig. 4(c). In this mode, both of the powerswitches SHI and SLO are turned OFF. Power diodes DHI andDLO will provide the current path for the inductor current. Bothof the voltage sources VHI and VLO are disconnected from theproposed double-input converter. The electric energy stored inL and C will be released into the load.

Mode IV (SHI: ON/SLO: ON): The equivalent circuit formode IV is shown in Fig. 4(d). In mode IV, both of SHI and SLO

are turned ON, and DHI and DLO are turned OFF with reverse-biased voltages. Two input voltage sources VHI and VLO areconnected in series to charge the inductor L. The demandedpower for the load is now provided by the capacitor C. Inthis operation mode, both of the high-/low-voltage sources willtransfer electric energy into the proposed double-input dc/dcconverter, simultaneously.

For the same switching frequency, SHI and SLO can besynchronized by the same turn-on transition with differentturn-off moment, or the same turn-off transition with differ-ent turn-on moment. Although either way can achieve thesynchronization of the switching control, only the latter onewith turn-off synchronization will be introduced in this paperfor further explanations. Fig. 5 shows the typical voltage andcurrent waveforms for the key components of the proposeddouble-input dc/dc converter under the condition of turn-offsynchronization. From the top to the bottom, there are wave-forms for gate signals νGSHI and νGSLO, inductor voltage νL,inductor current iL, high-voltage-source input current iHI, low-voltage-source input current iLO, unfiltered output current iO

′,and capacitor current iC . It can be seen from Fig. 5 that powerswitches SHI and SLO have different conducting time. In thisexample, the duty ratio for SHI is larger than the one for SLO.The voltage waveform across the inductor has three differentlevels which are determined by the ON–OFF status of the mainpower switches SHI and SLO. However, it still satisfies thevolt–second balance theorem. The inductor-current waveformhas three different current slopes based on the three differentinductor voltage values. The input currents reveal that two inputvoltage sources can provide electric energy for the proposeddouble-input dc/dc converter individually or simultaneously.The capacitor current will compensate the unfiltered outputcurrent; then, a stable dc load current can be obtained.

Under normal operation situation, the proposed double-inputdc/dc converter can transfer power from the two input voltagesources to the load simultaneously. If one of the voltage sourcesis disconnected, the other one can continue to deliver powerto the load normally. Hence, it can enhance the reliabilityof the power system. Fig. 6(a) shows the equivalent circuitof the proposed double-input dc/dc converter when only the

1540 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2006

Fig. 4. Different operation modes of the proposed double-input dc/dc converter. (a) Mode I. (b) Mode II. (c) Mode III. (d) Mode IV.

Fig. 5. Typical voltage and current waveforms for the key components of theproposed double-input PWM dc/dc converter.

low-voltage source is available, while Fig. 6(b) shows the onewhen only the high-voltage source is available. By removingthe open-circuit branches and replacing the forward-biased

Fig. 6. Equivalent circuits of the proposed double-input dc/dc converter whenonly one voltage source is available. (a) Only low-voltage source is available.(b) Only high-voltage source is available.

power diodes with short circuits, Fig. 6(a) and (b) can berecognized as the traditional buck–boost and buck converters,respectively.

CHEN et al.: DOUBLE-INPUT PWM DC/DC CONVERTER FOR HIGH-/LOW-VOLTAGE SOURCES 1541

TABLE ITHREE DIFFERENT POWER STATUS COMBINATION CASES

III. POWER MANAGEMENT

The input-power distribution of the two input voltage sourcesand the input–output power flow balancing are two importantpower management issues for the proposed two-input dc/dcconverter. Appropriate control methods are needed for differentpower distribution demands. Before determining the powermanagement strategy, the steady-state analysis of the proposedtwo-input PWM dc/dc converter should be derived in advance.

The input–output voltage relationship can be derived fromthe steady-state volt–second balance analysis of the inductor.For the voltage and current waveforms shown in Fig. 5, theequivalent operation circuit of the proposed converter duringone switching cycle will follow the sequence of modes I, IV,and III.

By applying the volt–second balance theorem on the inductorL, the following equation can be obtained:

(dHI − dLO)TS(VHI − VO) + dLOTS(VHI + VLO)

+ (1 − dHI)TS(−VO) = 0 (1)

where dHI and dLO are duty ratios for SHI and SLO, respec-tively, and TS is the switching period.

From (1), the output voltage expression can be obtained as

VO =dHI

1 − dLOVHI +

dLO

1 − dLOVLO. (2)

If SLO has a longer conducting time than SHI, then theequivalent operation circuit of the proposed converter in oneswitching cycle will follow another sequence of modes II, IV,and III. The volt–second balance equation for the inductorbecomes

(dLO − dHI)TSVLO + dHITS(VHI + VLO)

+ (1 − dLO)TS(−VO) = 0. (3)

From (3), the same output voltage expression of (2) can beobtained. It implies that power switches SHI and SLO can becontrolled independently.

On the other hand, by assuming that the inductance ofthe inductor is very large, the inductor ripple current can beneglected. When the power switches SHI or SLO are turned

ON, the input current of the high-voltage source or the low-voltage source will be equal to the inductor current. Hence, theaverage input current for high- and low-voltage sources can beexpressed as

IHI = dHI · IL (4)

ILO = dLO · IL. (5)

Besides, when the power switch SLO is turned OFF, the un-filtered output current iO

′, is equal to the inductor current iL.When SLO is turned ON, the unfiltered output current is zero.Hence, the average output current can be expressed as

IO = (1 − dLO)IL. (6)

From (4)–(6), the relationships between the average input andoutput currents can be obtained as

IHI =dHI

1 − dLOIO (7)

ILO =dLO

1 − dLOIO. (8)

From the above derived steady-state voltage and currentequations, different power distribution demands can be realized.

Since the proposed double-input dc/dc converter has twoinput voltage sources and one output load with a regulatedvoltage, where the power for each one of them can be eithercontrolled or undetermined, there are three different powerstatus combination cases which can be shown in Table I. In thefollowing, the power management and control strategy of thesethree different cases will be described in detail.

Case I: In case I, the load will consume the demanded powerby drawing constant current, while the high-voltage source willsupply a limited constant power and the low-voltage source willprovide the rest of the demanded power. For the power systemoperated under the situation of case I, the high-voltage sourceis the main power source for the load, and the low-voltagesource is the auxiliary power source. For the constant loadcurrent demand, if the input current of the high-voltage sourceis under controlled, then the low-voltage source will provide theremaining current demanded by the load current. This can beexplained by the aforementioned voltage and current equations.In order to control the input current IHI of the high-voltage

1542 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2006

Fig. 7. Control block diagram for three different cases. (a) Case I. (b) Case II. (c) Case III.

source with a constant load current IO, the duty ratio dHI in(7) can be expressed as

dHI =IHI

IO(1 − dLO). (9)

By substituting (9) into (2), the duty ratio dLO can be deter-mined with a specific VHI, VLO, and VO. As a result, the dutyratio dHI and the input current ILO of the low-voltage sourcecan then be decided.

Case II: Case II is similar to case I, where the low-voltagesource will supply limited constant power and the high-voltage

source will provide the inadequate power demanded by theload. The control strategy for case II is similar to that for case Iand can also be explained by the prementioned voltage andcurrent equations. In order to control the low-voltage inputcurrent ILO with a constant load current IO, the duty ratiodLO in (8) is determined. In (2), with constant input and outputvoltages and with the determined dLO, the duty ratio dHI fora high-voltage source can be decided. Once dHI and dLO areknown, the high-voltage-source input current IHI in (7) can bedetermined.

Case III: In case III, the load should have the ability toabsorb the power supplied by both of the high- and low-

CHEN et al.: DOUBLE-INPUT PWM DC/DC CONVERTER FOR HIGH-/LOW-VOLTAGE SOURCES 1543

Fig. 8. Proposed double-input dc/dc converter with one passive losslesssoft-switching cell.

Fig. 9. Measured waveforms of gate driving signals νGSHI, νGSLO, andinductor current iL.

voltage sources. A rechargeable battery is a typical load of thepower system for case III, where the two voltage sources canalways deliver their maximum power to the load. It is good forrenewable-energy applications, e.g., a PV-Wind hybrid powergeneration system. In this case, both of the input currents IHI

and ILO are under control, so that the relationship between theduty ratios dHI and dLO can be determined from (7) and (8) andexpressed as

dHI

dLO=

IHI

ILO. (10)

The output voltage of the proposed converter will be determinedby the electrical characteristics of the load. As a result, the dutyratios dHI and dLO can be decided from (2) and (10).

Fig. 7(a)–(c) shows the control block diagrams for cases I, II,and III. To save space, only case II will be addressed in detailin this paper. The control block diagram for case II is shownin Fig. 7(b). It is designed for a constant load current demandwith controlled low-voltage-source input current. That is, it willregulate the low-voltage source to provide the constant powerfor the load demand, and the rest of the demanded load powerwill be provided by the high-voltage source automatically.The compensated error-voltage signal νe is obtained from thefeedback output voltage and the reference voltage. The gatingsignals for SHI is generated by the error-voltage signal, whilethe ones for SLO is generated by the summation of the error-voltage and error-current signals.

Fig. 10. Measured voltage and current waveforms of power switches atturn-off transition. (a) Turn-off transition for SHI. (b) Turn-off transitionfor SLO.

Fig. 11. Waveforms of the step-load change response.

In practice, some protection circuits must be integratedwithin the control circuit of the proposed double-input dc/dcconverter. When one of the voltage sources fails, the controlcircuit will detect this source failure and disable the gatingsignal of the corresponding power switch, while the othervoltage source continues to deliver power to the load. Forinstance, when the high-voltage source fails, the control circuitwill disable the gating signal of SHI, resulting in an open circuitof the branch containing SHI. By removing the open-circuitedbranch, the proposed double-input dc/dc converter becomes thecircuit shown in Fig. 6(a) and can be equivalent to a buck–boostconverter. Thus, the equivalent operation circuit of the proposedconverter during one switching cycle will follow the sequenceof modes II and III. Meanwhile, (2) will become VO = dLO ·VLO/(1 − dLO), which is the same as the input–output voltagerelationship of the buck–boost converter.

1544 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2006

In order to reduce the switching loss of the power switchesSHI and SLO and improve the converter efficiency, the passivelossless soft-switching cell can be inserted in the appropri-ate position. Fig. 8 shows the proposed double-input dc/dcconverter with one single passive lossless soft-switching cellwhich can reduce the switching losses at the synchronous turn-off transition. Similarly, passive lossless soft-switching cell forturn-on transition can be adapted if the proposed two-inputconverter is operated with the synchronous turn-on transition.

IV. EXPERIMENTAL RESULTS

To verify the performance of the proposed double-inputPWM dc/dc converter shown in Fig. 8, a prototype circuit isimplemented with the following specifications and componentvalues:input dc voltage VHI = 60 V and VLO = 30 V;output dc voltage VO = 48 V;output power PO = 200 W;onput current ILO = 1 A;switching frequency fs = 50 kHz;SHI and SLO IRF630;DHI, DLO, D1, D2, and D3 MUR1020CT;C1 and C2 4.7 nF;C 470 µF;L1 52 µH;L 1 mH.

Fig. 9 shows the measured waveforms of gate driving signalsνGSHI of SHI, νGSLO of SLO, and inductor current iL. It showsthat the inductor current has two different charging stages andone discharging stage. The two different charging stages revealthat the proposed double-input dc/dc converter can deliverpower from high-/low-voltage sources to the load individuallyor simultaneously. The measured voltage and current wave-forms of power switches at turn-off transition are shown inFig. 10(a) and (b). It can be seen that the switching loss of eachpower switch can be reduced significantly at turn-off transitionwith the passive lossless soft-switching cell. The waveformsof the step-load change response of the proposed double-inputdc/dc converter are shown in Fig. 11. The output current iseither 3 or 1.5 A, and the output voltage is well regulated. Thelow-voltage-source input current remains at 1 A to provide theconstant power, and the high-voltage-source input current willchange automatically to supply the rest of the demanded powerof the load.

V. CONCLUSION

A novel double-input PWM dc/dc converter for high-/low-voltage sources is proposed in this paper. The opera-tion principle, including the operation modes, the steady-stateanalysis, and the power flow control, is explained in detail. Theexperimental results show that the proposed double-input PWMdc/dc converter can transfer the electric power from high-/low-voltage sources individually or simultaneously and deliverit to the load with low switching losses. Also, the step-loadchange response shows that the expected power managementcapability can be achieved.

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Yaow-Ming Chen (S’96–M’98–SM’05) receivedthe B.S. degree from National Cheng Kung Univer-sity, Tainan, Taiwan, R.O.C., in 1989, and the M.S.and Ph.D. degrees from the University of Missouri,Columbia, in 1993 and 1997, respectively, all inelectrical engineering.

From 1997 to 2000, he was with I-Shou Univer-sity, Taiwan, as an Assistant Professor. In 2000, hejoined National Chung Cheng University, Chia-Yi,Taiwan, where he is currently an Associate Professorwith the Department of Electrical Engineering. His

research interests include power electronic converters, power system harmonicsand compensation, and intelligent control.

CHEN et al.: DOUBLE-INPUT PWM DC/DC CONVERTER FOR HIGH-/LOW-VOLTAGE SOURCES 1545

Yuan-Chuan Liu was born in Chia-Yi, Taiwan,R.O.C., in 1973. He received the B.S. degree in1996 from the Department of Electrical Engineering,National Chung Cheng University, Chia-Yi, Taiwan,where he is currently working toward the Ph.D.degree.

His research interests include developing and de-signing of converter topologies, power-factor correc-tors, and electronic ballasts.

Sheng-Hsien Lin was born in Tainan, Taiwan,R.O.C., in 1979. He received the B.S. degree fromI-Shou University, Kaohsiung, Taiwan, in 2001 andthe M.S. degree from National Chung Cheng Uni-versity, Chia-Yi, Taiwan, in 2003, both in electricalengineering.

In 2003, he joined the Cyntec Company, Ltd.,Hsinchu, Taiwan, as a Senior Engineer. His researchinterests include intelligent power module applica-tion systems and microprocessor-based applicationsystems.