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A New 98% Soft-Switching Full-Bridge DC-DC Converter based on Secondary-Side LC

Resonant Principle for PV Generation Systems

Daisuke Tsukiyama*, Yasuhiko Fukuda*, Shuji Miyake*, Saad Mekhilef**, Soon-Kurl Kwon*** and Mutsuo Nakaoka***

* Dispersed Power System Division, Daihen Corporation, Osaka, Japan ** University of Malaya, Malaysia

*** Kyungnam University, Republic of Korea/Yamaguchi University, Yamaguchi, Japan d-tsukiyama@daihen.co.jp

Abstract — This paper is mainly concerned with the state-of-the-art feasible development of a novel prototype high-efficiency phase-shift soft-switching pulse modulated full-bridge DC-DC power converter with a high-frequency power transformer, which is designed for utility-grid tied photovoltaic (PV) power inverters. The proposed high-frequency transformer (HFTR) link DC-DC converter topology is based upon a new conceptual secondary-side series resonant principle and its inherent nature. All the active power switches in the HFTR primary-side can achieve lossless capacitive snubber-based ZVS with the aid of transformer parasitic inductances. In addition to this, passive power switches in its secondary-side can also perform ZVS and ZCS transitions for input voltage and load variations.

In the first place, the operation principle of the newly-proposed DC-DC converter and some remarkable features are described in this paper on the basis of the simulation analysis. In the second place, the 5 kW experimental setup of the DC-DC converter treated here is demonstrated and its experimental results are illustrated from a practical point of view. Finally, some comparative evaluations between simulation and experimental data are actually discussed and considered in this paper, together with its future works.

Index Terms---DC-DC power converter, full bridge topology, high-frequency transformer link, secondary-side LC resonant principle, primary-side ZVS, secondary-side ZCZVS, photovoltaic generation system

I. INTRODUCTION

In recent years, a variety of collaborated developments on renewable, sustainable energy conversion devices technologies and advanced power electronics have been strongly required toward concrete realization of non-carbon society from a global point of view. Of these relating to energy electronics, effective utilizations of the clean PV generating power and the energy with the aid of the latest energy storage devices such as batteries and/or capacitors have attracted special interest in the fields of power electronic distributed power supply systems applications and DC feeding smart grid. Under such conditions mentioned above, next generation developments of high-efficiency, high-power

density and high performance solar converters have been practically needed so far, which include high frequency switching pulse modulated DC-DC converters and utility interactive sinewave modulated inverters on the basis of digital control schemes.

In particular, the front-end high-efficiency, high frequency switching boost DC-DC power converters such as non-isolated and isolated circuit topologies have been considered and discussed for solar power converters in order to improve their efficiency, power density and control performances including noise issue. The authors have proposed several circuit topologies of high- frequency switching DC-DC high-power converter circuits operating under the conditions of soft commutation schemes for high power industrial applications [1-2]. In addition, the authors have also

Fig. 1. Overview of utility-grid tied photovoltaic (PV) system with high-frequency isolation and, circuit diagram of the new phase-shift soft-switching pulse modulated full-bridge DC-DC converter, which is based on the secondary-side series LC resonant principle.

IEEE PEDS 2011, Singapore, 5 - 8 December 2011

978-1-4577-0001-9/11/$26.00 ©2011 IEEE 1112

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developed utility-grid tied three-phase sinewave inverter operating under the conditions of high efficiency control strategy [3]. To further decrease the power consumption, advanced high efficiency and high-power density soft-switching DC-DC converter topology with high frequency transformer link is proposed for a new issue on power solution, which is based on its secondary-side LC resonant circuit principle for the front-end solar converter in PV generation systems.

Also, some previous researches about a variety of DC-DC converter with high-frequency transformer are introduced to improve their efficiency and power density [4-14]. It includes series-resonant soft-switching DC-DC converter with high-frequency transformer [13-14]. In these researches, DC-DC converter utilizes the leakage inductance of transformer for energy storing and/or soft commutation. However, although it is useful for realizing soft switching DC-DC converter, a large leakage inductance more than 2 μH or presence of parasitic capacitance more than 1 μF have to decrease magnetic coupling coefficient of the transformer. This result leads to not only reducing the high-frequency transformer efficiency but also increasing the surge voltage applied to the high-frequency full bridge inverter circuit which is introduced into this DC-DC converter with high-frequency transformer. In addition to this, it is extremely difficult that some series resonant DC-DC converters which utilize the leakage inductance of transformer is adopted as high voltage and high power applications above 1 kW, due to the reduction of power transmission efficiency from primary-side to secondary-side.

This paper presents an improved efficiency phase shift soft-switching pulse modulated full-bridge DC-DC converter with a high frequency transformer stage and front-end boost converter cascade stage, which includes soft-switching full bridge diode rectifier operating on the basis of the resonant operating principle and inherent nature of the secondary-side LC series resonant circuit. This new DC-DC power converter suitable for solar converter can achieve not only soft-switching transition based on ZVS in the primary-side, but also ZVS and ZCS commutation for the full-bridge diode rectifier in secondary-side. The operating principle of this DC-DC converter in a periodic steady-state is described by using switching mode equivalent circuits and simulation analysis, along with its inherent remarkable features as compared with conventional ones. The simulated operating voltage and current waveforms are comparatively illustrated in experimental ones. The actual efficiency vs. output power characteristics and power loss analysis are demonstrated from an experimental point of view. The practical effectiveness of the proposed converter for PV generation systems are confirmed and verified by means of 5 kW setup implementation and simulation analysis.

II. NEW SOFT-SWITCHING DC-DC CONVERTER

A. Circuit Description

Figure 1 shows a proposed high-frequency transformer link phase-shift soft-switching pulse modulated full-bridge DC-DC converter with the front-end boost converter cascade stage which incorporates soft-switching full bridge rectifier

operating at the secondary-side LC resonant principle. The LC resonant components are located across the output diode rectifier. This high frequency isolated DC-DC converter designed for PV generation systems can operate under ZVS condition of the primary-side active switches S1 ～ S4 in H-Bridge arms with the aid of lossless capacitor, leakage inductor Ll and magnetizing inductance Lm of the two-winding high-frequency transformer with ferrite core.

On the other hand, the passive switches in the secondary-side full-bridge rectifier can also operate under a principle of ZVS and ZCS transitions due to the secondary-side LC resonant circuit property.

B. Principle of Converter Operation

Figure 2 illustrates the relevant voltage and current operating waveforms during a complete switching period for the gate driving pulse sequences. The switching operating modes of the soft-switching full-bridge DC-DC converter with a high frequency transformer are divided into 6 operations modes from mode 1 to mode 6 in accordance with operational timing points from t0 to t6. As can be seen in Fig. 3, the operation principle is described with the equivalent circuits corresponding to each operating mode.

●Mode 1 (t0～t1):

During an active state the corresponding set of the primary switches and secondary rectifier diodes (S2, S3 and D2, D3) conduct simultaneously so that the secondary voltage and current have the same polarity. The output power is delivered from the DC source to the load. Positive voltage magnitude Vin is applied to the high-frequency transformer. Both VS1 and VS4, the voltages across S1 and S4, respectively, are also equal to the source voltage Vin. ●Mode 2 (t1～t2): The turn-off signal is applied to S3 at time t1. After S3 is turned off, VS1 begins to decrease gradually because the capacitor paralleled with S1, previously charged to source voltage Vin, is linearly discharging until zero level is reached. At the same time, the capacitor paralleled with S3, previously discharged to zero level, is linearly charging toward positive voltage. Therefore, low-side switch S3 is able to be turned off with ZVS due to current flowing into the capacitor paralleled with S3. On the other hand, diode currents (iD2, iD3) form sinusoidal wave approache to zero so that not only D2 but also D3 can be easily turned off with extremely soft recovery during this period. ●Mode 3 (t2～t3): After the capacitor voltage VS1 reaches to zero, the diode paralleled with S1 (DS1) starts to conduct and clamps voltage on the high-side switch S1 at zero. And consequently, the voltage across S3 is clamped at source voltage Vin. On the other hand, the turn-off gate pulse signal is applied to S2 at time t2. After S2 is turned off, VS4 begins to decrease gradually because the capacitor paralleled with S4, previously charged to source voltage Vin, is linearly discharging until zero level is reached. At the same time, the capacitor paralleled with S2, previously discharged to zero level, is linearly charging toward positive voltage. Therefore, high-side switch S2 is turned off with almost ZVS due to current flowing into the capacitor paralleled with S3. Although all primary switches (S1 ～ S4) are turned off, primary current is

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maintained

Fig. 2. Operating voltage and current waveforms of soft-switching high-frequency link phase-shift full-bridge DC-DC converter based on the secondary-side LC resonant principle.

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Fig. 3. Switching mode equivalent circuits of the phase-shift soft-switching full-bridge DC-DC power converter with two winding high-frequency transformer link and soft-switching full-bridge rectifier.

Mode 1 (t0 ≦t < t1 )

Mode 2 (t1 ≦t < t2 )

Mode 3 (t2 ≦t < t3 )

Mode 4 (t3 ≦t < t4 )

Mode 5 (t4 ≦t < t5 )

Mode 6 (t5 ≦t < t6 )

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maintained in the same direction during this state. ●Mode 4 (t3～t4): Similarly to Mode 3, the diode DS4 begins to flow current and clamps voltage on the bottom switch S4 at zero after the capacitor voltage VS1 previously reaches to zero. On the other hand, the current through the secondary-side diodes (D1 and D4) start to flow after VD1 and VD2 are equal to zero at the same time when both VS2 and VS3, the voltage across S2 and S3, respectively, reach to the source voltage Vin. At the time t4, primary current is still flow through diodes paralleled with S1 and S4 (DS1 and DS4) while the turn-on signal is applied to S1. ●Mode 5 (t4～t5):

The current through the diodes of DS1 and DS4 decreases gradually toward zero as the secondary-side current of D1 and D4 increases. At the time t5, Both S1 and S4 are turned on with ZCS simultaneously. ●Mode 6 (t5～t6):

The sinusoidal resonant current flows through primary-side active switches S1, S4 and the winding in primary-side of high-frequency transformer. The current through secondary-side diodes D1, D4 and the secondary-side winding of the transformer also forms sinusoidal resonant wave as well. The current through the primary-side is lagging behind the voltage while the phase between the voltage and current through the secondary-side is close to zero, because the resonant capacitor is located between the transformer and the full-bridge diode rectifier acts to form a leading current. This nature is effective to improve the converter efficiency. In other words, there is no phase difference between the voltage and the current in the secondary-side so that diodes set in this side are able to be turned off without generating reverse recovery current. In addition to this, all the diodes paralleled with the primary-side active switches utilize the lagging current during free-wheeling states and can be turned off with zero reverse recovery current at time t5.

C. Unique Features

The specific advantageous points of the proposed DC-DC converter in Fig. 1 are as follows;

(1) Primary-side ZVS transition of all the active switches. (2) Secondary-side ZVS and ZCS transition of all the

passive switches. (3) Minimum reverse recovery current switching loss at

the diode full bridge stage. (4) Higher converter efficiency realization (98%, 380 V

DC output). (5) Flexible design of high voltage conversion ratio. (6) Ground leakage current protection from PV panel to

power conversion circuit portion. (7) Utilization of the high-efficiency transformer with less

Fig. 3. Simulated results of (a) gate driving signal S1 and S3, (b) gate driving signal S2 and S4, (c) voltage and current of D1, (d) voltage and current of S1, (e) resonant current of high-frequency transformer and (f) inductor current and capacitor voltage of LC resonant circuit under the conditions of in 380 V and 5.5 kW.

0

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600I_S1*20 V_S1

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Itrans_pri

0.19994 0.19995 0.19996 0.19997 0.19998 0.19999Time (s)

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-100

50100150

I_Lout*5 Vcap

S2 S4

iD1×20 (A)VD1 (V)

VS1 (V)iS1×20 (A)

itr (A)

iLout×5 (A)Vcap (V)

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MOSFETFull-bridge circuit

Series resonantinductor and capacitor

Diode rectifier

HF-Transformer

Fig. 4. Whole exterior appearance of boost converter-fed soft-switching full-bridge high-frequency inverter link DC-DC converter with soft-switching full bridge rectifier based on the secondary-side series LC resonant principle.

Fig. 5. Two winding high-frequency isolated power transformer is set to the DC-DC high power converter.

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leakage inductance and parasitic capacitance. (8) High DC voltage step-up due to front-end boost

converter cascade topology.

III. CONVERTER SIMULATION

Real time-based computer simulations were carried out in PSIM version 9.0 to reveal the operation of this converter under constant load conditions. Figure 3 shows the waveform of gate driving pulse signal (a) (b), the output rectifier (c), the MOSFET primary switch (d), transformer secondary current (e) and voltage across the resonant capacitor connects series with the transformer secondary (VCr) and the inductor current which composes output filter connected to load (iLout) (f) with 5 kW output specifications. As shown in Fig. 3 (c), diode D1 is turned off with ZVS and turned on with ZCS. In Fig. 3 (d), switch S1 is turned off after iS1 reaches to zero due to the capacitor paralleled with it. The current through the transformer forms sinusoidal wave due to series LC resonant circuit is depicted in Fig. 3 (e). Figure 3 (f) indicates the energy commutation between the resonant capacitor (1/2CV2) and the resonant inductor (1/2Li2) which is connected to the output diode rectifier.

IV. EXPERIMENTAL RESULTS AND EVALUATIONS

A 380 V input DC-5 kW soft-switching phase-shift pulse modulation full bridge DC-DC converter with soft-switching full bridge diode rectifier operating on the basis of the secondary-side LC resonant topology was built and tested in experiment. Figure 4 shows the whole appearance of the proposed soft-switching full-bridge DC-DC converter based on the secondary-side LC resonant principle for PV generation systems. The two-winding high-frequency transformer with ferrite core which is introduced in the DC-DC converter is shown in Fig. 5. The design specifications and circuit parameters of the converter are listed in Table 1. The measured voltage and current waveforms of this prototype converter are illustrated in Fig. 6. Comparing Fig. 6 to Fig. 3 (c), it can be seen that these observed voltage and current waveforms have good agreements with the simulated ones.

The actual efficiency vs. output power characteristic is illustrated in Fig. 7, under the conditions of constant output voltage and some varying specifications. Observing Fig. 7, the actual efficiency reaches maximum value of 98% around the 3-4 kW output power ranges indicated by white-diamond marks. Over higher output power range, the efficiency slightly decreases mainly because the total conduction loss of the converter increases in accordance with the load current increase. However, 97.8% efficiency can be still measured at 5.2 kW outputs. The actual efficiency above 97% is measured in the range of 1-5.2 kW outputs. Figure 7 also depicts the efficiency in black-circle marks. In this condition, terminals of these components are connected directly to the measuring instruments by lengthening cables for examining losses of each component which compose the soft-switching DC-DC converter with high-frequency

transformer. The maximum decrease in efficiency of 0.3% was measured over 3 kW because of increasing actually conduction losses.

VS2

VD1 ID1(5 A/Div)(100 V/Div)

(250 V/Div)

(10 V/Div)VG_S2

Fig. 6. Experimental waveforms of gate-source voltage of S2 (VG_S2, 10 V/div), drain-source voltage of S2 (VS2, 250 V/div), diode D1

current (ID1, 5 A/div) and anode-cathode voltage (VD1, 100 V/div).

Table 1. Design specifications and circuit parameters

Item Symbol Value UnitDC input voltage

(from boost converter) Vin380～388 V

DC output voltage Vout 380 VMaximum input power Pin 5.5 kWSwitching frequency fsw 30.6 kHz

Switching period Ts 16.33 μsPhase shift time Tφ 0.95 μs

Dead time Td 1.6 μsLeakage inductance of

high-frequency transformer Ll < 1.3 μH

Magnetizing inductance of high-frequency transformer Lm 640 μH

Magnetic coupling coefficient of high-frequency transformer k 0.998 -

Inductance of output filter(series resonant inductor) Lout 20 μH

Capacitance of capacitors paralleled with transistors

CS1

～CS45 nF

Capacitance ofinput/output capacitors

Cin,Cout

50 μF

Capacitance of series resonant capasitor Cr 0.88 μF

Turn ratio ofhigh-frequency transformer N1 : N2 1 : 1 -

Item Symbol Product type

Primary-side MOSFET switches

S1

～S4FCA76N60N

Secondary-side rectifier diodes

D1～D4

RHRP3060

Core material ofhigh-frequency transformer - ferrite

(PQ107)

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By connecting each input/output terminal of each power circuit component composes the phase shift soft-switching pulse modulated full-bridge DC-DC converter with a high frequency transformer to the measuring instrument, input and/or output power of each component can be measured directly so that efficiencies and losses of these components can also be investigated. Power efficiency of each component was measured are shown in Fig. 8 as a function of the output power. Total efficiency (white-circle marks) in Fig. 8 is the same as black-circle marks in Fig. 7. As shown in Fig. 8, all elements except full-bridge inverter which is constructed by MOSFET reach to high efficiency of 99.5% around 4 kW or more. Although the efficiency of the rectifier with the filter (black-circle marks) is extremely high in the area of 0.5-2 kW, it slightly decreases as output power becomes larger than 3 kW. The efficiency of the full-bridge inverter shows 98.6% maximum at 3 kW and appears to maintain almost the same value even when output power is more increased.

Power loss analysis about components of the converter was investigated, the result of which is illustrated in Fig, 9. As seen in Fig. 9, the sum of these losses is increased with increasing output power. In the experimental condition of Vin = 380 V, total loss of the soft-switching full-bridge DC-DC converter with secondary-side resonant circuit is increased from 30.7 W to 144 W (about 4.7 times growth) with increasing output power from 0.5 kW to 5.5 kW (11 times growth). In the MOSFET full-bridge circuit part, the loss is increased from 24.5 W to 81 W (3.3 times growth). Similarly, 8.8 (from 4.1 W to 36 W) and 12.9 (from 2.1 W to 27 W) values as the growth rate of power loss are calculated about the HF-transformer and the rectifier with the output filter, respectively.

Figure 10 shows loss ratio of elements which compose the converter. The primary-side MOSFET transistors which construct the full-bridge circuit in the soft-switching DC-DC converter account for 80% of a whole consumption in the range of 1 kW or less and shows about 60% of that at 1.6 kW

9494.5

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Efficiency (%)

Output Power (kW)

Vin = 380 Vfsw= 30.6 kHzLout = 20 µH

MOSFET switchesTotal system efficiency

Rectifier + Output Filter

HF-Transformer with resonant capacitor

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Vin = 380 Vfsw = 30.6 kHzLout = 20 µH

HF-Transformer with resonant capacitor

Rectifier + Output filter

MOSFET switches

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Vin = 380 Vfsw= 30.6 kHzLout = 20 µH

HF-Transformer with resonant capacitor

Rectifier + Output filter

MOSFET switches

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Output Power (kW)

Vin = 380 Vfsw = 30.6 kHzLout = 20 µH

Without wiring for measurement

With wiringfor measurement

Fig. 7. Measured actual efficiency of the trial-produced soft-switching full-bridge DC-DC power converter with secondary-side series LC resonant rectifier as a function of the output power.

Fig. 8. Measured actual efficiencies versus output power characteristics of, diode rectifier with output filter (black-circle marks), high-frequency-transformer (white-triangle marks), MOSFET full-bridge inverter circuit (black-square marks) and total efficiency of the proposed converter with cables length consideration (white-circle marks).

Fig. 9. Power loss analysis about, full-bridge inverter circuit which is consists of MOSFET primary switches, high-frequency transformer, and diode rectifier with output filter.

Fig. 10. Power loss analysis represented by p. u. of converter components for, full-bridge inverter circuit adopts power MOSFETs as the primary-side switches, high-frequency transformer, and diode rectifier with output filter.

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or more. This ratio still shows 56.3% at a full-load. These results indicate that most of the total converter loss is obviously due to the full-bridge circuit which consists of MOSFET switches. The loss ratio of HF-transformer decreases from about 30% to 25% with increasing output power, in the range of 1.6 kW or more. On the contrary, the loss ratio of the rectifier with the filter grows from 7.5% to 18.8% due to increasing conduction loss. Comparing Fig. 7 and Fig. 10, these results indicate that the effect of increasing loss of the rectifier with the filter on efficiency degradation of the converter becomes more significant as output power increases.

V. CONCLUSION

In this paper, the latest high-efficiency phase-shift soft-switching transition pulse modulation DC-DC power converter with high-frequency transformer for PV generation systems has been presented, which is based on the secondary-side LC series resonant circuit principle.

The operation principle of this new DC-DC converter in a steady state has been schematically described with the aid of simulation analysis and confirmed in experiment. This new conceptual high-frequency link is achieved by the high-efficiency power conversion processing based on the secondary-side LC resonant principle. The comparative key operating waveforms between simulation and experimental results have been discussed and evaluated in details. The actual efficiency vs. output power characteristics have been demonstrated and discussed in this paper.

Further considerations for this new conceptual soft-switching DC-DC converter proposed have should be discussed as the following topics;

(1) The soft-switching operating ranges for some circuit parameters, lossless snubbing capacitors dead time setting leakage and magnetizing inductance of high-frequency transformer, and the secondary-side LC resonant constants.

(2) SiC power MOSFETs and SiC-SBD Integration. (3) Feasibility study of this DC-DC converter for utility-

grid tied PV generation systems. (4) Multi-phase DC-DC converter system integration and

practical evaluation for Power Electronic Building Block (PEBB).

REFERENCES

[1] K. Fathy, K. Morimoto, T. Doi, H. W. Lee, and M. Nakaoka, “An asymmetrical switched capacitor and lossless inductor quasi-resonant snubber-assisted ZCS-PWM DC-DC converter with high frequency Link,” The 12th International Power Electronics and Motion Control Conference (IPEMC 2006), Portoroz (Slovenia), Vol. 2, pp. 1-5, Aug/Sep. 2006.

[2] K. Fathy, K. Morimoto, A. N. Ahmed, H. W. Lee, and M. Nakaoka, “A novel soft-switching PWM DC/DC converter with DC rail series switch-parallel capacitor edge resonant snubber assisted by high-frequency transformer components,” The 3rd International Conference on Power Electronics, Machines and Drives (PEMD 2006), Dublin (Ireland), pp. 242-246, April 2006.

[3] N. Hattori, N. Morotomi, S. Miyake, and M. Nakaoka, “Utility interactive 3-phase pulse modulated PV inverter embedding neutral point voltage

shifting scheme into instantaneous current control implementation,” Power Conversion Intelligent Motion EUROPE (PCIM EUROPE 2010), Nuremberg (Germany), May 2010.

[4] L. Zhu, “A novel soft-commutating isolated boost full-bridge ZVS-PWM dc-dc converter for bidirectional high power applications,” IEEE Transactions on Power Electronics, vol. 21, no. 2, pp. 422–429, Mar. 2006.

[5] R. Li, A. Pottharst, N. Frohleke, and J. Bocker, “Analysis and design of improved isolated full-bridge bidirectional DC-DC converter,” in Proc. IEEE PESC Conf. Rec., pp. 521–526, 2004.

[6] S. Inoue and H. Akagi, “A bidirectional isolated DC-DC converter as a core circuit of the next-generation medium-voltage power conversion system,” IEEE Transactions on Power Electronics, vol. 22, no. 2, pp. 535–542, Mar. 2007.

[7] D. Xu,C. Zhao, andH. Fan, “A PWM plus phase-shift control bidirectional DC-DC converter,” IEEE Transactions on Power Electronics, vol. 19, no. 3, pp. 666–675, May 2004.

[8] C. G. Oggier, R. Ledhold, G. O. Garcia, A. R. Oliva, J. C. Balda, and F. Barlow, “Extending the ZVS operating range of dual active bridge high-power DC-DC converters,” in Proc. IEEE PESC 2006, pp. 1–7, Jun. 2006.

[9] F. Krismer, J. Biela, and J.W.Kolar, “Acomparative evaluation of isolated bi-directional DC-DC converters with wide input and output voltage range,” in Proc. IEEE IAS 2005, pp. 599–606, 2005.

[10] H. Xiao and S. Xie, “A ZVS bidirectional DC-DC converter with phaseshift plus PWM control scheme,” IEEE Transactions on Power Electronics, vol. 23, no. 2, pp. 813–823, Mar. 2008.

[11] H. Bai and C. Mi, “Eliminate reactive power and increase system efficiency of isolated bidirectional dual-active-bridge DC-DC converters using novel dual-phase-shift control,” IEEE Transactions on Power Electronics, vol. 23, no. 6, pp. 2905–2814, Nov. 2008.

[12] S. Inoue and H. Akagi, “A bidirectional DC-DC converter for an energy storage system with galvanic isolation,” IEEE Transactions on Power Electonics, vol. 22, no. 6, pp. 2299–2306, Nov. 2007.

[13] X. Li and A. K. S. Bhat, “Analysis and Design of High-Frequency Isolated Dual-Bridge Series Resonant DC-DC Converter,” IEEE Transactions on Power Electronics, vol. 25, no. 4, pp. 850-862, Nov. 2010.

[14] A. K. S. Bhat, “A Comparison of Soft-Switched DC-DC Converters for Fuel Cell to Utility Interface Application,” IEEJ Transactions on Industry Applications, vol. 128, no. 4, pp. 450-458, 2008.

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