FUEL CELLS are considered to be one of the most promising

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1 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 41, NO. 6, NOVEMBER/DECEMBER A Novel Three-Phase High-Power Soft-Switched DC/DC Converter for Low-Voltage Fuel Cell Applications Changrong Liu, Member, IEEE, Amy Johnson, and Jih-Sheng Lai, Senior Member, IEEE Abstract An efficient dc/dc converter is needed as the interface between a low-voltage fuel cell source and a high-voltage bus for inverter operation. In this paper, a three-phase transformerisolated dc/dc converter utilizing phase-shift (PS) modulation is proposed. The converter must be able to boost the voltage significantly and operate at current levels above 240 A on the source side. Key features of the proposed converter include reduced transformer turns ratio by a factor of two while maintaining the same output voltage, reduced size of passive components including output filter and input dc bus capacitor using three-phase interleaving, eliminated inductor current ripple at PS angles above 120, and achieved soft switching over a wide load range without auxiliary circuitry. The proposed converter has been analyzed, simulated, and implemented in hardware. An efficiency of above 96% was achieved using the prototype unit. Experimental results were used to verify all designs and analyses. Index Terms Converter, dc/dc converter, multiphase, phase shift, soft switching, zero-current switching (ZCS), zero-voltage switching (ZVS). I. INTRODUCTION FUEL CELLS are considered to be one of the most promising future energy generation devices due to their energy efficiency and environmental friendliness [1], [2]. Major applications were identified in various areas such as transportation, stationary power, and portable power. For portable power applications, the general structure is to have a low-voltage fuel cell as the primary source, a dc/dc converter to obtain isolated high voltage, and a dc/ac inverter to obtain ac voltage. Using the specifications of the 2003 International Future Energy Challenge, the nominal fuel cell dc output voltage is Paper IPCSD , presented at the 2004 IEEE Applied Power Electronics Conference and Exposition, Anaheim, CA, February 22 26, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Industrial Power Converter Committee of the IEEE Industry Applications Society. Manuscript submitted for review March 8, 2004 and released for publication August 29, This work was supported by the U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) Solid-State Energy Alliance Program (SECA) under Award DE-FC26-02NT C. Liu is with the Future Energy Electronics Center, Electrical and Computer Engineering Department, Virginia Polytechnic Institute and State University, Blacksburg, VA USA and also with Maxim Integrated Products, Inc., Sunnyvale, CA USA ( changrong_liu@maximhq.com). A. Johnson is with the Future Energy Electronics Center, Electrical and Computer Engineering Department, Virginia Polytechnic Institute and State University, Blacksburg, VA USA and also with the GE Energy, Salem, VA USA ( ajohnson@vt.edu). J.-S. Lai is with the Future Energy Electronics Center, Electrical and Computer Engineering Department, Virginia Polytechnic Institute and State University, Blacksburg, VA USA ( laijs@vt.edu). Digital Object Identifier /TIA V and the ac load is 120/240 V at 5-kW continuous and 10-kW peak. In order to provide a dual ac output, an isolated dc/dc converter is needed to convert low-voltage dc to a dc voltage higher than 400V, sufficient for a 240-V ac output. This dc/dc converter inevitably sees more than 240 A on the fuel cell side, making the design of a dc/dc converter with low voltage and high current very challenging. The converter needs to be capable of high-power operation with a high voltage conversion ratio. A transformer is needed for both voltage boost and isolation. However, a high turns ratio is not favored due to potentially high leakage inductances. Furthermore, a high switching frequency is preferred to reduce the passive component size. In order to achieve a high switching frequency while improving converter efficiency, soft switching is necessary. Among the soft-switching techniques suitable for high-power converter applications, phase-shift (PS) control has been the favorite. However, for a single-phase full-bridge PS converter, zero-voltage switching (ZVS) is achieved over a limited load range. Past efforts have focused on solving this problem. The most popular solutions are to add a saturable core or make some devices switch under zero-current switching (ZCS) condition with added auxiliary circuitry [3] [9]. In this paper, a three-phase transformer isolated PS dc/dc converter is proposed. Major features of the converter include: 1) increase converter power rating by paralleling phases, not by paralleling multiple devices; 2) double output voltage by transformer delta wye connection, thus lowering the turns ratio; 3) reduce size of output filter and input dc bus capacitor with interleaved control; and 4) achieving zero-voltage zero-current switching (ZVZCS) over a wide load range without auxiliary circuitry. Due to these advantages, this converter is highly recommended as the interface between a low-voltage highpower fuel cell source and an inverter load. It is also suitable for other low-voltage sources such as batteries and photovoltaics to supply high-voltage high-power dc to other circuits. The operating modes of this converter are discussed in this paper and a qualitative analysis is presented. A prototype hardware unit is built and tested. Experimental results are used to verify all designs and analyses. II. PROPOSED MULTIPHASE DC/DC CONVERTER Fig. 1 shows the proposed converter. It consists of three fullbridge converters whose outputs are connected to a three-phase full-bridge diode rectifier through a set of transformers. The proposed transformer secondary Y connection is capable of /$ IEEE

2 1692 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 41, NO. 6, NOVEMBER/DECEMBER 2005 Fig. 1. Proposed dc/dc converter topology. Fig. 2. Gate signals and filter inductor current under 60 PS angle. boosting the output voltage without increasing the transformer turns ratio. In the ideal case, if 1 per-unit (pu) voltage is applied to one transformer input and 1 pu voltage is applied to the other, then there will be a 2-pu voltage imposed on the secondary output. For example, if V a1a2 = V dc and V b1b2 = V dc, then the voltage at the rectifier output will be 2nV dc, which is twice the single-phase transformer output. Moreover, the three full-bridge single-phase converters can be controlled in an interleaved manner, which means the phase of their output waveforms will be 120 apart from each other. With this method, the rectifier output ripple frequency will be increased to six times the switching frequency, thus reducing the size of the output filter significantly. III. STEADY-STATE OPERATING MODE ANALYSIS For the proposed dc/dc converter, interaction occurs among phases from the connection of the transformer secondary windings. This interaction is determined by the PS angle between the legs of each full-bridge converter. Complementary gate signals are adopted to control top and bottom switches for each leg. Based on the PS modulation angle α, the converter operating mode can be analyzed for three cases: 0 <α<60,60 < α<120, and 120 <α<180. Fig. 3. Case 1 topological changes in different modes. (a) Mode 0. (b) Mode 1. (c) Mode 2. (d) Mode 3. Case 1: 0 <α<60 : The gate signals and inductor current waveforms are shown in Fig. 2. a 1p represents the upper switch gate signal of phase leg a 1. Similarly, a 2p, b 1p, b 2p, c 1p, and c 2p are the upper switch gate signals of phase leg a 2, b 1, b 2, c 1, and c 2, respectively. Fig. 3 shows different circuit structures corresponding to various operation modes. Fig. 4 displays the voltage across each transformer primary and the corresponding voltage at the output of the rectifier. Mode 0 (t 0 t 1 ): In this mode, phase A applies a positive voltage V dc to the transformer, which causes the output inductor current, thus phase A current, to increase linearly. Meanwhile, phases B and C are under freewheeling conduction due to the reflected current from the transformer secondary side. The sum of phase B and C currents is equal to the negative of phase A current. The duration for this mode is t = Tα/360.

3 LIU et al.: A THREE-PHASE HIGH-POWER SOFT-SWITCHED DC/DC CONVERTER FOR FUEL CELL APPLICATIONS 1693 Fig. 4. Transformer primary voltages and rectifier output for case 1. Fig. 6. Transformer primary voltages and rectifier output for case 2. Fig. 5. Case 2 upper switch gate signals and inductor current waveform. Fig. 7. Case 3 upper switch gate signals and filter inductor current waveforms. Mode 1 (t 1 t 2 ): This is an idle mode, where all three phases are under freewheeling conduction. Current freewheels through phases A and C upper switches and diodes and phase B bottom switches and diodes. The transformer output voltages are all zero, thus causing the output filter inductor current to decrease with the slope of V o /L f. The duration for this mode is t = T (60 α)/360. Mode 2 (t 2 t 3 ): During this mode, phases A and B continue to be in the freewheeling mode. However, the leg C 1 upper switch turns OFF and the bottom switch turns ON, applying a negative voltage 0 V dc to the phase C transformer primary. Voltage nv dc is applied to the output filter inductor and causes the current to increase with a slope of (nv dc V o )/L f.this current also flows through the phase C transformer secondary winding in the negative direction and is reflected to the primary side. This mode lasts for t = Tα/360. Mode 3 (t 3 t 4 ): At the beginning of this mode, the leg C 2 upper switch turns OFF and the bottom device turns ON, driving phase C into freewheeling mode. Since phases A and B continue in freewheeling mode, there is no voltage output to the transformer secondary side, causing the output filter inductor current to decrease with a slope of V o /L f. The duration for this mode is t = T (60 α)/360. The steady-state operation analysis for modes 4 11 are similar to those for the above modes 0 3, but with current flowing through different phases. In this case, at most one phase transfers dc bus voltage to the output side. Thus, the ideal maximum voltage transferred to the output is nv dc and the minimum voltage is 0. Hence, the averaged output voltage is V o = nv dc α/60. Case 2: 60 <α<120 : Fig. 5 shows the top-switch gate signals and inductor current waveforms for case 2. The voltage across each transformer primary and the resulting voltage at the output of the rectifier are shown in Fig. 6. In this case, at least one phase and at most two phases transfer dc bus voltage to the output side. Thus, the ideal maximum voltage transferred to the output is 2nV dc and the minimum voltage is nv dc. Through similar analysis as in case 1, the durations for this mode are t = T (α 60 )/60 and t = T (120 α)/60, respectively. Therefore, the averaged output voltage can be derived as V o = nv dc + nv dc (α 60 )/60. Case 3: 120 <α<180 : Fig. 7 shows the upper-switch gate signals and inductor current waveforms for case 3. The voltage across each transformer primary and the resulting voltage at the output of the rectifier are shown in Fig. 8. Notice how the voltage at the output of the rectifier is constant.

4 1694 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 41, NO. 6, NOVEMBER/DECEMBER 2005 Fig. 8. Transformer primary voltages and rectifier output for case 3. Fig. 9. Voltage conversion ratio. In this case, at least two phases and at most three phases transfer dc bus voltage to the output side. The output voltage overlaps, thus the ideal voltage transferred to the output is 2nV dc at any time. This also accounts for the filter inductor current being constant for the ideal case. Therefore, the averaged output voltage is uncontrollable and is always equal to V o = 2nV dc. Since the voltage conversion ratio is fixed, the converter behaves like a transformer, but with dc input/output voltage. Therefore, this mode is defined as the dc/dc transformer mode. From the above analysis of all three cases, the voltage conversion ratio as a function of PS modulation angle α can be illustrated as in Fig. 9. The case 1 condition happens when the load is light. Without overlap, the entire output power is carried by one phase subconverter. However, it is well below the rated power and is not a concern. Cases 2 and 3 happen when the output load is high. Since the load current is shared by at least two subconverters, each phase subconverter carries no more than half of the output power under high-load condition. IV. ZVZCS SOFT-SWITCHING VERIFICATION Similar to the single-phase PS converter, the proposed multiphase converter can easily achieve ZVS turn ON for the lagging leg switches in each phase. For the leading leg, during its switching transition, since three phases are coupled through the transformer secondary side Y connection, the phase current is affected by the other two phases. The distribution of the current heavily depends on the phase inductance, which consists of the transformer leakage inductance and any other stray inductances in each phase. If the phase inductance is large enough, the switches should be able to turn ON with a fairy low current, or nearly ZCS. The proposed converter is simulated for the three operating cases mentioned above under the following conditions: 25-V input dc voltage, resistive 30-Ω output load, 50-kHz switching frequency, and 500-ns deadtime control for each leg. Other circuit parameters are 23-nH leakage inductance on the primary side of each phase, 70-µH filter inductor, and 2.2-mF filter capacitor. For the PS modulation angle, 40 is used for case 1, 80 is used for case 2, and 150 is used for case 3. Since phases A, B, and C are identical except for the 120 phase displacement from each other, the resulting waveforms are similar. Therefore, only phase A waveforms are shown in Fig. 10. The phase current and device drain-to-source voltages are recorded. Simulation results verify that the lagging leg switches are operating under ZVS condition. Also, from the simulation results, we can see that for all three cases the leading leg a 1 switches are turned ON under zero-current condition. When a 1p and a 2p or a 1n and a 2n are conducting simultaneously, the phase current i a would have been continuously flowing in the conventional full-bridge converter. With a three-phase structure, the coupled currents from the other two conducting phases will assume the duty and reset this freewheeling current, and thus the ZCS condition for the leading leg is naturally created without additional resetting circuitry. In cases 1 and 2, to get the same output voltage, a larger PS modulation angle is needed at heavy loads due to component voltage drop. Therefore, the converter operates under soft switching for a wide load range with closed-loop regulation. Another simulation comparison has been done by varying load resistance to get a 400-V output with the same PS modulation angles as stated in the above cases. The simulation results also confirm that the ZVZCS soft-switching operation can be achieved with load-varying conditions. V. P ROTOTYPE EXPERIMENTAL VERIFICATION To verify this new topology, a prototype unit is built and tested. Fig. 11 illustrates the assembly of the prototype converter. This unit is rated at 3 kw due to the current limitation on power device pins. It consists of three major parts: a sixleg converter power board, a set of three transformers, and an output rectifier/filter board. In this test unit, three fullbridge single-phase converters are synchronized by an external clock signal and are controlled by the same reference signal. Thus, with well-tuned ramp signals, the PS modulation angles between two legs for each phase are identical. This timing is critical; if it is slightly unmatched, it may cause large circulating energy among the transformer primary sides. The actual conversion ratio for the prototype is shown in Fig. 12. As can be seen, the experimental results match very closely with the ideal ratio. The discrepancy is the power loss in the circuit, caused by the impact of deadtime control in the leg switches and duty-cycle loss caused by circuit inductance.

5 LIU et al.: A THREE-PHASE HIGH-POWER SOFT-SWITCHED DC/DC CONVERTER FOR FUEL CELL APPLICATIONS 1695 Fig. 11. Illustration of prototype test unit. Fig. 12. Conversion ratio versus PS of prototype converter. Fig. 10. Soft-switching operation for each case. (a) Case 1. (b) Case 2. (c) Case 3. The experimental results also verify the converter softswitching operation. Device switching waveforms are shown in Fig. 13. With current circuit parameters, all the devices turn OFF with ZVS, as illustrated in Fig. 13(a). Fig. 13(b) shows the turn ON switching waveforms for the leading leg devices; the lagging leg devices turn ON waveforms are shown in Fig. 13(c). It should be noticed that these turn ON waveforms are acquired with only 10% load condition and the ZVZCS operation is apparent. An increased load helps to achieve device ZVZCS operation, confirmed by tests not presented here. The soft-switching operation discussed above improves system efficiency. The curve in Fig. 14 shows the measured system efficiency for different loads, with a margin of error of ±1%. The heat sink temperature rise was less than 20 C at the 2-kW condition with natural convection. Experimental results for the dc/dc transformer mode are shown in Fig. 15. The device voltages are very clean, resulting in low voltage stress. The inductor current ripple is not noticeable. This implies that the inductor size can be further reduced. The rectifier output voltage, the voltage before LC filter, is continuous. It would have a clean dc output if there is no parasitic ringing. However, the high-frequency ringing on the rectifier voltage is less than 50 V for a 200-V output; thus, no voltage clamp is needed. Here, 200 V is used to generate a 120-Vrms inverter ac output. The input current between switching devices and the low-side dc bus capacitor is not shown because it is not accessible with the printed circuit board as the interconnection. However, the evidence of a flat output inductor current implies clearly that the input ripple current is also canceled with interleaved operation. With reduction on the major passive components including both output filter inductor

6 1696 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 41, NO. 6, NOVEMBER/DECEMBER 2005 Fig. 15. Experimental waveform at α = 150. Fig. 13. Device switching waveforms. (a) Device ZVS turns OFF waveforms. (b) Leading leg device turns ON switching waveforms with 10% load. (c) Lagging leg device turns ON switching waveforms with 10% load. Overall, the major design features and advantages are low transformer turns ratio achieved by delta wye connection, reduced size of output filter and input capacitor with interleaved control, zero-voltage zero-current switching (ZVZCS) for a wide load range without auxiliary circuit, and high system efficiency. These merits make this converter extremely suitable for low-voltage fuel cell applications. Analyses should be done to further reduce the parasitic losses and increase the efficiency. Efficiency is emphasized because it implies better operating condition on fuel cell, savings on hydrogen fuel, and smaller heat sinking requirement. Further reductions in the output filter may be possible since the inductor current ripple is practically eliminated by interleaved operation. The reduction in passive components and heat sinking also implies potential reduction in size and weight and savings in cost. Further discussion about modeling and control issue of the system, including the fuel cell source and inverter load, can be found in [11], in which the simulation and experimental results are provided to verify the control design and system transient response. ACKNOWLEDGMENT Fig. 14. Experimental data and trendline for converter efficiency at α = 150. and the input dc bus capacitor, a significant cost reduction can also be expected. VI. CONCLUSION A novel three-phase transformer-isolated dc/dc converter with efficiency above 96% is proposed in this paper. Phase-shift (PS) modulation is used to achieve device soft switching. Also, the multiphase structure reduces the rms current per phase, thus reducing the I 2 R conduction loss, without paralleling multiple devices. Moreover, a Y connection on the three-phase transformer secondary side doubles the output voltage without increasing the turns ratio. Therefore, this converter is favored as the choice of high-power converters that have a low voltage source and high input current. The authors would like to thank D. Collins of the U.S. DOE National Energy Technology Laboratory for his technical support and guidance on fuel cell requirements. REFERENCES [1] EG&G Technical Services, Inc. Science Applications International Corporation, Fuel Cell Handbook, 7th ed. Morgantown, WV: U.S. Dept. of Energy, Office of Fossil Energy, National Energy Technology Laboratory, Nov [2] H. B. Puttgen, P. R. MacGregor, and F. C. Lambert, Distributed generation: Semantic hype or the dawn of a new era?, IEEE Power Energy, vol. 1, no. 1, pp , Jan./Feb [3] J. A. Sabate, V. Vlatkovic, R. B. Ridley, F. C. Lee, and B. H. Cho, Design considerations for high-voltage high-power full-bridge zerovoltage-switched PWM converter, in Proc. IEEE Applied Power Electronics Conf. and Exposition, Los Angeles, CA, Mar. 1990, pp [4] G.Hua,F.C.Lee,andM.M.Jovanović, An improved full-bridge zerovoltage-switched PWM converter using a saturable inductor, IEEE Trans. Power Electron., vol. 8, no. 4, pp , Oct [5] R. Watson and F. C. Lee, A soft-switched full-bridge boost converter employing an active-clamp circuit, in Conf. Rec. IEEE Power Electronics Specialists Conf., Baveno, Italy, Jun. 1996, pp

7 LIU et al.: A THREE-PHASE HIGH-POWER SOFT-SWITCHED DC/DC CONVERTER FOR FUEL CELL APPLICATIONS 1697 [6] S.-J. Jeon and G.-H. Cho, A zero-voltage and zero-current switching full bridge DC DC converter with transformer isolation, IEEE Trans. Power Electron., vol. 16, no. 5, pp , Sep [7]J.G.Cho,J.W.Baek,D.W.Yoo,H.S.Lee,andG.H.Rim, Novel zero-voltage and zero-current-switching (ZVZCS) full bridge PWM converter using transformer auxiliary winding, in Conf. Rec. IEEE Power Electronics Specialists Conf., St. Louis, MO, 1997, pp [8] J. G. Cho, G. H. Rim, and F. C. Lee, Zero voltage and zero current switching full bridge PWM converter using secondary active clamp, in Conf. Rec. IEEE Power Electronics Specialists Conf., Baveno, Italy, 1996, pp [9] P. K. Jain, W. Kang, H. Soin, and Y. Xi, Analysis and design considerations of a load and line independent zero voltage switching full bridge DC/DC converter topology, IEEE Trans. Power Electron., vol. 17, no. 5, pp , Sep [10] D. S. Oliveira, Jr. and I. Barbi, A three-phase ZVS PWM DC/DC converter with asymmetrical duty cycle for high power applications, in Conf. Rec. IEEE Power Electronics Specialists Conf., Acapulco, Mexico, 2003, vol. 2, pp [11] C. Liu, A. Johnson, and J.-S. Lai, Modeling and control of a novel six-leg three-phase high-power converter for low voltage fuel cell applications, in Conf. Proc. IEEE 35th Power Electronics Specialist Conf., Aachen, Germany, Jun , 2004, vol. 6, pp Changrong Liu (S 99 M 05) was born in Fujian, China, in He received the B.S. and M.S. degrees in electrical engineering from Tsinghua University, Beijing, China, in 1994 and 1997, respectively, and the Ph.D. degree from Virginia Polytechnic Institute and State University, Blacksburg, in In 1997, he was a Graduate Research Assistant at the Center for Power Electronics System (CPES), Virginia Polytechnic Institute and State University. After that, he joined Maxim Integrated Products, Inc., Sunnyvale, CA, working as a Corporate Application Engineer on notebook power products. His research interests include portable system power management, modeling and control of power converter systems, converter system stability analysis, soft-switching technique, power converter applications in fuel cell systems, and motor drive systems. Amy Johnson received the B.S. degree in electrical engineering from Valparaiso University, Valparaiso, IN, in 2002, and is currently working toward the M.S. degree in electrical engineering at Virginia Polytechnic Institute and State University, Blacksburg. She is currently a Power Conversion Design Engineer with the Wind Energy Group, GE Energy, Salem, VA. She is interested in the area of renewable energy sources, working with both wind and fuel cell applications. Jih-Sheng (Jason) Lai (S 84 M 87 SM 93) received the M.S. and Ph.D. degrees in electrical engineering from the University of Tennessee, Knoxville, in 1985 and 1989, respectively. From 1980 to 1983, he was the Head of the Electrical Engineering Department, Ming-Chi Institute of Technology, Taipei, Taiwan, R.O.C., where he initiated a power electronics program and received a grant from his college and a fellowship from the National Science Council to study abroad. In 1986, he was a Staff Member at the University of Tennessee, where he taught control systems and energy conversion courses. In 1989, he joined the Electric Power Research Institute (EPRI) Power Electronics Applications Center (PEAC), where he managed EPRI-sponsored power electronics research projects. In 1993, he was a Power Electronics Lead Scientist at the Oak Ridge National Laboratory, where he initiated a high-power electronics program and developed several novel high-power converters including multilevel converters and soft-switching inverters. In 1996, he joined Virginia Polytechnic Institute and State University, Blacksburg, where he is currently a Professor and the Director of the Future Energy Electronics Center. His main research areas are in high-efficiency power electronics conversions for high power and energy applications. He has published more than 135 technical papers and two books and received 11 U.S. patents. Dr. Lai chaired the 2000 IEEE Computers in Power Electronics (COMPEL 2000). He was the Founding Chair for the 2001 IEEE/DOE Future Energy Challenge and the General Chair of the 2005 IEEE Applied Power Electronics Conference and Exposition (APEC 2005). His work brought him several distinctive awards including a Technical Achievement Award at Lockheed Martin Award Night, two IEEE Industry Applications Society (IAS) Conference Paper Awards from the Industrial Power Converter Committee, and one IEEE Industrial Electronics Society (IECON) Best Paper Award.