Fuel Cell Generation System With a New Active ... - IEEE Xplore

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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 22, NO. 2, JUNE 2007

Fuel Cell Generation System With a New Active Clamping Current-Fed Half-Bridge Converter Su-Jin Jang, Student Member, IEEE, Chung-Yuen Won, Senior Member, IEEE, Byoung-Kuk Lee, Senior Member, IEEE, and Jin Hur, Senior Member, IEEE

Abstract—In this paper, a new active clamping current-fed halfbridge converter is proposed, which is suitable for fuel cell power generation systems. The proposed converter is superior to conventional dc–dc converters in terms of efficiency and component utilization. The overall efficiency is estimated to be 94% at full load. Index Terms—Active clamping, current-fed half-bridge converter, fuel cell power generation system.

I. INTRODUCTION O effectively utilize energy resources, the development of fuel cell generation systems is becoming increasingly important for global environment. The fuel cell, a clean and renewable energy source, has recently been revived and shows promising results for applications as small as cellular phones to as large as utility power generations. This particular fuel cell system is intended for household stand-alone power generation. A fuel cell stack consists of an individual low-voltage cell; therefore, from a cost standpoint, the use of a minimum number of cells makes the best sense. Furthermore, fuel cell manufacturers have chosen a standard voltage of 28–43 Vdc . This low-voltage characteristic requires that the output voltage of the fuel cell stack (input voltage of the inverter) should be boosted before it is inverted to ac source, which means that the inverter must also have a dc–dc converter on the front end. Until now, several dc–dc converter and inverter topologies have been presented and compared based on their performance and cost [1]. However, most topologies have been developed without considering the dynamics of the fuel cell stack and have been only tested with constant battery and dc link. Therefore, the actual performance and efficiency cannot be evaluated for fuel cells. Also, compared to other applications, in the fuel cell applications, voltage-fed converter configurations may not be optimal due to the severe ripple current characteristic of the fuel cell. In order to handle the ripple current, a large number of electrolyte capacitors are essentially required, resulting in an increase in the overall system size and the manufacturing cost. Moreover,

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Manuscript received November 28, 2005; revised November 28, 2005. This work was supported by the Ministry of Commerce, Industry, and Energy through the Advanced Electric Energy Industry Research Center Program. Paper no. TEC-00089-2005. S. J. Jang, C. Y. Won, and B. K. Lee are with the School of Information & Communication Engineering, Sungkyunkwan University, Suwon 440-746, Korea (e-mail: [email protected]; [email protected]; [email protected]). J. Hur is with the Intelligent Mechatronics Research Center, Korea Electronics Technology Institute, Puchon 422-010, Korea (e-mail: [email protected]). Digital Object Identifier 10.1109/TEC.2006.874208

Fig. 1.

Comparison of (a) voltage- and (b) current-fed dc–dc converter.

in voltage-fed converters, high winding ratio between the primary and secondary sides of the high-frequency transformer is necessary because the boosting action is only performed by the winding ratio and it also causes the snubber to be enlarged to handle the surge at turn-off switching instants. Otherwise, in current-fed converters, using an inductor decreases the current ripple as well as the electrolyte capacitor size. An active boosting action can also be achieved with relatively low winding ratio. Therefore, for the fuel cell system, currentfed converter is a better choice than the conventional voltagefed converter [2]. The converter circuit can be broadly divided into voltage-source and current-source topologies, as shown in Fig. 1. The current-fed converter shown in Fig. 1(b) provides high conversion ratio, simplicity of construction, reduction of component stress, and minimization of conduction loss. However, it suffers from severe voltage overshoots at turn-off due to the storage energy in the leakage inductor of the transformer. Fig. 2(a) and (b) show converters with lossless snubbers, which can solve the inherent problem of the circuit in Fig. 1(b). The main problems of Fig. 2(a) and (b) are that the snubber

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JANG et al.: FUEL CELL GENERATION SYSTEM WITH A NEW HALF-BRIDGE CONVERTER

Fig. 2. Current-fed converter topologies for fuel cell applications. (a) Case of lossless snubber. (b) Case of active lossless snubber.

circuit in Fig. 2(a) is only operated at turn-off and its circuit is too much complicated, and in Fig. 2(b), even though zero-voltage switching (ZVS) operation can be achieved both at turn-on and turn-off, the voltages across the auxiliary switches are twice as those of the main switches [2]. In this paper, a fuel cell generation system with a new active clamping current-fed half-bridge converter has been presented. The proposed converter boosts the low output voltage (28–43 Vdc ) of fuel cell stacks to high dc voltage, such as 380 Vdc in a highly efficient manner. Particularly, the developed converter is tested with 1.2-kW Ballard Nexa polymer electrolyte membrane (PEM) fuel cell stacks, so that the dynamic performance is verified, along with its efficiency. The rest of the paper addresses the operational principle, design consideration and simulation, and experimental results. II. CHARACTERISTICS OF THE PROPOSED FUEL CELL GENERATION SYSTEM A. Configuration of the Proposed DC–DC Converter As shown in Fig. 3, the developed fuel cell system consists of a PEM fuel cell (Ballard Nexa), an active clamping dc–dc converter, which is proposed in this paper, and a conventional full-bridge dc–ac inverter. The overall circuit diagram of the proposed active clamping current-fed half-bridge dc–dc converter is shown in Fig. 4. The proposed converter consists of a single clamp capacitor (Cclamp ) and two auxiliary switches (QA1 and QA2 ) based on the conventional converter circuit. The main advantages of the proposed circuit are the ZVS operation of all switches, constant clamping voltage (Vclamp )

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Fig. 3.

Configuration of fuel cell generation system.

Fig. 4.

Proposed active clamping current-fed half-bridge converter.

across all switches, and the simple gate drive implementation, which are suitable for low-cost and high-performance fuel cell generation systems. B. Operational Principle of the Proposed Converter The switching operation can be divided into two symmetrical half cycles, and the overall voltage and current waveforms are displayed in Fig. 5. Also, based on the operational mode, the detailed circuit operations are summarized in–Fig. 6. The analysis of the steady-state operation of the proposed converter can be explained during half cycle as follows [3], [4]. 1) Mode 1—Charging Current (t0 < t < t1 ): The main switches QM 1 and QM 2 , are turned on. The energy is stored in the boost inductors L1 and L2 , and the voltages across the auxiliary switches (QA1 and QA2 ) are clamped to the recharged capacitor voltage Vclamp . 2) Mode 2—Charging Capacitor (t1 < t < t2 ): The main switch QM 1 , is turned off and the current IL1 flows through the parasitic capacitor CM 1 of QM 1 . The voltage across CM 1 (VM 1 ) rises to the value of Vclamp and VA1 of the auxiliary switch QA1 , declines to zero. IL1 VM 1 = (t − t1 ) (1) Ctotal Ctotal = CM 1 + (CA1 //Cclamp ).

(2)

3) Mode 3—Turn on Anti-Parallel Diode (t2 < t < t3 ): At t2 , the antiparallel diode DA1 , is about to conduct, and QA1 can be turned on with zero voltage at this moment. 4) Mode 4—Charging Clamp Capacitor (t3 < t < t4 ): The auxiliary switch QA1 , is turned on with ZVS. From mode 3 to 6,

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Fig. 5.


Voltage and current waveforms according to the operational modes.

the current iclamp and voltage vclamp can be described by the following equations: iclamp (t) = IL1 cos[ω0 (t − t2 )]

(3)

vclamp (t) = IL1 Z0 sin[ω0 (t − t2 )] + Vout n (4) where angular resonance frequency ω0 = 1/ Llk Cclamp , the characteristic impedance Z0 = Llk /Cclamp , and the initial current condition of the leakage inductor is Ilk at t4 . 5) Mode 5—Commutation (t4 < t < t5 ): At t4 , the clamp capacitor starts to transfer the energy into the primary winding during the resonant operation through QA1 . The current of the leakage inductor Llk , is expressed by ilk (t) = IL1 − IL1 cos[ω0 (t − t2 )].

(5)

6) Mode 6—Discharging Capacitor (t5 < t < t6 ): The auxiliary switch QA1 is turned off. The parasitic capacitor CA1 is charged by the leakage inductor current ilk . Voltage across CA1 (VA1 ) first rises to the value of Vclamp and CM 1 of the main switch QM 1 , and then declines to zero. 7) Mode 7—Reset Leakage Inductor Current (t6 < t < t7 ): At t6 , the leakage inductor current ilk discharges CM 1 , turns on the antiparallel diode DM 1 , and QM 1 can be turned on with zero voltage at this moment. It can be described by the following: ilk (t) =

nVout (t − t5 ) + ilk (t5 ) Llk

iDM 1 (t) = ilk (t5 ) −

nVout (t − t5 ) − IL1 . Llk

(6) (7)

Fig. 6. Detailed circuit operations according to the operational modes. (a) Mode 1 (t0 < t < t1 ). (b) Mode 2 (t1 < t < t2 ). (c) Mode 3 (t2 < t < t3 ). (d) Mode 4 (t3 < t < t4 ).


Fig. 7.

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Current through the leakage inductor (primary winding current).

Fig. 8. Time delay (dead time) of the main and the auxiliary switch. (a) ZVS turn-on of the main switch. (b) ZVS turn-on of the auxiliary switch.

continuous conduction mode and the average value of Ilk is equal to that of the load current reflected to the primary side. The voltage conversion ratio µ, can be derived from the volt– sec (Fig. 7) product of the inductor L1 (or L2 ) and leakage inductor Llk , as follows: Vin = VFuelCell ,

Vc = Vclamp

Vin (1 − D1 ) = (Vc − Vin )D1 Vc − nVout nVout D1 = D2 Llk Llk Ilk,peak =

III. DESIGN METHODOLOGY FOR THE PROPOSED DC–DC CONVERTER

Vc − nVout × D1 Ts Llk 1 µ= , (1−D)n (1−D)2 n2 + + F 2 4

In this section, the detailed design process of the proposed converter is explained for practical applications and a design example is illustrated based on a 500-W rated converter [5]–[10]. A. Voltage Conversion Ratio Fig. 7 shows that the current flowing through the leakage inductor (transformer primary winding current) flows in a dis-

F =

(9)

1 Vout 1 × × Rout n D1 + D2

=

Fig. 6. (Contd.) Detailed circuit operations according to the operational modes. (e) Mode 5 (t4 < t < t5 ). (f) Mode 6 (t5 < t < t6 ). (g) Mode 7 (t6 < t < t7 ).

(8)

Llk . Ts Rout

(10)

(11)

B. Dead Time for ZVS The delay time between the main switch (dead time) and the auxiliary switch is a critical parameter for ensuring the ZVS operations (Fig. 8). To achieve ZVS for the main switch, it must be turned on during t6 –t7 after the auxiliary switch has been turned off. The optimum value of this delay should be selected

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so as not to exceed one-quarter of the resonant period formed by Llk and CM 1 . The delay time can be calculated by the following equation: Tdelay,QA 1 −QM 1 =

π Llk CM 1 . 2

(12)

The auxiliary switch must be turned on during t2 –t3 after the main switch has been turned off. Therefore, the value of the delay time for the auxiliary switch should be less than onequarter of the resonant period, which is decided by Llk and Cclamp , and can be calculated by the following equation: Tdelay,QM 1 −QA 1 =

π Llk Cclamp . 2

(13)

Fig. 9.

Block diagram of the proposed current-fed half-bridge converter.

C. Leakage Inductor (Llk ) To obtain the condition of ZVS, the minimum value of Llk must sufficiently discharge the parasitic capacitor CA1 (or CA2 ), and then Llk can be derived as follows: ELl k ECA 1 2 Llk Ilk,peak

Llk

(at t5 )

2 CA1 VQM 1

CA1 (IL1 Z0 + nVout )2 . 2 IM 1,peak

(14) (15)

D. Clamp Capacitor (Cclamp ) The value of Cclamp should be selected so that one-half of the resonant periods formed by Cclamp and Llk exceed the maximum turn-off time of the main switches. This value is described by the following equation: (1 − D)Ts > π

Llk Cclamp

(1 − D) 2 T . π 2 Llk s

(16)

2

Cclamp >

(17)

Based on (1)–(17), a 500-W converter with an input voltage ranging from 28 to 43 Vdc (output voltage of the fuel cell) and an output voltage of 380 Vdc can be designed for practical applications by using the following parameters: Inductor 200 µH, leakage inductor 6 µH, clamp capacitor 4 µF, winding ratio 1:3.5, and switching frequency 50 kHz. As explained in the previous section, the proposed currentfed converter is superior to the voltage-fed converter in terms of its winding ratio. In this example, if it is implemented using a voltage-fed converter, the winding ratio should be 1:14, which causes the transformer to be enlarged. Fig. 9 shows the digital/analog controller of the proposed converter and this controller is mainly implemented with a TL494 PWM controller and an electrically programmable logic device (EPLD). The EPLD is designed to convert one gate pulse generated by the TL-494 to the four gate pulses required by the proposed converter. Also, to protect and restart the converter, the protection circuit is operated in the over-current and overvoltage modes.

Fig. 10.

Block diagram of the single-phase dc–ac inverter.

In the proposed fuel cell generation system, a conventional dc–ac inverter is implemented. The block diagram of the fullbridge dc–ac inverter including the DSP (TMS320C31) control block is shown in Fig. 10. The controller consists of a DSP board, a digital signal processor that processes the voltage feedback signal, and an I/O control unit. The output voltage of the inverter can be controlled so as to produce a constant voltage and constant frequency. The control technique of the inverter uses the instantaneous voltage control method and sinusoidal pulse width modulation (SPWM) [11]–[14]. IV. SIMULATION AND EXPERIMENTAL RESULTS The proposed system was simulated with PSIM 6.0 software and the prototype was fully built and tested with actual PEM fuel cell stack. In the simulation experiment, the dc–dc converter was operated at 50 kHz and the single-phase inverter was operated at 4.5 kHz. A resistor bank was connected to the output terminal of the single-phase inverter as a load. Fig. 11 shows the active clamping current-fed half-bridge converter and single-phase inverter. Table I shows the design parameters of the proposed fuel cell generation system. The simulation voltage and current waveforms of the proposed dc–dc converter are shown in Fig. 12. The main switches


Fig. 11.

Simulation schematic of the proposed fuel cell generation system.

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Fig. 13.

Output waveforms of the inverter (top: voltage; bottom: current).

Fig. 14.

Photograph of the fuel cell generation system.

TABLE I PARAMETERS OF FUEL CELL GENERATION SYSTEM

are successfully operated under the ZVS condition at the turn-on switching transients, as certified by the simulation results shown in Fig. 12. Output waveforms of the inverter in Fig. 13 are voltage and current in case a resistor load is connected to the inverter. As shown in Fig. 13, the voltage and current are well controlled in phase. In the experimental prototype, an actual fuel cell (1.2-kW Ballard Nexa Power Module), which is displayed in Fig. 14, is implemented in order to test the proposed converter and the overall system performance under consideration of the nonlinear V –I characteristics of the PEM fuel cell. The output voltage and current waveforms from the PEM fuel cell stack are depicted in Fig. 15. Fig. 16 shows the experimental result of the proposed dc–dc converter. Fig. 16(a) shows the voltage and current waveforms across the main switches and Fig. 16(b) shows the waveforms across the auxiliary switches. These results are in good agreement with the theoretical results. With the switching signals, it is noted that the voltages across all of the switches are clamped to the same value of the clamping capacitor (Cclamp ) and, with the help of the lossless snubber, the overshoot voltages of all of the switches could be successfully eliminated, as shown in Fig. 16. Fig. 12. Simulation voltage and current waveforms of the proposed currentfed half-bridge converter.

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Fig. 15. Experiment voltage and current waveforms of the Nexa fuel cell stack (top: voltage; bottom: current; 20 V/div., 4 A/div., 5 µs/div.). (Top to bottom: Gate signals, transformer primary voltage, transformer primary current, clamp capacitor current, main and auxiliary switch voltage, and main switch current).

Fig. 17. Expanded voltage and current waveforms of (a) the main switch and (b) the auxiliary switch (100 V/div., 10 A/div., 1 µs/div.).

Fig. 18. Fig. 16. Voltage and current waveforms of (a) the main switch and (b) the auxiliary switch (100 V/div., 10 A/div., 5 µs/div.).

Fig. 17 shows the ZVS operation capability of the main switch QM 1 , and the auxiliary switch QA1 . Due to this ZVS operation, the power loss of the converter can be dramatically reduced. Hence, the overall efficiency of the converter is increased. The performance of the dc–ac inverter can be confirmed with the experimental result as shown in Fig. 18. In this figure, a single-phase 60-Hz, 220-V ac output is realized and the voltage waveform is generated in a sinusoidal manner. The dynamic performance of the proposed system is examined according to the load variation. The load is changed as step function from 160 to 240 W and the voltage and current waveforms are investigated and summarized in Fig. 19. In this

Output waveforms of the inverter (200 V/div., 1 A/div., 5 ms/div.).

figure, even though the load is varied, the converter is robustly controlled and generates stable outputs. Moreover, the efficiency of the proposed converter is measured and the result is summarized in Fig. 20. In this figure, the proposed converter shows high-efficiency characteristics through the wide load ranges. Fig. 21 shows the theoretical and experimental comparison of the voltage conversion ratio at load 313 Ω. From this result, the theoretical analysis is well matched with the experimental result and it is supposed that the small difference is due to the errors of the circuit parameters, etc. Fig. 21 shows the theoretical and experimental comparison of the voltage conversion ratio at load 313 Ω. From this result, the theoretical analysis is well matched with the experimental result and it is supposed that the small difference is due to the errors of the circuit parameters, etc.


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V. CONCLUSION In this paper, an active clamping current-fed half-bridge dc– dc converter was proposed for PEM fuel cell applications. In particular, compared with the previous converter topologies, the proposed converter is actually tested under a real fuel cell stack, so that the dynamic performance can be examined with consideration of V –I characteristics of fuel cell. Therefore, it is highly expected that the proposed converter can be utilized in high-efficiency and high-performance fuel cell applications. REFERENCES

Fig. 19. Output and input waveforms according to variation of the loads from 160 to 240 W (time: 0.5 µs/div.). (a) Converter output voltage (100 V/div.). (b) Converter output current (0.5 A/div.). (c) Fuel cell output current (2 A/div.). (d) Fuel cell output voltage (10 V/div.).

Fig. 20.

Measured efficiency of the proposed converter by fuel cell.

[1] B. K. Lee, D. W. Yoo, J. Hur, G. H. Rim, and C. Y. Won, “Cost effective power conversion topologies for fuel cell applications,” presented at the 2004 Fuel Cell Seminar, San Antonio, TX. [2] J. T. Kim, B. K. Lee, T. W. Lee, S. J. Jang, S. S. Kim, and C. Y. Won, “An active clamping current-fed half-bridge converter for fuel-cell generation systems,” in Conf. Rec. IEEE-PESC, 2004, pp. 4709–4714. [3] S. K. Han, H. K. Yoon, G. W. Moon, M. J. Youn, and Y. H. Kim, “A new active clamping non-dissipative snubber for ZVS PWM current-fed half-bridge converter,” presented at the EPE, Toulouse, France, 2003. [4] 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,” in Conf. Rec. IEEE Power Electron. Transp., 1998, pp. 47–51. [5] G. Ivensky, I. Elkin, and S. Ben-Yaakov, “An isolated DC–DC converter using two zero current switched IGBTs in a symmetrical topology,” in Conf. Rec. IEEE-PESC, 1994, pp. 1218–1225. [6] K. Morimoto, N. A. Ahmed, H. W. Lee, and M. Nakaoka, “A novel type of high-frequency transformer linked soft-switching PWM DC-DC power converter for large current applications,” J. Electr. Eng. Technol., vol. 1, no. 2, pp. 216–225, 2006. [7] R. Watson and F. C. Lee, “A soft-switched full-bridge boost converter employing an active-clamp circuit,” in Conf. Rec. IEEE-PESC, 1996, pp. 1948–1954. [8] F. J. Nome and I. Barbi, “A ZVS clamping mode-current-fed push-pull DC–DC converter,” in Conf. Rec. IEEE-ISIE, 1998, pp. 617–621. [9] S. W. Lee, S. R. Lee, and C. H. Jeon, “A new hig efficient bi-directional DC–DC converter in the dual voltage systems,” J. Electr. Eng. Technol., vol. 1, no. 3, pp. 343–350, 2006. [10] J. Wang, P. Z. Peng, J. Anderson, A. Joseph, and R. Buffenbarger, “Low cost fuel cell converter system for residential power generation,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1315–1322, Sep. 2004. [11] T. A. Nergaard, J. F. Ferrell, L. G. Leslie, and J. S. Lai, “Design considerations for a 48V fuel cell to split single phase inverter system with ultracapacitor energy storage,” in Conf. Rec. IEEE-PESC., 2002, pp. 2007–2012. [12] A. M. Tuckey and J. N. Krese, “A low-cost inverter for domestic fuel cell applications,” in Conf. Rec. IEEE-PESC, 2002, pp. 339–346. [13] C. Jeraputra, I. H. Hwang, S. W. Choi, E. C. Aeloiza, and P. N. Enjeti, “An improved anti-islanding algorithm for utility interconnection of multiple distributed fuel cell powered generations,” J. Electr. Eng. Technol., vol. 1, no. 2, pp. 192–199, 2006. [14] F. Santi, D. Franzoni, A. Monti, D. Patterson, F. Pconci, and N. Barry, “A fuel cell based domestic uninterruptible power supply,” in Conf. Rec. IEEE-APEC, 2002, pp. 605–613.

Su-Jin Jang (S’04) was born in Korea, in 1976. He received the M.S. degree from Sungkyunkwan University, Suwon, Korea, in 2004, in energy system engineering. Currently, he is working toward the Ph.D. degree at the Department of Mechatronics Engineering, Sungkyunkwan University. His research interests include fuel cell generation system and regeneration inverter system. Fig. 21.

Voltage conversion ratios of the proposed converter.

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Chung-Yuen Won (S’85–M’88–SM’05) was born in Korea, in 1955. He received the B.S. degree from the SungKyunKwan University, Suwon, Korea, in 1978, and received the M.S. and Ph.D. degrees from the Seoul National University, Seoul, Korea, in 1980 and 1988, respectively, all in electrical engineering. From 1990 to 1991, he was a Visiting Professor at the Department of Electrical Engineering, the University of Tennessee, Knoxville. Since 1988, he has been with the faculty of SungKyunKwan University, where he is a Professor at the School of Information & Communication Engineering. His research interests include dc–dc converters for fuel cells, electromagnetic modeling and prediction for motor drive, and control systems for rail power delivery applications.

Byoung-Kuk Lee (S’97–M’02–SM’04) received the B.S. and M.S. degrees from Hanyang University, Seoul, Korea, in 1994 and 1996, respectively, and the Ph.D. degree from Texas A&M University, College Station, in 2001, all in electrical engineering. During 2002, he was a Postdoctoral Research Associate with the Power Electronics and Motor Drives Laboratory and Advanced Vehicle Systems Research Program, Texas A&M University. From 2003 to 2005, he was a Senior Researcher at Power Electronics Group, Korea Electrotechnology Research Institute, Changwon, Korea, where he was engaged in fuel cell generation systems. Since 2006, he has been an Assistant Professor at the School of Information and Communication Engineering, Sungkyunkwan University, Suwon, Korea. His current research interests include sensorless drives for high-speed PM motor drives, power conditioning systems for fuel cells, modeling and simulation, and power electronics. Since 2000, he has been a Reviewer for the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, the IEEE TRANSACTIONS ON POWER ELECTRONICS, the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, the IEEE TRANSACTIONS ON ENERGY CONVERSION, the Institute of Electrical Engineers Electronics Letters, and the Proceedings of the Electric Power Applications. Prof. Lee is a Member of the IEEE Industry Applications Society (IAS) Industry Drive Committee and Industry Power Converter Committee. He is also the General Secretary of the International Conference on Electric Machines and Systems (ICEMS) 2007.


Jin Hur (S’93–M’98–SM’03) received the Ph.D. degree in electric engineering from Hanyang University, Seoul, Korea, in 1999. From 1999 to 2000, he was a Postdoctoral Research Associate at the Department of Electric Engineering, Texas A&M University, College Station. From 2000 to 2001, he was a Research Professor of electrical engineering for BK21 projects, Hanyang University. Currently, he is a Managerial Researcher at the Intelligent Mechatronics Research Center, Korea Electronics Technology Institute, Puchon, Korea, where he is engaged in the development of special electric machine and systems. He is the author of more than 100 publications in electric machine design, analysis and control, and power electronics. He is the holder of 10 granted and pending Korea patents, one patent pending in the United States, and one in Japan. His current research interests include high-performance electrical machines, modeling, drives, new concept actuator for special purpose, and power conditioning systems for fuel cells. He is also a Reviewer for the IEEE TRANSACTIONS ON ENERGY CONVERSION, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, IEEE TRANSACTIONS ON POWER SYSTEM, and Power Engineering Society Letters.