Journal of Electrical Engineering & Technology Vol. 5, No. 2, pp. 319~328, 2010 319 An Optimal Design Methodology of an Interleaved Boost Converter for Fuel Cell Applications Gyu-Yeong Choe*, Jong-Soo Kim*, Hyun-Soo Kang** and Byoung-Kuk Lee† Abstract – In this paper, an optimal selection methodology for the number of phases will be proposed for an interleaved boost converter (IBC). Also, the analysis of the input current ripple according to CCM and DCM is carried out. The proposed design methodology will be theoretically analyzed, and its validity verified by simulation as well as with experimental results. Moreover, a comparison of cost and efficiency based on a 600W laboratory prototype using the Ballard NEXA 1.2kW PEMFC system is demonstrated. Keywords: Optimal Design Methodology, Interleaved Boost Converter, Fuel Cell Converter, Ripple Analysis 1. Introduction Recently, the interest in fuel cells has been increasing due to the potential exhaustion of fossil fuel, concerns over global warming and increases in power demands. An annual meeting for fuel cell systems has been initiated by the U.S Department of Energy (DOE), and this has resulted in much information relating to the increase of fuel cell projects [1]-[2]. These fuel cell systems mainly consist of a fuel cell stack, an EBOP (Electrical BOP), and a MBOP (Mechanical BOP). In an EBOP, a PCS (Power Conditioning System) plays an important role in order to deliver high quality and stable power from the fuel cell to various loads. The PCS of a fuel cell system is composed of a dc-dc converter and a dcac inverter. In fuel cell systems designed for small/medium power, the dc-dc converter topologies are found to be either a full-bridge, a half-bridge or a push-pull type [3]. However, in high power fuel cell systems, such as those that operate over 250kW MCFC, the dc-dc converter needs to handle a much higher current, therefore an interleaved boost converter (IBC) is considered a good solution with respect to efficiency and current distribution. As mentioned above, the IBC has been much studied due to its merits, such as a high efficiency, a high current distribution, a low ripple reduction and a small component capacity. Previous research on the IBC can be summarized as follows: In order to decrease the input current ripple and improve the inductor utilization, a coupling inductor is used [4]-[5]. Power factor correction has been studied employing an IBC [6]-[8]. An IBC was operated by DCM (Discontinuous Conduction Mode) in order to reduce the inductor size and to improve efficiency as a result of a de† Corresponding Author: School of Information and Communication Engineering, Sungkyunkwan University, Korea. (bkleeskku@skku.edu) * School of Information and Communication Engineering, Sungkyunkwan University, Korea. ** Korea Railroad Research Institute (hanmin@krri.re.kr, cmlee@krri.re.kr, gdkim@krri.re.kr) Received : January 13, 2010; Accepted : March 31, 2010 crease in diode reverse recovery loss [8]. Also, there has been research into improving the control performance of the converter by the analysis of a small signal model [9] and by analyzing the input current ripple and output voltage ripple according to inductor coupling [10]-[12]. Current distribution control for solving the current imbalance of each phase has also been studied [13]-[14]. For the optimal design of an IBC, based on these previous works, the decision on the number of phases needs to be considered in the first design stages of the IBC. Therefore, in this paper, an optimal selection methodology for the number of phases is proposed for the IBC with respect to the input current and the output voltage ripples, taking into account the duty ratio at the CCM and the DCM, the switching frequency, and the inductance. Also, design guidelines to select the passive components of the IBC, such as the inductor and the capacitor are presented. The proposed design methodology is theoretically analyzed, and its validity is verified by simulation and experimental results based on a 600W laboratory prototype using the Ballard NEXA 1.2kW PEMFC system. 2. Ripple Analysis of Interleaved Boost Converters 2.1 Configuration and Operation Principle of the IBC Fig. 1 shows the circuit configuration of a 3-phase IBC Fuel Cell iin Vin i3 L3 D3 i2 L2 D2 i1 L1 D1 SW3 SW 2 SW1 iO C Vo LOAD Fig. 1. Configuration of 3-phase interleaved boost converter. 320 An Optimal Design Methodology of an Interleaved Boost Converter for Fuel Cell Applications where it can be seen that the inductors, switches and diodes are connected in parallel and then to a single output capacitor. Each phase’s switching frequency is identical and each switch has the same phase shift angle, namely, 360°/N. According to the duty ratio, the switching sequences of each phase can either be overlapped or not, resulting in variations of the input current ripple; it is evident that the input current ripple frequency will be N times the frequency of the inductor current. The analysis of the current and voltage ripples according to the duty ratio is derived under the following assumptions: 1) The resistances of the inductors and capacitors are negligible. 2) Stray inductances and capacitances are negligible. 3) The switches are ideal. The input current ripple of the single-phase boost converter is expressed by multiplying the time by the rising slope, or the falling slope, of the inductor current. The rising slope of the inductor current in the boost converter, and the falling slope of the inductor current are expressed by (1) and (2). (1) VoT DD ' L 360° OFF Switching Pattern ON S2 S3 I1 Inductor Current D I2 D I3 D' T d tr Iin d' ΔIin τ ΔVout Vo Output Voltage t1 t2 t0 (a) 0° 240° 120° S1 ON Switching Pattern 360° OFF S2 S3 D Inductor Current Input Current Output Voltage (2) t4 t5 t3 (b) where, D is the ON duty ratio of the switch, D ' is the OFF duty ratio of the switch, and iL is the inductor current. The input current ripple of the single-phase boost converter is expressed as shown in (3) [15]. ΔI in = 240° 120° S1 Input Current 2.2 Analysis of the Input Current Ripple at CCM diL Vin = dt L diL Vin − Vo − DVin = = dt L LD ' 0° (3) Unlike the single-phase boost converter, the slope of the input current ripple of a 3-phase IBC consists of three parts according to the duty ratio. Fig. 2 depicts the input current and the output voltage waveforms according to the duty ratio where τ is the period of the input current, d and d ' are the rising and falling period of the input current , ret spectively ( d = r , d ' = 1 − d ). τ In the case of 0<D<0.33, as shown in Fig. 2(a), only one of the inductor currents is increased during t0 ~ t1 because only one switch is turned on. As all the switches are turned off during t1 ~ t2 , the inductor currents and the input currents are seen to decrease. In the other cases, such as 0.34<D<0.66 and 0.67<D<1, the variation of the input current ripple can be explained in similar ways. As a result, 0° 240° 120° S1 360° ON Switching Pattern OFF S2 S3 D Inductor Current Input Current Output Voltage t6 t7 t8 (c) Fig. 2. Input current and output voltage waveforms of 3phase IBC according to duty ratio: (a) 0<D<0.33 (b) 0.34<D<0.66 (c) 0.67<D<1. the input current ripple can be expressed by (4), (5) and (6) according to their duty ratios, and equations (7) and (8) can be generalized by using (4), (5) and (6) as the functions of input and output voltages, respectively. Gyu-Yeong Choe, Jong-Soo Kim, Hyun-Soo Kang and Byoung-Kuk Lee ΔI in = Vin ⎛ 1 − 3D ⎞ T d (0 < D < 0.33) L ⎜⎝ D ' ⎟⎠ N (4) Vin ⎛ 2 − 3D ⎞ T d (0.34 < D < 0.66) L ⎜⎝ D ' ⎟⎠ N V ⎛ 3 − 3D ) ⎞ T ΔI in = in ⎜ d (0.67 < D < 1) L ⎝ D ' ⎟⎠ N ΔI in = ⎛ N on _ sw − ND ⎞ T ⎜⎜ ⎟⎟ d D' ⎝ ⎠N V T ΔI in = o N on _ sw − ND d L N ΔI in = Vin L ( be depicted as shown in Fig. 4, and can be expressed by (10) [12]. { (5) (6) (7) 321 QC = ( N off _ sw + 1) I L − I o } TN d ' (10) Vo Vo is the output current, I L = is the RD ' N R average current of the inductor, and N off _ sw is the number where, I o = of OFF switches during τ . ) (8) Equation (11) represents the charge of the output current ripple, and can be calculated using (12). where, N is the number of the phase, T is the switching period, and N on _ sw is the number of ON switches during τ . Fig. 3 shows the variation of the input current ripple according to the duty ratio under the same inductors. The input current ripple becomes zero at D/N times duty (for example, at 0.33, 0.66 duty in 3ph-IBC), and is proportionally reduced according to the increase in the number of phases. In the case of an even number of phases, the input current ripple becomes zero at a 0.5 duty ratio in common and above 3-phase, it is considerably reduced compared with a single-phase boost converter. Input c urrent ripple ac c ording to duty ratio QC = TVo dd ' (11) N 2 RD ' Q TVo dd ' Δvo = C = C RCN 2 D ' (12) Fig. 5 shows the variation of the output voltage ripple according to the number of phases under the same capacitance. From (12), it is noted that the output voltage ripple is decreased by 1/N2 times according to the increase in the number of phases, and it becomes zero at specific duty ratios (at the D/N times). Therefore, in the case of the IBC 1 1 -P h 2 -P h 3 -P h 4 -P h 5 -P h 0.9 0.8 Input current ripple 0.7 V o C o n s ta n t IB C IB C IB C IB C IB C ( N off _ sw + 1) I L Io QC 0.6 dτ Io 0.5 0.4 τ 0.3 t 0.2 0.1 0 Fig. 4. Charge of output voltage. 0 0.1 0.2 0.3 0.4 0.5 0.6 Duty ratio 0.7 0.8 0.9 1 Output voltage ripple acc ording to duty ratio 0.35 Fig. 3. Input current ripple variation according to duty ratio. 1-Ph IBC 2-Ph IBC 3-Ph IBC 4-Ph IBC 5-Ph IBC 0.3 2.3 Analysis of the Output Voltage Ripple at CCM Ic = I L − Io (9) where, I c is the capacitor current, and I o is the average output current. The charge of the output capacitor, QC , can Output voltage ripple 0.25 The output voltage ripple is determined by the output current charge (Q), which is caused by the charging/discharging currents of the capacitor. Even though the shape of the output voltage of the N-phase IBC is different, the average charge is identical. The capacitor current can be obtained by (9) which is the difference between the average current of the inductor and the average output current. Vo Constant 0.2 0.15 0.1 0.05 0 0 0.1 0.2 0.3 0.4 0.5 0.6 Duty ratio 0.7 0.8 0.9 1 Fig. 5. Variation of output voltage ripple according to duty ratio. 322 An Optimal Design Methodology of an Interleaved Boost Converter for Fuel Cell Applications with multiple phases, the output voltage ripple becomes dramatically reduced compared with a conventional singlephase boost converter. 0° 240° 120° S1 360° OFF Switching Pattern S2 2.4 Analysis of the Input Current Ripple at DCM S3 The rising slope of inductor current at the DCM is same as the rising slope of inductor current of single boost converter at the CCM. However, the falling slope of inductor current is different and is expressed by (13). − DVin diL Vin − Vo = = dt L L(t RF − D) ON (13) Inductor Current t RF Input Current ΔI in ΔVout Output Voltage (a) where, t RF is the duration of the inductor current. Using (13), the input current ripple of the single-phase boost converter at the DCM is represented by (14). VT ΔI in = o D (t RF − D) L 0° S1 Switching Pattern (14) Inductor Current Input Current Output Voltage (b) 0° 120° S1 Vin ⎛ 2t RF − 3D ⎞ T ⎜ ⎟ d L ⎝ (t RF − D) ⎠ N (16) D S3 Input Current Output Voltage (c) 0° (17) S1 Switching Pattern 120° ON 240° OFF S2 D S3 (18) Inductor Current Input Current • Case of constant falling slope at 0.67<D<1 V ⎛ 2t − 3D ⎞ T ΔI in = in ⎜ RF ⎟ d' L ⎝ (t RF − D) ⎠ N S2 Inductor Current • Case of constant rising slope at 0.34<D<0.66 ΔI in = 360° OFF (15) • Case of constant falling slope at 0.34<D<0.66 V ⎛ t − 2D ⎞ T ΔI in = in ⎜ RF ⎟ d' L ⎝ (t RF − D) ⎠ N 240° ON Switching Pattern • Case of constant rising slope at 0<D<0.33 V ⎛ t − 3D ⎞ T ΔI in = in ⎜ RF ⎟ d L ⎝ (t RF − D) ⎠ N ON S2 • Case of constant falling slope at 0<D<0.33 ⎛ −2 D ⎞ T ⎜ ⎟ d' ⎝ (t RF − D) ⎠ N 360° OFF S3 As shown in Fig. 6, the input current ripple can be categorized as five different cases according to duty ratio and shape of rising and falling slope. Therefore, the input current ripple should be calculated at a constant slope to obtain accurate input current ripple. As a result, the input current ripple at the DCM can be expressed by (15)-(19) according to duty ratio. V ΔI in = in L 240° 120° (19) Output Voltage (d) 360° Gyu-Yeong Choe, Jong-Soo Kim, Hyun-Soo Kang and Byoung-Kuk Lee 0° S1 Switching Pattern 240° 120° Input c urrent ripple ac c ording to duty ratio 360° ON 323 1.5 1-Ph IBC: L 2-Ph IBC: L/2 3-Ph IBC: L/3 4-Ph IBC: L/4 5-Ph IBC: L/5 OFF S2 Input current ripple S3 Inductor Current Input Current 1 0.5 Output Voltage 0 (e) The calculation method of output voltage ripple at DCM is almost same as method at CCM. Equation (20) represents the output voltage ripple at DCM. { Vo d ' T t RF N off − (t RF − D) N Q Δvo = C = C RC (t RF − D) N 2 } (20) 2.5 Design of the Inductor and Capacitor Elements at the CCM The input current ripple at the CCM of the IBC is the sum of each inductor current. Therefore, a specific design guideline is needed for selecting the inductor, compared to the conventional single-phase boost converter. From (8), the input current ripple of the IBC is inversely proportional to the number of phases, and it can be noted that during the duty ratio 0.4<D<0.7, the inductor at the L/N times is used and the input current ripple becomes almost the same as the single phase boost converter. Therefore, the inductor of the IBC can be designed by using the L/N times. Fig. 7 shows the variation of the input current ripple for the N-phase IBC with 1/N times the inductance. On the other hand, during the duty ratio 0.4<D<0.7, the output voltage ripple is almost identical to the single-phase boost converter. With the same capacitance, in the IBC, the output voltage ripple is decreased by 1/N2 times from (12). Therefore, in the IBC, the output capacitor can be reduced by 1/N2 times. Fig. 8 shows the variation of the output voltage ripple for the N-phase IBC with 1/N2 times capacitance. 0.1 0.2 0. 3 0.4 Duty ratio 0.5 0.6 0.7 0.8 Fig. 7. Variation of input current ripple according to inductance. Output voltage ripple according to capacitor 0.45 1-Ph IBC: C 2-Ph IBC: C/4 3-Ph IBC: C/9 4-Ph IBC: C/16 5-Ph IBC: C/25 0.4 0.35 Output voltage ripple Fig. 6. Input current and output voltage waveforms of 3phase IBC according to duty ratio at DCM: (a) Case of constant falling slope at 0<D<0.33 (b) Case of constant rising slope at 0<D<0.33 (c) Case of constant falling slope at 0.34<D<0.66 (d) Case of constant rising slope at 0.34<D<0.66 (e) Case of constant falling slope at 0.67<D<1. 0 0.3 0.25 0.2 0.15 0.1 0.05 0 0 0.1 0.2 0. 3 0.4 Duty ratio 0.5 0.6 0.7 0.8 Fig. 8. Variation of output voltage ripple according to capacitance. 3. Design Example of the IBC Based on the analysis presented in section II, a design example of the IBC for fuel cell power generation is explained as follows where the design specification is that output voltage of fuel cell stack is 27V~40V, the output voltage of the IBC is 90V, and the power rating is 600W. The duty ratio of the IBC can be calculated as 0.44~0.7 from (21). D = 1− Vin Vo (21) Considering input current ripple and output voltage ripple during this specific duty ratio, one can select 2-phase, 3-phase, and 4-phase as shown in Figs. 9 and 10. For a the fuel cell normally operated fuel cell at the rated power, the 3-phase and 4-phase IBCs have a lower minimum input current ripple than the 2-phase IBC. An Optimal Design Methodology of an Interleaved Boost Converter for Fuel Cell Applications 324 Consequently, for this application, 3-phase IBC can be selected with respect of cost reduction. Input current ripple variation (0.44<D<0.7) 1.5 Input current ripple 1-Ph IBC: L 2-Ph IBC: L/2 3-Ph IBC: L/3 4-Ph IBC: L/4 5-Ph IBC: L/5 1 0.5 Rating Power 0 0.45 0.5 0.55 Duty ratio 0.6 0.65 0.7 Fig. 9. Variation of input current ripple during 0.44<D<0.7. Output voltage ripple variation (0.44<D<0.7) 0.3 Output voltage ripple 0.25 1-Ph IBC: C 2-Ph IBC: C/4 3-Ph IBC: C/9 4-Ph IBC: C/16 5-Ph IBC: C/25 0.2 0.15 0.1 0.05 Rating Power 0 0.45 0.5 0.55 Duty ratio 0.6 0.65 0.7 Fig. 10. Variation of output voltage ripple during 0.44<D<0.7. error signal becomes the current reference through the voltage PI controller. The final reference is generated after the same sequence, and then compared with a 120° shifted saw-toothed waveform, from which the 120° shifted PWM of the 3-phase IBC is generated. The EPLD makes the 120° shifted PWM of the 3-phase IBC. Figs. 12 and 13 represent the simulation and experimental results of the 3-phase IBC, respectively, at 0.5, 0.6, and 0.66 duty ratios. As expected from Fig. 9, the input current ripple is reduced, and it is dramatically decreased at the 0.66 duty ratio. Fig. 14 represents the experimental results of the 3-phase IBC applied to the proposed optimal design methodology where the voltage of the fuel cell is 27V and current is at 23A and the output voltage and current of the 3-phase IBC is 90V and 6.6A respectively. Fig. 15 shows the simulation and experimental results of the inductor and input current ripple at 600W. Compared with Figs. 15(a) and (b), it is noted that the simulation results have a good agreement with the experimental ones. Finally, in order to certify the validity of the proposed analysis and design methodology, all the results from the theoretical analysis, simulations and experimental studies have been compared, as shown in Fig. 16. Using a YOKOGAWA WT3000 Power Analyzer, the efficiency of the 3-phase IBC is also measured and compared with the single-boost converter under several operational conditions and the results are summarized in Fig. 17. As the result of the efficiency comparison, the efficiency of the 3-phase IBC is about 2% higher than that of the single-phase boost converter. Fig. 18 shows the efficiency map according to the each duty ratio in case of the singlephase boost converter and the 3-phase IBC. Table II shows the estimated cost of the 3-phase and the single phase boost converter. As shown in Table 2, the estimated total cost of the 3phase IBC is more economical than that of the single boost converter. From these comparisons, it is possible to verify that the analysis and the proposed design methodology has been well developed and can be utilized for the various applications of designing and specifying the required IBCs. 4. Simulation and Experimental Results In order to verify the analysis of the input ripple current for the IBC, simulations and experimental studies are performed. A PSIM 6.0 is used as the simulation tool, with the system parameters shown in Table 1. A Ballard NEXA 1.2kW PEMFC system is utilized to implement the experimental testbed. Fig. 11 shows the 600W rated 3-phase IBC laboratory. The controller of the 3-phase IBC consists of an inner current loop and an outer voltage loop configuration. The outer voltage loop controls the voltage level to stay constant at 90V. First, the output voltage is sensed and this becomes the real voltage in the controller, and then it is compared with the voltage reference to generate the voltage error. This 3-phase IBC Control board Fig. 11. Experimental testbed of 3-phase IBC. Gyu-Yeong Choe, Jong-Soo Kim, Hyun-Soo Kang and Byoung-Kuk Lee (a) 325 (b) (c) △Iin = 0.2A △Iin = 0.315A △Iin = 0.04A Duty ratio = 0.5 Duty ratio = 0.6 Duty ratio = 0.66 Fig. 12. Simulation waveforms of ripple variation of the input current: (a) 0.5 duty ratio (b) 0.6 duty ratio (c) 0.66 duty ratio (250mA/div, 50us/div). (a) (b) (c) △Iin = 0.334A △Iin = 0.218A △Iin = 0.05A Duty ratio = 0.6 Duty ratio = 0.5 Duty ratio = 0.66 Fig. 13. Experimental waveforms of ripple variation of the input current: (a) 0.5 duty ratio (b) 0.6 duty ratio (c) 0.66 duty ratio (200mA/div, 20us/div). Input current Table 1. Simulation Parameter Parameter Value Fuel Cell Voltage 23-40 [V] Output Voltage of 3-phase IBC 90 [V] Rating Power 600 [W] Switching Frequency 20 [kHz] Inductor 1.19 [mH] Capacitor 940 [uF] Inductor current Fuel Cell Voltage (a) Simulation waveforms Fuel Cell Current 23A Converter Output Voltage 7.6A Input current (500mA/div, 20us/div) Inductor current (1A/div, 20us/div) Converter Output Current Fig. 14. Current and voltage waveforms of 600W rated 3phase IBC (Above: 10V/div., 10A/div., 20us/div. Below: 50V/div., 5A/div., 20us/div.). (b) Experimental waveforms Fig. 15. Input current and inductor current waveforms. An Optimal Design Methodology of an Interleaved Boost Converter for Fuel Cell Applications 326 Input current ripple 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Experiment 1ph Simulation 1ph Theory 1ph Experiment 3ph Simulation 3ph Theory 3ph 0 0.05 0.1 0.15 0.2 0.25 0.3 0.33 0.35 0.4 0.45 Duty ratio 0.5 0.55 0.6 0.66 0.7 0.75 0.8 Fig. 16. Comparison of theory, simulation, and experimental results. Efficiency Efficiency 100 Efficiency 100 98 100 3-phase IBC 98 3-phase IBC 96 94 96 94 92 Single-phase Boost 90 92 90 90 88 88 86 86 86 84 84 Duty = 0.74 80 60 120 180 240 300 360 420 480 540 84 82 82 Duty = 0.5 80 600 Single-phase Boost 94 Single-phase Boost 92 88 82 3-phase IBC 98 96 60 120 180 240 Power[W] 300 360 420 480 540 Duty = 0.35 80 600 60 120 Power[W] (a) 180 240 300 360 420 480 540 600 Power[W] (b) (c) Fig. 17. Efficiency comparison: (a) Duty = 0.74 (b) Duty = 0.5 (c) Duty = 0.35. Table 2. Cost Comparison Efficiency 100 98 3-phase IBC 96 94 92 Cost (Unit: US$) Inductor 3 15 Diode 3 2.19 Switch 3 Single-phase Boost 90 88 86 Single-phase Boost Converter Quantity Quantity Cost (Unit: US$) Inductor 1 14.29 Diode 1 5.75 13.54 Switch 1 22.5 1 6.17 4 48.7 84 82 80 60 120 180 240 300 D=0.35 D=0.5 360 Power[W] 420 480 540 D=0.74 600 (a) Efficiency 100 Capacitor 1 6.17 Capacitor Total 10 36.9 Total 98 96 Source - http://dkc1.digikey.com/kr/digihome.html 94 3-phase IBC 92 90 88 5. Conclusion 86 84 82 80 60 120 180 240 300 D=0.35 D=0.5 360 Power[W] 420 480 540 D=0.74 600 (b) Fig. 18. Efficiency mapping according to duty ratio: (a) Single-phase boost converter (b) 3-phase IBC. This paper has presented a detailed analysis and an optimal design methodology for effectively utilizing interleaved boost converters in fuel cell applications. Based on the proposed method, the IBC with a minimum number of switches and other parasitic components can be designed and successfully operated to minimize input current and output voltage ripples. Gyu-Yeong Choe, Jong-Soo Kim, Hyun-Soo Kang and Byoung-Kuk Lee Therefore, the proposed design methodology can be used in various applications that use fuel cells and photovoltaic devices with broad system specifications from low to high power situations. References J. M. Carrasco et al., “Power-electronic systems for the grid integration of renewable energy sources: A survey,” IEEE Trans. Ind. Electron., Vol. 53, Issue 4, pp. 1002-1016, Jun., 2006. [2] U.S Department of Energy, Summary of annual merit review fuel cells subprogram, [Online]. Available: http://www.hydrogen.energy.gov/pdfs/review07/4207 2-05_fuel_cells.pdf [3] S. J. Jang, T. W. Lee, K. S. Kang, S. S. Kim, C. Y. Won, “A new active clamp sepic-flyback converter for a fuel cell generation system,” in Proc. Ind. Electron. 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Enjeti, “Design of a wide input range DC-DC converter with a robust power control scheme suitable for fuel cell power conversion,” IEEE Trans. Ind. Electron., Vol. 55, Issue 3, pp. 1247-1255, March, 2008. [14] C. T. Pan, Y. H. Liao, “Modeling and control of circulating currents for parallel three-phase boost rectifiers with different load sharing,” IEEE Trans. Ind. Electron., Vol. 55, Issue 7, pp. 2776-2785, July, 2008. [15] N. Mohan, T. M. Undeland, W. P. Robbins, “Power electronics converter,” application and design, 3rd ed. JOHN WILEY & SONS, INC., ch.7, 2003 Gyu-Yeong Choe received the M.S. degrees from Sungkyunkwan University, Suwon, Korea, in 2008, in electrical engineering. From 2008, he is working toward the Ph.D. degree at electrical engineering, Sungkyunkwan University. His research interests include renewable energy source modeling, renewable energy hybrid system, and battery charger for PHEV/EV and interleaved dc-dc converters. Jong-Soo Kim received the B.S. degrees from Seoul National University of Technology, Seoul, Korea, in 2006, and received the M.S. degree from Sungkyunkwan University, Suwon, Korea, in 2008, all in electrical engineering. From 2008, he is working toward the Ph.D. degree at electrical engineering, Sungkyunkwan University. His research interests include eco-friendly vehicle technologies, power conditioning systems for renewable energy and PM motor drives. Hyun-Soo Kang received the B.S. and the M.S. degrees from Hanyang University, Seoul, Korea, in 1994 and 1996, respectively, and the Ph.D. degree from Sungkyunkwan University, Suwon, Korea, in 2008, all in electrical engineering. From 1996 to 1999, he has been an Associate Research Engineer at Power Electronics Lab., LGIS R&D Center, Anyang, Korea. From 2000 he joins at ADT co., Ltd, and now he is a Principal Engineer in R&D Center, ADT co., Ltd. His research interests include sensorless drives for IM and PM motor drives, power conditioning systems for renewable energy sources and power electronics. 328 An Optimal Design Methodology of an Interleaved Boost Converter for Fuel Cell Applications Byoung-Kuk Lee received the B.S. and the M.S. degrees from Hanyang University, Seoul, Korea, in1994 and 1996, respectively and the Ph.D. degree from Texas A&M University, College Station, TX, in 2001, all in electrical engineering. From 2003 to 2005, he has been a Senior Researcher at Power Electronics Group, KERI, Changwon, Korea. From 2006 Dr. Lee joins at School of Information and Communication Engineering, Sungkyunkwan University, Suwon, Korea, as an Assistant Professor. His research interests include electric vehicles, sensorless drives for high speed PM motor drives, power conditioning systems for renewable energy, modeling and simulation, and power electronics. Prof. Lee is a recipient of Outstanding Scientists of the 21stCentury from IBC and listed on 2008 62nd Ed. of Who’s Who in America. and 2009 26th Ed. of Who's Who in the World. Prof. Lee is an Associate Editor in the IEEE Transactions on Industrial Electronics and is the IEEE Senior Member.