Dr.R.Seyezhai* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 7, Issue No. 2, 323 - 329
Design Consideration of Interleaved Boost Converter for Fuel Cell Systems Dr.R.Seyezhai Associate Professor, Department of EEE, SSN College of Engineering, Chennai , India. e-mail: seyezhair@ssn.edu.in
T
literature. But these topologies add objectionable ripples in the current flowing out of the fuel cell. To minimize the ripples, an IBC has been proposed as a suitable interface for fuel cells. In addition, interleaving provides high power capability, modularity and improved reliability [1]. A two-phase directly coupled IBC using CoolMOS and SiC diodes has been proposed in this paper compared to the conventional boost converter. The performance of IBC has been investigated over the operating range of the PEM fuel cell. The relationship between phase current ripple, input current ripple versus duty ratio has been analyzed. Mathematical analysis of overall current ripple and the design equations for IBC has been presented. The switching losses of IBC have been studied. Simulation of IBC interfaced with PEM fuel cells has been performed using MATLAB/SIMULINK. In order to get the desired value of output voltage at the interleaved boost converter stage and to get a regulated voltage at the output, a simulation study of PI controller has been studied. Hardware prototype has been built to validate the results.
A
ES
Abstract— Proton Exchange Membrane (PEM) fuel cell is one of the promising technologies for the distributed power generation. For designing high efficiency fuel cell power systems, a suitable DC-DC converter is required. Among the various topologies, Interleaved Boost converter (IBC) is considered as a better solution for fuel cell systems due to improved electrical performance, reduced weight and size. Detailed analysis has been done to investigate the benefits of interleaved boost converter compared to the conventional boost converter. The design equations for IBC have been presented. In this paper, a two phase interleaved boost converter employing CoolMOS transistor and SiC diode has been suggested for fuel cells, considering variation of input voltage, inductor and switching frequency. Simulation study for the closed loop control (PI) of IBC interfaced with fuel cells has been studied using MATLAB. Theoretical analysis and hardware prototype has been performed to validate the results. Key words — PEM fuel cells, IBC, ripple, PI controller. I.
INTRODUCTION
IJ
Fuel cell is one of the most important sources of distributed energy because of its high efficiency, high energy density, plus high reliability and long life due to few moving parts. Comparison with the other types of fuel cells, PEM fuel cell shows charming attraction with its advantages such as low temperature, high power density, fast response and zero emission. Fuel cells operate at low DC voltages (typically 600 mV per cell) and therefore a number of cells are connected in series. As a long string of cells is difficult to operate, DC-DC boost converter is generally used to further boost the voltage to the required level. Various topologies such as boost, buck and series resonant full-bridge and pushpull converters have been proposed in the
ISSN: 2230-7818
II. INTERLEAVED BOOST CONVERTER A two-phase interleaved boost converter is usually employed in high input-current and high input-to-output voltage conversion applications. It is used to eliminate reverse-recovery losses of the boost rectifier by operating the two boost converters at the boundary of continuous conduction mode (CCM) and discontinuousconduction mode (DCM) so that the boost switches are turned on when the current through the corresponding boost rectifier is zero. In addition, interleaving is also employed to reduce
@ 2011 http://www.ijaest.iserp.org. All rights Reserved.
Page 323
Dr.R.Seyezhai* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 7, Issue No. 2, 323 - 329
III. DESIGN CONSIDERATIONS OF IBC The interleaved boost converter design [3,4] involves the selection of the number of phases, the inductors, the output capacitor, the power switches and the output diodes. Both the inductors and diodes should be identical in all the channels of an interleaved design. In order to select these components, it is necessary to know the duty cycle range and peak currents. Since the output power is channeled through ‗n‘ power paths where ‗n‘ is the number of phases, a good starting point is to design the power path components using 1/n times the output power. Basically, the design starts with a single boost converter operating at 1/n times the power.
T
the input current ripple, and therefore to minimize the size of the input filter that would be relatively large if a single boost converter was used [2].Interleaving adds additional benefits such as reduced ripple current in both input and output circuits. Higher efficiency is realized by splitting the output current into ‗n‘ paths, substantially reducing I2R losses and inductor losses. The circuit diagram of two-phase IBC is shown in Fig.1.
ES
1) Choosing the number of phases:
Fig.1. Circuit Diagram of 2-Phase IBC
IJ
A
The gating pulses of the switches of the two phases are shifted by 360/n, i.e., 360/2 for n= 2, which is 180 degrees and it is shown in Fig.2.
This paper utilizes two phases since the ripple content reduces with increase in the number of phases. The ripple reduces to 12% of that of a conventional boost converter. If the number of the phases is increased further, without much decrease in the ripple content, the complexity of the circuit increases very much, thereby increasing the cost of implementation. Hence, as a tradeoff between the ripple content and the cost and complexity, number of phases is chosen as two. The number of inductors, switches and diodes are same as the number of phases and switching frequency is same for all the phases [5]. 2) Selection of duty ratio: The decision of the duty cycle is based on the number of phases. This is because depending upon the number of phases, the ripple is minimum at a certain duty ratio [6,7]. For two phase interleaved boost converter, the ripple is minimum at duty ratio, D = 0.45.Hence, the design value of the duty ration is chosen as 0.45.
Fig. 2 Switching pattern for 2- phase IBC
ISSN: 2230-7818
@ 2011 http://www.ijaest.iserp.org. All rights Reserved.
Page 324
Dr.R.Seyezhai* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 7, Issue No. 2, 323 - 329
The selection of capacitance and inductance is done using the formulae [8,9]. C = VoDF/RΔVo
(1)
where Vo represents the output voltage (V), D represents the duty ratio, F represents frequency (Hz), R represents resistance (Ω) and ΔVo represents the change in the output voltage (V). L = VsD/ΔiLF (2) where Vs represents the source voltage and iL represents the inductor current ripple.
IV.SIMULATION RESULTS Two –phase IBC with fuel cell as power source is simulated in MATLAB with the parameters as shown in Table-I. Table 1 : Simulation Parameters for 3-phase uncoupled and directly coupled IBC
ES
4) Selection of power devices:
Due to absence of reverse recovery current, there is no need of active snubber circuit for protection. The loss reduction in percentage using SiC diode is shown in Fig 3. Hence, proper choice of semiconductor device is important in improving the performance of the converter.
T
3) Selection of capacitance and inductance:
IJ
A
The semiconductor devices chosen for constructing the 2-phase interleaved boost converter is the Coolmos transistor and Silicon carbide (SiC) diode. The main benefits of Coolmos are lower on- state résistance, lower conduction losses and high switching operation. The performance of CSD100060 SiC Schottky diode is compared with that of MUR1560 Si diode for IBC using PSPICE. The SiC diode has less forward voltage, high reverse breakdown voltage and less reverse recovery current which results in reduced switching loss.
Fig.3. Loss reduction in percentage using SiC diode
ISSN: 2230-7818
Parameters
Values
Input Voltage (PEM fuel cell)
26 -43V
Output Voltage
70V
Switching Frequency
10kHz
Duty Ratio
0.45
Inductance, L
3.3mH
Capacitance
2000uF
The input current ripple, and output voltage ripple obtained from PEMFC connected to interleaved boost converter are shown in Fig. 4 and Fig. 5.
@ 2011 http://www.ijaest.iserp.org. All rights Reserved.
Page 325
Fig. 7 Output voltage ripple for Boost converter
ES
Fig. 4 Input current ripple for 2- phase IBC
T
Dr.R.Seyezhai* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 7, Issue No. 2, 323 - 329
The simulation results of IBC are compared to that of a PEMFC connected to a boost converter which is shown in Table -II.
Table II Comparison of IBC and Boost Converter
A
Parameters
Fig. 5 Output voltage ripple of 2-phase IBC
IJ
The input current ripple and output voltage ripple for a boost converter interfaced with the fuel cell are shown in Figs. 6 and 7.
Fig. 6 Input current ripple for boost converter
ISSN: 2230-7818
1.78 %
Interleaved Boost Converter 0.08 %
13.8 %
7.35 %
0.87 %
0.03 %
Boost Converter
Input current ripple Inductor current ripple Output voltage ripple
The combination of CoolMOS transistor and SiC diode for the proposed IBC topology results in a reduced switching loss compared to IRFP460A MOSFET and Si diode. Switching loss for the main power device is calculated based on the equation given below:
1 I Psw Vo in ton tr toff t f f s 2 N
@ 2011 http://www.ijaest.iserp.org. All rights Reserved.
(3)
Page 326
Dr.R.Seyezhai* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 7, Issue No. 2, 323 - 329
Parameter
IRFP460A MOSFET 120ns 550ns 500uJ
V. EXPERIMENTAL PROTOTYPE OF 2PHASE IBC A prototype of a 2- phase IBC has been designed as shown in Fig.8 in order to verify the simulation results. The hardware set-up consists of the main power circuit, PIC microcontroller board for pulse generation and power supply circuit for optocouplers. The main power circuit consists of three boost converters in parallel with CoolMOS transistors and SiC diodes. The optocoupler 6N137 is used to isolate the power circuit from the PIC microcontroller circuit. PIC18F4550 is employed to generate the pulses required to trigger the CoolMOS transistor .
ES
Table III Comparison of turn-on, turn-off tim and switching energy for CoolMOS and IRFP460A MOSFET (simulation results)
Table IV shows that SiC Schottky diode has less reverse recovery time compared to the conventional Si diode.
T
where, Psw represents the switching loss of the main power semiconductor device, N represents the number of phases, fs represents the switching frequency, Iin represents the current through the device, Vo represents the voltage, ton represents the turn-on time of the device, toff represents the turn-off time of the device, tr represents the current rise time, tf represents the voltage fall time.The simulated turn-on time, turn-off time and switching energy for CoolMOS transistor and IRFP460A MOSFET is shown in Table III.
CoolMOS Transistor 120ns 400ns 310uJ
A
Turn-on time Turn-off time Switching Energy From TableIII, the simulation results shows that CoolMOS transistor has lower switching energy compared to MOSFET. The SiC diode chosen for IBC has high reverse breakdown voltage, less reverse recovery current, less reverse recovery time and the simulated results are shown in Table IV.
IJ
Table IV Comparison of Irr and trr for Si and SiC diode Parameter
Si Diode
SiC Diode
Reverse recovery current (Irr)
100A
20A
Reverse recovery time (trr)
60ns
20ns
ISSN: 2230-7818
Fig. 8 Hardware prototype for 2-phase IBC Figs. 9 and 10 shows the ripple in output voltage and input current for IBC.
@ 2011 http://www.ijaest.iserp.org. All rights Reserved.
Page 327
Dr.R.Seyezhai* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 7, Issue No. 2, 323 - 329
ES
Fig.9 Ripple factor for output voltage ripple
T
A suitable PI controller has been designed to regulate the output voltage of IBC .The PI controller has been tuned using Zeigler‘s Nichols method to determine the values of Kp and Ki .The simulation has been carried out using MATLAB/SIMULINK and the closed loop output voltage response is shown in Fig.12.
Fig.12.Closed loop output voltage response of IBC using PI controller. VI.CONCLUSION
A
Fig. 10 Ripple factor for input current ripple
IJ
Fig 11 shows the inductor current ripple which is below the designed value (10.2%).
Fig 11 Ripple factor for inductor current ripple
ISSN: 2230-7818
This paper has investigated the performance of two-phase IBC and conventional boost converter for fuel cells. The relationship between phase current ripple, input current ripple versus duty ratio is analyzed. The design equations for IBC have been presented. It is found that IBC effectively reduces the overall current ripple compared to that of boost converter. The choice of SiC diode and CoolMOS transistor for IBC has led to reduced switching losses. From these results, two-phase IBC with CoolMOS transistor and SiC diode proves to be a good candidate for fuel cell interface. Acknowledgements The author wish to thank the management of SSN College of Engineering, for providing the computational and laboratory facilities to carry out this work.
@ 2011 http://www.ijaest.iserp.org. All rights Reserved.
Page 328
Dr.R.Seyezhai* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 7, Issue No. 2, 323 - 329
REFERENCES Gyu-Yeong Choe, Hyun-Soo Kang, Byoung-Kuk Lee and Won-Yong Lee, ― Design Consideration of Interleaved Converters for Fuel Cell Applications‖, in Proceedings of International Conference on Electrical Machines and Systems 2007, 8-11 Oct. 2007,Seoul, Korea, pp.238-243. P.A.Dahono, S.Riyadi , A.Mudawari and Y.Haroen,‗‗Output ripple analysis of multiphase DC–DC converter‘‘. IEEE Int. Conf. Power Electrical and Drive Systems, Hong Kong, pp. 626–631, 1999.
3.
H.Kosai, S.McNeal, Austin Page, Brett Jordan, Jim Scofield and B.Ray , ‗Characterizing the effects of inductor coupling on the performance of an interleaved boost converter,‘ Proc. CARTS USA 2009, pp. 237–251, 2009.
4.
H.Xu, E.Qiao, X.Guo, X.Wen and L.Kong , ‗Analysis and Design of High Power Interleaved Boost Converters for Fuel Cell Distributed Generation System‘, Int. Conf. IEEE Power Electronics Specialists Conference (PESC), pp. 140 – 145, 2005.
5.
P.Lee, Y.Lee, D.K.W. Cheng and X.Liu , ‗Steadystate analysis of an interleaved boost converter with coupled inductors‘. IEEE Trans. Industrial Electronics, pp. 787–795, 2000.
6.
R.J.Wai and R.Y.Duan , ‗High step-up converter with coupled-inductor,‘ IEEE Trans. Power Electronics, Vol. 20, No.5, pp. 1025-1035, 2005.
7.
Laszlo Huber, T.Brian Irving and M.Milan Jovanović, ‗Closed-Loop Control Methods for Interleaved DCM/CCM Boundary Boost PFC Converters,‘ Int. Conf. IEEE Applied Power Electronics , Washington, pp. 991-997, 2009.
8.
P.Thounthong, P.Sethakul, S.Rael and B.Davat , ― Design and implementation of 2- phase interleaved boost converter for fuel cell power source,‘ Int. Conf. Power Electronics, Machines, and Drives,PEMD 2008, pp. 91–95, 2008.
9.
M.Veerachary, T.Senjyu and K.Uezato , ‗Modeling and analysis of interleaved dual boost converter‘, Int. Conf. IEEE International Symposium on Industrial Electronics,Vol. 2, pp 718 – 722, 2001.
IJ
A
ES
2.
Dr. R.Seyezhai obtained her B.E. (Electronics & Communication Engineering) from Noorul Islam College of Engineering, Nagercoil in 1996 and her M.E in Power Electronics & Drives from Shanmugha College of Engineering, Thanjavur in 1998. She has been working in the teaching field for about 12 Years. She has published 65 papers in the area of Power Electronics & Drives. Her areas of interest include SiC Power Devices & Multilevel Inverters.
T
1.
BIOGRAPHY
ISSN: 2230-7818
@ 2011 http://www.ijaest.iserp.org. All rights Reserved.
Dr.B.L.Mathur obtained his B.E. (Electrical Engineering) from University of Rajasthan, in 1962 and his M.Tech in Power Systems from IIT, Bombay in 1964.He completed his Ph.D. in 1979 from IISc, Bangalore. His Ph.D. thesis was adjudged as the best for application to industries in the year 1979 and won gold medal. He has been working in the teaching field for about 44 Years. He takes immense interest in designing Electronic circuits. He has published 50 papers in National and International journals and 100 in National and International conferences. His areas of interest include Power Devices, Power Converters, Computer Architecture and FACTS.
Page 329