Project title: A Fully Soft Switched Two Quadrant bidirectional Soft Switching converter for ultracapacitor interface circuit
Isfahan University of technology
By Amin Mirzaei
Importance and statement of topic:
This is so obvious that the total efficiency of energy storage system in terms of size and cost can be increased by combination of batteries and ultracapacitors. The required system energy is provided by battery, while ultracapacitor is used at high load power pulses. Ultracapacitor voltage changes during charge and discharge modes, therefore an interface circuit is required between ultracapacitor and battery. This interface circuit must have good efficiency, while providing bidirectional power conversion and protecting ultracapacitor from immediate discharge.
Review of literature and relevant topics:
Many articles tried to consider about interface circuits. Most of them applied soft switching techniques to increase efficiency, but they used many elements to achieve their aims, so their control systems were complex and they could not increase efficiency as they wished.
Aims and Hypothesizes:
At first, I studied about soft switching techniques from my professor’s class notes and then, I searched and read the paper(s) about interface circuits. After 6 months I started to develop my idea and my professor Dr. H. Farzaneh-fard proposed to give me a team of students for help. I really liked team work so I accepted. We designed bidirectional converter in about 4 months and orcad Software show(ed) good simulation results
In my M.S. thesis I designed and implemented a two quadrant bidirectional soft switched converter for ultracapacitor interface circuit. We tried to implement appropriate soft switching techniques to increase efficiency without complexity and we wanted to have a converter with good properties like: bidirectional power conversion, soft switching and limited elements. It can be used in feature hybrid vehicles. A laboratory prototype converter is designed and realized for hybrid vehicle applications. Experimental results confirm theoretical and simulation results. We implemented the converter among 2 months.
I divided my thesis in 6 sections. First section was about introduction. In this section, I explained about batteries and their structures, then I introduced ultracapacitors and their structures and benefits and finally I talked about interface circuits. In this part I gave a simple view about why we needed interface circuits in hybrid power source.
In section two I introduced ultracapacitors completely. In this section, I considered about ultracapacitors in many aspects like; basic performance, energy and power density, benefits, application and electrical model of ultracapacitors. I explained about bank of ultracapacitors too. I studied many articles and books about batteries and ultracapacitors.
In section 3, I introduced 6 interface circuits. For each circuit, I showed its structure and then I mentioned about its benefits, faults and applications. I studied many articles and after about 4 months, I selected these six circuits from among them. Finally I introduced soft switching techniques at the end of this section.
In section 4, I described my interface circuits and I considered about its action completely. I applied ZVT (Zero Voltage Transient) and ZCT (Zero Current Transient) techniques to increase efficiency. The proposed converter acts as a ZCT buck to charge ultracapacitor. On the other hand, it acts as a ZVT boost to discharge ultracapacitor.
Figure 1 showed our proposed converter. It is obvious that our interface circuit has auxiliary switch, resonant inductor and resonant capacitor. I explained about our interface circuit when it acts as a ZCT buck to charge ultracapacitor.
In figure 2, S1 is the main switch and D2 is the main diode.
In one period of switching, our proposed converter has 6 conditions. I showed each step by electrical model.
Prior to t0, D2 is turned on and output current flows through it. At t0, S1 is turned on and the current through the resonant inductor increases linearly. S1 is softly turned on under ZCS by resonant inductor Lr. At t1, the resonant inductor current reaches Io.
Figure 3: Mode1 (t0-t1)
At t1, the main diode current reaches zero and D2 is softly turned off under ZCZVS and then the resonance between the resonant inductor and resonant capacitor starts through S1 and Da1.
Figure4: Mode2 (t1-t2)
At t2, the resonant capacitor current reaches zero and then the resonance between the resonant inductor and resonant capacitor stops because the auxiliary switch is turned off. The resonant capacitor voltage reaches 2Vs and the resonant inductor current reaches Io in this period.
Figure5: Mode3 (t2-t3)
At t3, the auxiliary switch is softly turned on under ZCS and the resonance between the resonant inductor and resonant capacitor starts so the resonant capacitor discharges. In this mode the resonant inductor current reaches zero so we can softly turned off S1 under ZCS.
Figure6: Mode4 (t3-t4)
At t4, the resonant inductor current reaches zero and the output inductor current flows through the resonant capacitor so the resonant capacitor voltage decreases linearly. At the end of this mode the resonant capacitor voltage reaches zero.
Figure7: Mode5 (t4-t5)
At t5, the resonant capacitor voltage reaches zero so D2 is turned on under ZVS condition and we can turn off auxiliary switch under ZVS condition too.
Figure8: Mode6 (t5-t6)
In section 5, I described my method in several aspects like; design of resonant capacitor, design of resonant inductor, experimental results, comparison between proposed converter with other converters and its benefits.
In section 6, I have some conclusions and give some proposals for future projects.
Morcos, M. M. and Dillman, N. G. and Mersman, C. R., “Battery charger for electric vehicles”, IEEE Power Engineering Review, Vol. 20, No. 11, pp. 8-11, 2000.
Bullard, G. L. and Sierra, H. B. and Lee, H. L. and Morris, J. L., “Operating principles of the ultracapacitor”, IEEE Transactions on Magnetics, Vol. 25, No. 1, pp. 102-106, 1989.
Pay, S. and Baghzouz, Y., “Effectiveness of battery-supercapacitor combination in electric vehicles”, IEEE Proceeding of the Power Tech Conference., Vol. 3, pp. 23-26, 2003.
Arnet, B. J. and Haines, L. P., “High power DC-DC converter for supercapacitors”, IEEE International Electric Machines and Drives Conference. IEMDC, pp. 985-990, 2001.
Mar, M. and Schroder, D., “ A novel zero-current-transition full bridge DC-DC converter”, IEEE Power Electronics Specialists Conference, Vol. 1. pp. 664-669, 1996.
Guichao, H. and Ching, S. L. and Yimin, J. and Lee, F. C. Y., “Novel zero-voltage-transition pwm converters”, IEEE Transactions on Power Electronics. Vol. 9. No. 2. pp. 215-219, 1994.
Gomez, J. L. and Enjeti, P. N. and Jouanne, A. V., “An approach to achieve ride-through of an adjustable-speed drive with flyback converter modules powered by supercapacitors”, IEEE Transactions on Industry Applications, Vol. 38, No. 2, pp. 515-516, 2002.
I really enjoy to research about some topics of power electronics like, DC to DC Converters, Soft Switching Techniques, Electric Vehicle, High Frequency Power Conversion, wind farm power stations and my M.S. thesis include(d) many of them. I mention that my B.S. thesis was about feasibility study of replacing asynchronous generators with synchronous generators in wind farm power stations. Consequently, I want to pursue my research in Ph.D. in these fields and some fields that correspond to my Master of Science thesis. Because I have good experience about these topics and I studied them very well. I really like to improve implementation of converters and find new ways that cause to increase efficiency and reduce cost and size