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Top1. Introduction
To reduce air pollution, noise and oil consumption, conventional vehicle can be replaced by the electrical vehicle in the near future. A lot of research and development have been conducted to explore the alternative energy sources. The fuel cell has been widely considered as one of the most promising solutions in automobile applications thanks to its high energy density, zero emissions and sustainable fuels it employs. Fuel cell systems are considered as the most attractive solution by the Automotive and Power Generation industry, and by many research/academic organizations (Pianese, Sorrentino, 2010). However, the cost and low power density of the fuel cell are the major obstacles for its commercialization (Chen, 2009). Fuel cell does not release storage of energy; instead it convert energy from hydrogen-rich fuel directly into electricity and they have a large time constant to respond to an increase or decrease in power output demand transients (Ayed, Becherif & Henni, 2010). The disadvantages could be partly overcome by hybridization with ultra-capacitor. There are two potential major benefits of combining a FC with an UC as shown Figure 1. Firstly, the durability of the FC stack could be improved because the UC can fulfill the transient power demand fluctuations. Secondly, the ability of the energy storage source to recover braking energy (Xural, Erdinc & Uzunoglu, 2010). UC has a nominal single cell voltage of 2.3 to 2.75V. In order to obtain higher voltages, a DC-DC converter must been connected between the UC and the DC bus. This bidirectional converter operates in two modes of charge and discharge in order to absorb or deliver power.
Figure 1. Bloc diagram of an electrical vehicle
Several DC-DC converters, such as push-pull, half bridge and full-bridge converters can be used to boost the low voltage of the UC to the required level. The mathematical models of these converters are very important for engineers to study the system’s dynamic behavior. However, the power converter models are normally time varying due to the switching action (Ghdimi, Rastegan & Keyham, 2007). Several methods have been studied to modeling the DC-DC converter. In reference (Rathore, Bhat & Oruganti, 2010), a small signal model of full-bridge dc-dc converter is studied. In this study, the parasitic resistances of switches are considered. the small signal model and closed loop control design using average current control for an L-L type active-clamped zero-voltage switching (ZVS) current-fed isolated DC-DC converter are presented in (Qin & Kimball, 2012). The small-signal control-to-output transfer function is proposed of a Dual active bridge DC-DC converter (Vijayal & Ghosh, 2012). Full-order small-signal modelling and dynamic analysis of zero-voltage-switching (ZVS) phase-shift bidirectional DC–DC converters are studied in (Zhao, Round & Kolar, 2010). Dynamic performance of PWM dc-dc converter has been analyzed using state space averaging method in continues and discrete time domain (Iannello, Luo & Batarseh, 2003; Zarkowski & Czuk, 1992).
The paper is organized as following: section 2 presents the choose topology of the converter. Average model is developed using state space in section 3. Small signal and transfer function are derived in section 4. The converter with the designed closed loop control system has been simulated in section 5. Results are presented in section 6. The Appendix contains the notation used in this paper for reference.
Top2. Topology Of Converter
Preferably, bi-directional energy transfer should be achieved using a bi-directional power electronic conversion system to reduce size, weight and cost of the system, while still maintaining a high operating efficiency respectively of the direction of energy flow. There are many possible alternative topologies that can be used to construct a higher power bidirectional DC-DC Converter. Compared to traditional dc–dc converter circuits, isolated bidirectional DAB dc–dc converters illustrated in Figure 2 have many advantages, such as electrical isolation, high reliability, ease of realizing soft-switching control, and bidirectional energy flow. The converter can be operated in two modes of operation boost and buck. It is composed of: