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This sub-section presents different control techniques of DC-DC converter using analog and digital domain. A boost converter is a power converter with output DC voltage which is greater than its input DC voltage. A boost converter steps up the output voltage; it stores energy by passing current(s) through an inductor and that energy is then delivered at intervals by a MOSFET regulated by PWM (Pulse Width Modulation) to a capacitor [12]. The charged capacitor will then supply a higher voltage at lower current to the load. Boost converter can be isolated or non-isolated type.

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A zero-voltage switching (ZVS) three level DC/DC resonant converter for high-power operation is analysed at fixed and variable frequencies in [13] . The converter operates with wide input voltage variations without penalizing the efficiency. As a result, the converter is suitable for applications in which high efficiency and high power density are required. Experimental results for a 2.7 kW prototype verify the operation of the converter performance as designed. A new high efficiency transformer less DC/DC converter is proposed in [14] in which large step-up rations is achieved with low duty cycle. The design structure integrates a multiphase voltage multiplier that allows high static gain with low voltage stress in all the semiconductors used. The main advantages include, low voltage and current stresses, reduction of the turn-on and turn-off losses, low input voltage and current ripples, high efficiency and design modularity. An experimental result confirms the basic operation of the designed converter and theoretical analysis developed. A current-fed full bridge boost converter with zero current switching (ZCS) based on constant on-time for high voltage application is presented in [15]. The proposed converter utilizes the leakage inductance and the winding parasitic capacitance resonant tank to achieve zero current switching. In order to achieve zero current switching under wide load range, the turn-on time of the full bridge boost converter is kept constant and the output voltage is regulated via frequency modulation. With careful design of the circuit parameters, the proposed converter can be operated with ZCS under wide load range without the use of series connected diodes. A laboratory prototype implemented verifies the ZCS performance. In [16], the principle and electrical characteristics of the fuel cell has been discussed. They have proposed a DC-DC converter scheme to combine the fuel cell with storage system. They have used shifted pulse width modulation technique for the DC-DC converter fuel cell. State and transfer function model of the set made up of a proton exchange membrane (PEM) fuel cell and DC-DC converter is presented in [17]. The set is modelled as plant controlled by the converter duty cycle. This model describes the relationship between different electrical variables and is valid for any operating of fuel cell. The linearization technique is applied in order to obtain the entire system’s transfer function. Digital control of DC-DC converter on fuel cell vehicles is developed in [18]. Based on the half bridge topology, the control circuit adopting DSP and the control method carried out. TMS320F2812, a highly integrated and high performance DSP is employed as the control core chip. This paper has described the hardware design and control methods. Finally it represents the control method of main DC-DC converter and inverter of Fuel Cell Vehicles.

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Currently, there is a trend in which embedded control is applied to most power converters for various applications. As a result, control circuit size is significantly reducing as entire control circuit is configured into a single chip, system on chip (SOC) or FPGA. Therefore, some researchers have initiated research work in this area for making the control circuit simpler, effective and reconfigurable. In literature [19] the authors implemented a 1 kW power conditioner for fuel cell power system with lower ripples and faster dynamics. Poly-phase boost converter is used for power conditioning system which is controlled using current mode control technique. The DC-DC converter and inverter topology is controlled using digital controller implemented in DSP and FPGA. A detailed method of Hardware-in-the-Loop real-time simulation of switch-mode converters in FPGA is reported in [20]. A mathematical description of DC-DC boost converter model, its FPGA-based implementation and debugging results are presented. The results are compared with Simulink model and practical converter. The presented method of simulation can be used for verification of discrete control in designed converters and also as an educational platform. Digitally controlled DC-DC buck converter performed by FPGA circuitry is presented in [21]. All tasks, analog to digital conversion, control algorithm and pulse width modulation, were implemented in the FPGA. This approach enables high-speed dynamic response and programmability by the controller, without external passive components. In addition, the controller’s structure can be easily changed without external components.

Fuel cell provides a low voltage (approx. 1.2 V DC) output at a reasonably high current. However, this does not suit many applications. Therefore, this voltage has to be stepped up using DC-DC converter. There are numerous DC-DC converter topologies available for this purpose. To provide a regulated DC voltage at the output a closed loop control is essential. Conventionally, the closed loop control is implemented in analog mode but there are certain limitations of analog mode control. In recent times, in most of the cases control law is implemented in digital domain.

Considering the above discussion, we agree that several topologies are available for dc- dc boost conversion. However, control laws are important as this would determine several performance metrics (ripple, settling time, response time) of the topology in general. Moreover, in the present dissertation our objective is to operate fuel cell based dc power system. Therefore, we do use the existing topologies, use suitable control structures that work well with the source. In our study we have used PI controller and

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SMC controller. Furthermore, considering application perspective, it is also important to investigate implementation strategies. In our work, we have adopted digital domain; we have implemented PI controller and PWM control using FPGAs that provides flexibility for further modification through reconfigurable computing.