WIDE INPUT-VOLTAGE RANGE BOOST THREE-LEVEL DC-DC CONVERTER WITH QUASI-Z SOURCE FOR FUEL CELL VEHICLES
Abstract:
To solve the problem of the mismatched voltage levels between the dynamic lower voltage of the fuel cell stack and the required constant higher voltage (400V) of the DC link bus of the inverter for fuel cell vehicles, a Boost three-level DC-DC converter with a diode rectification quasi-Z source (BTL-DRqZ) is presented in this paper, based on the conventional flying-capacitor Boost three-level DC-DC converter. The operating principle of a wide range voltage-gain for this topology is discussed according to the effective switching states of the converter and the multi-loop energy communication characteristic of the DRqZ source. The relationship between the quasi-Z source net capacitor voltages, the modulation index and the output voltage, is deduced and then the static and dynamic self-balance principle of the flying-capacitor voltage is presented. Furthermore, a Boost three-level DC-DC converter with a synchronous rectification quasi-Z source (BTL-SRqZ) is additionally proposed to improve the conversion efficiency. Finally, a scale-down 1.2 kW BTL-SRqZ prototype has been created, and the maximum efficiency is improved up to 95.66% by using synchronous rectification. The experimental results validate the feasibility of the proposed topology and the correctness of its operating principles. It is suitable for the fuel cell vehicles.
EXISTING SYSTEM:
surely the conventional Boost two-level DC-DC converter is employed due to its simple structure, but, it suffers from disadvantages including limited voltage-gain, and high voltage stress for its power semiconductors. To alleviate the problem of mismatched voltage levels, the rated voltage of the fuel cell stack has to be increased (increasing the difficulty of assembling the fuel cell stack). At the same time, power semiconductors with higher rated blocking voltage need to be employed and consequently the conduction losses can be improved. In order to reduce the high voltage stress of power semiconductors, Boost three-level DC-DC converters have been proposed, and then the voltage stress can be reduced by half. However, there remain two essential problems concerning the interface between the fuel cell stack and the DC-link bus, namely the same limited voltage-gain with that of the Boost two-level converter, and the complicate control required for the flying-capacitor voltage balance of the Boost three-level converter, especially the voltage imbalance of the flying-capacitor in the transient state- this latter may cause power semiconductor failure. It is therefore necessary to solve these problems for fuel cell vehicles, which use the Boost three-level DC-DC converter with a flying capacitor.
PROPOSED SYSTEM:
In this paper, a wide input-voltage range Boost three-level DC-DC converter with a diode rectification quasi-Z source (BTL-DRqZ) is proposed as a solution which can reduce the voltage stress of all semiconductors to half of the output voltage; it also has a common ground for the input and output by using the flying-capacitor three-level structure, and operates well with a high voltage-gain, proper duty cycles (0.5<=d<0.75), and balancing of the voltage of the flying capacitor without additional hardware. Although one more power switch and diode are employed compared to the conventional quasi-Z source Boost DC-DC converter, the lower rated voltage semiconductors with lower on-resistance can replace the higher rated voltage devices. In addition, the equivalent frequency of the inductor current and the capacitor voltage ripple in the proposed topology is double the switching frequency due to using one additional power switch, diode and flying capacitor, achieved by using the flying-capacitor three-level structure with two phase-shifted 180 degree gate driving signals. These features are beneficial to improve efficiency. In order to improve the efficiency of the proposed converter further, the Boost three-level DC-DC converter with a synchronous rectification quasi-Z source (BTL-SRqZ) is additionally proposed, based on the BTL-DRqZ. This paper is organized as follows: in Section II, the topology of the BTL-DRqZ for fuel cell vehicles is presented.
CONCLUSION
The topology of the BTL-SRqZ is proposed in this paper. It has the advantages of lower voltage stress for the power semiconductors and the common ground between the input and output sides, as well as the wider range of the voltage-gain with modest duty cycles 0.5, 0.75 for the power switches. In addition, the voltage of the flying-capacitor can be clamped well at half the output voltage by the capacitor voltages of the quasi-Z source net in both the static and dynamic states. At the same time, the synchronous rectification power switch operates with ZVS turn-on and turn-off, and the losses of the quasi-Z source circuit can be reduced by the synchronous rectification operation. Therefore, it is suitable to vehicles powered by a fuel cell stack which has a soft output characteristic.
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