A High Efficiency Step-Up Current-Fed Push-Pull Quasi-Resonant Converter with Fewer Components for Fuel Cell Application

A High Efficiency Step-Up Current-Fed Push-Pull Quasi-Resonant Converter with Fewer Components for Fuel Cell Application
Abstract
In this paper, a new high efficiency step-up current-fed push-pull quasi-resonant converter is proposed, which is suitable for low-voltage fuel cell power conditioning system. The proposed converter conserves inherent advantages of low input-current stress and high voltage conversion ratio of the conventional current-fed push-pull converter. All of power devices can achieve soft-switching at light load improving the overall efficiency. Moreover, similar features have been obtained with fewer components in comparison with the active-clamped current-fed push-pull converter [37] and current-fed push-pull resonant converter [40], that enabling to reduce the cost and improve system reliability. In addition, the voltage-doubler rectifier is adopted to eliminate the reverse-recovery problem of secondary diodes and provides much higher voltage conversion ratio resulting in small turn-ratio of the high frequency transformer. Detailed operation, analysis, design, comparative study, experimental results and loss breakdown for the proposed converter are presented in the article. A 510 W prototype verifies the theoretical analysis and the effectiveness of the proposed concept.
PROPOSED SYSTEM:
It is worth noticing that one other drawback of the push-pull converter is the high turn-off voltage spike across the switch. Normally, the passive snubber and active clamp circuits the energy recovery snubber are employed to absorb this spike. In passive snubber, such as a resistor-capacitor-diode (RCD), the energy absorbed by snubber capacitor is dissipated in the resistor causing low efficiency. The energy recovery snubber in uses many devices and the method in utilizes a switching converter to transfer the absorbed power back to the input voltage source. They are rather more complicated. The active snubber has more attractive since it can absorb the turn-off voltage spike, as well as assisting in soft-switching of switches. The voltage stress of primary switches can be suppressed and is less than two times of the input voltage. However, it needs two switches conduction simultaneously causing more conduction loss when the energy transfers to the load. The ZVS clamping-mode current-fed push-pull converter in, although can clamp a surge voltage and achieve ZVS of switches, ZVS does not enhance the efficiency sufficiently since the energy stored in the output capacitances of switches is small in low input voltage, moreover, the ZVS condition needs a larger leakage inductance or an resonant inductor resulting in additional power loss. The active soft-commutation technique is proposed. which diverts the switch current each other by transformer to realize natural commutation, thereby reducing the requirement of snubber. However, it just can be achieved through the control of secondary switch and, thus, it is only suitable for the bidirectional converter.
EXISTING SYSTEM:
these numerous dc/dc converters, the step-up converter suitable for fuel cell applications can be categorized as either voltage-fed. An important advantage of voltage-fed type is the low switch voltage rating that enabling the use of devices with low R . This is greatly beneficial in the low-voltage high-current application such as fuel cells. In addition, this type does not have a self-start problem reducing the complexity of system. However, the voltage-fed step-up converters have several limitations in the fuel cells application, i.e., high transformer turns ratio which results in large leakage inductance leading to large duty cycle loss (if inductive output filter), high pulsating input current which requires an LC filter causing additional power loss and size, high circulating current through primary switches and the windings of transformer and severe ringing on the secondary rectifier diodes. Compared with voltage-fed type, the current-fed type has inherently a smaller input ripple and a lower HF transformer turns ratio due to the input inductor providing filtering and voltage boosting, a lower rectifier diode rating due to the negligible diode ringing and effective voltage clamping, a low risk of transformer saturation and no the problem of duty cycle. Therefore, the current-fed step-up converters may be meritorious over the voltage-fed converters for low-voltage high-current input applications.
CONCLUSION
A simple high step-up current-fed push-pull quasi-resonant converter for fuel cell power system was presented in this paper. By employing the current-fed structure and voltage-doubler rectifier, a much higher voltage conversion ratio was achieved without large turns-ratio of transformer. ZVS can be realized for all of three primary switches at light load and ZCS can be achieved for the secondary diodes in the full load range by the resonant condition. The voltage-doubler rectifier eliminates the reverse-recovery problem of rectifier diodes and the auxiliary active-clamp circuit suppresses the voltage spike of switches and recycled the energy stored in the leakage inductance. Moreover, compared with the reported topologies, this converter realizes the similar features with fewer components, which reduces the cost and improves system reliability. A 510W prototype was implemented to verify the analysis and performance. The prototype achieved a high efficiency of 96.12% at an input voltage of 50 V. o =510W, Total loss: 22.530W Switches turn-on loss Switches turn-off loss Switches conduction loss Driving loss Diodes Conduction loss Input inductor loss High frequency transformer loss
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