Analysis and Design of Current-Fed High Step Up PWM Controlled Quasi-Resonant DC-DC Converter for Fuel Cell Applications
Abstract:
A quasi-resonant DC-DC Converter with high voltage gain and low current stresses on switches is proposed in this paper. This Converter preserved inherent advantages of current-fed structures, for instance: zero magnetizing DC offset, low input ripple and low transformer turn ratio. Moreover, by employing active clamp circuit, the voltage spikes across the main switch, due to existence of leakage inductance of isolating transformer is absorbed and switches work in zero voltage switching (ZVS). Since quasi-resonant switching strategy is empl oyed turn-off current (TOC) and losses of switches are considerably reduced. Because of zero current switching (ZCS) reverse recovery problem of diodes is alleviated. Experimental results on a 150 W prototype are provided to validate the proposed concept.
EXISTING SYSTEM:
These converters are more suitable for high-power and low-voltage application, for ins tance, FC’s and PV’s . The most utilized topologies for current-fed converters are full-bridge, half-bridge and push-pull converters. Full-bridge converters are used in high -power applications. In , a full-bridge current-fed converter with active clamp is introduced. Although zero voltage switching (ZVS) is employed for main switches, active clamp has high current, and its switching frequency is three times the main switching frequency, but by employing ZCS turn-off of primary switches and ZVS turn-on of secondary switches in a wide variation of output power in , switching low efficiently reduced and allows the converter to operate at high frequencies. The proposed push-pull converter in adopted the minimum number of components. However, it operates under hardswitching condition. So, the voltage spikes on switches increase converter losses dramatically. In an active clamped current-fed push-pull converter is introduced which adopted ZVS and lossless clamp circuit, which have improved its efficiency. Although a current-fed push-pull converter has been proposed in which adopted ZVS and ZCS for switches, it suffers from high voltage stress on switches. On the other side, the converter’s component number and its complexity has increased which increases the cost. L-type half-bridge currentfed converters have the lowest transformer turn ratio which causes low ohmic loss. Although they require additional snubber circuits, they cannot operate under ZVS condition for the wide range of input voltage and load variation . In LLC resonant converters are proposed which are well suited for high-voltage and high-frequency applications
PROPOSED SYSTEM:
To obtain high voltage gain, transformer magnetizing inductance must be small which rises magnetizing current and higher conduction and core losses. In this paper, a half-bridge current-fed quasi-resonant is proposed (Fig.1). An active clamp circuit is added to a conventional boost converter. Adopting active clamp circuit introduces soft switching for the converter. A high-frequency transformer provides galvanic isolation and improves voltage gain. Moreover, a voltage doubler circuit at the output eliminates the reverse-recovery problem of the output diode and doubles the voltage gain. Furthermore, the converter operates in quasi-resonant condition. In this situation, switches current stress and loss will be reduced to its minimum value while the converter still can be controlled by the pulse width modulation (PWM) method. Actually, the proposed converter adopts the soft-switching merit of resonant converters, and control and implementation simplicity of the conventional PWM converters. By reducing the switching loss of the proposed converter, it can operate in higher switching frequency. Also, the number of the components is low. Consequently, proposed converter’s volume and weight are decreas ed and its power density is increased. Almost the same topology is presented as dual active bridge converter for aerospace application. This converter retains soft switching in a wide range of load variation, but switches suffer from high switches current stress. The same topologies are proposed in.The converter is proposed for Plug in Hybrid Electric Vehicle (PHEV), and it is bi-directional, but just like has high current stress in switches. The converter is proposed for Photovoltaic (PV) system. Although it operates in resonant mode, main switch has zero current switching (ZCS) in both turn-on and turn-off moments and auxiliary switch operates in hard switching condition, which result in high switching loss The proposed converter circuit is shown in Fig. 1. Input boost circuit consists of inductance L B and switch S . Leakage inductance of transformer in primary side is modeled as L and resonant circuit is provided by C 1 1 and leakage inductance L . Active clamp circuit is made by C 2 and S . In secondary side of the transformer, diodes D 1 and D 2 2 make a full wave rectifier which provide voltage doubler circuit with capacitors C and C o2 in output. Because of ZCS on the output diodes, fast diodes are not necessary anymore. Moreover, load resistance is modeled as R L . N 1 and N are primary and secondary of transformer turn ratio, respectively. “n” Is defined as n = N 2 2 / . f are switching and resonant frequency, respectively. N 1 The operation of proposed converter is described by 6 intervals. In important currents and voltage curves are plotted, and circuit operating modes At the time before t 0 , S is turned ON. It is assumed that input current has constant value I 2 in because inductance L is large enough to alleviate input current ripple.
Conclusion
In this paper, analysis and experimental results of a new quasi-resonant converter has been presented. Both switches of the proposed converter work under ZVS condition. Also, the main switch operates in below-resonant, while the active clamp switch operates in above-resonant in order to retain ZVS condition for the main switch, and switching losses are minimum. The leakage inductance of transformer, has been employed to make the resonant circuit with clamp capacitor. Low input current ripple which is appropriate for fuel cell applications has made by using a boost circuit at the input of converter. At output, to obtain high voltage gain, a voltage doubler circuit has been used. The output diodes working under ZCS alleviates their reverse recovery problem and reduce losses. Finally, the proposed converter power loss has been compared with a L-L type half-bridge converter, and it was concluded that by employing quasi-resonant performance total power loss has been reduced effectively.
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