[u]A Soft-Switching DC/DC Converter With High Voltage Gain[/u]

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I. INTRODUCTION
RECENTLY, the demand for dc/dc converters with high
voltage gain has increased. The energy shortage and the
atmosphere pollution have led to more researches on the renewable
and green energy sources such as the solar arrays and the
fuel cells [1]–[5]. Moreover, the power systems based on battery
sources and supercapacitors have been increased. Unfortunately,
the output voltages of these sources are relatively low.
Therefore, the step-up power conversion is required in these systems
[6], [7]. Besides the step-up function, the demands such as
low current ripple, high efficiency, fast dynamics, light weight,
and high power density have also increased for various applications.
Input current ripple is an important factor in a high
step-up dc/dc converter [8], [9]. Especially in the fuel cell systems,
reducing the input current ripple is very important because
the large current ripple shortens fuel cell’s lifetime as well as
decreases performances [10]–[14]. Therefore, current-fed converters
are commonly used due to their ability to reduce the
current ripple [15].


II. ANALYSIS OF THE PROPOSED CONVERTER
Fig. 1 shows the circuit diagram of the proposed softswitching
dc/dc converter with high voltage gain. Its key waveforms
are shown in Fig. 2. The switches S1 and S2 are operated
asymmetrically and the duty ratio D is based on the switch S1 .
D1 and D2 are intrinsic body diodes of S1 and S2 . Capacitors
C1 and C2 are the parasitic output capacitances of S1 and S2 .
The proposed converter contains a CCM boost cell. It consists
of LB , S1 , S2, Co1 , and Co2 . The CCM boost cell provides a
continuous input current. When the switch S1 is turned on, the
boost inductor current iLB increases linearly from its minimum
value ILB2 to its maximum value ILB1 . When the switch S1
is turned off and the switch S2 is turned on, the current iLB
decreases linearly from ILB1 to ILB2 .



Voltage Stress of Devices
Generally, high output voltage will impose high-voltage stress
across the switching devices in dc/dc converters. In the proposed
dc/dc converter, the voltage stresses across the switching devices
are smaller than the output voltage. Maximum values of vS1 and
vS2 are confined to the output of the CCM boost cell as follows:
vS1,Max = vS2,Max = Vo1 + Vo2 = Vin

Fig. 5 shows the voltage stresses of the power switches and the
output diodes that are normalized with Vo .