I. Introduction
The dc–dc converter with high step-up voltage gain is widely used for many applications, such as fuel-cell energy-conversion systems, solar-cell energy-conversion systems, and high-intensity-discharge lamp ballasts for automobile headlamps. Conventionally, the dc–dc boost converter is used for voltage step-up applications, and in this case, this converter will be operated at extremely high duty ratio to achieve high step-up voltage gain [1], [2]. However, the voltage gain and the efficiency are limited due to the constraining effect of power switches, diodes, and the equivalent series resistance (ESR) of inductors and capacitors. Moreover, the extremely high duty ratio operation will result in a serious reverse-recovery problem. Some literatures have researched high step-up dc–dc converters that do not incur an extremely high duty ratio [3]–[25]. The transformerless dc–dc converters include the cascade boost type [3], the quadratic boost type [4], the switched-inductor type [5], [6], the voltage-lift type [7], [8], the voltage-doubler type [9]–[11], the capacitor–diode voltage-multiplier type [12], and the boost type that is integrated using a switched-capacitor technique [13]. These converters can provide higher voltage gain than the conventional dc–dc boost converter. However, the voltage gain of these converters is only moderately high. If higher voltage gain is required, these converters must cascade more power stages, which will result in low efficiency.
Circuit configuration of the conventional high step-up dc–dc converter.
The dc–dc flyback converter is adopted to achieve high step-up voltage gain by adjusting the turns ratio of the transformer. This converter has the merits of simple topology, easy control, and low cost, but the fact that the leakage-inductor energy of the transformer cannot be recycled results in low efficiency and high voltage stress on the active switch. In order to reduce the voltage stress, an snubber is used [14]. However, this decreases the efficiency. Some active-clamp techniques are adopted to recycle the leakage-inductor energy of the transformer and to minimize the voltage stress on the active switch, but this approach requires an additional switch [15]–[17]. Some converters, which include the clamp-mode boost type, the integrated boost–flyback type, and the integrated boost–sepic type, are developed to achieve high step-up voltage gain by using the coupled-inductor technique [18]–[22]. The leakage-inductor energy of the coupled inductor can be recycled, and the voltage stress on the active switch is reduced. Much higher voltage gain is achieved by using the coupled-inductor and the voltage-multiplier or voltage-lift techniques [23]–[25]. However, the active switch will suffer high current stress during the switch-on period.