I. Introduction
High-Efficiency power supplies have been presented for distribution power systems such as telecommunication applications. For medium- and high-power applications, full-bridge converters with phase-shift pulsewidth modulation (PWM) [1], [2] have been presented. The phase-shift PWM has features of zero-voltage switching (ZVS) and high efficiency. However, the high cost and the narrow ZVS range for the lagging leg of the phase-shift full-bridge converter are the main disadvantages. Asymmetrical PWM schemes for dc/dc converters [3]–[5] were proposed to achieve ZVS operation during the commutation stage. The drawback of the asymmetrical converter is that the voltage and current stresses of switching devices are related to the duty cycle. The active-clamp techniques [6]–[8] have been proposed to absorb the surge energy stored in the leakage inductance so that the voltage stress of switching devices can be reduced. Recently, two series-connected transformers [9] are presented in the full-bridge-converter application. Each transformer can be operated as an inductor or a transformer. No output inductor is needed in the secondary side of the transformer.