\High Step-Down Isolated PWM DC-DC Converter Based on Combining a Forward Converter With the Series-Capacitor Structure

Incorporating switched-capacitor structures into isolated dc-dc converters is a promising approach to alleviate the limitations of topologies fully based on the use of high step-down transformers. In this paper, the combination of a forward converter with a series-capacitor structure is proposed for applications that require a very high step-down conversion ratio, low output voltage ripple, high output current and isolation. The result of the combination only adds one series-capacitor, one inductor, one switch and one diode (or synchronous rectifier switch) to the component count of a conventional forward converter, thus avoiding the use of a complete second phase. The topology provides high step-down conversion ratio and low output voltage ripple, a characteristic that can be used to decrease the total energy stored by inductors (i.e., higher power density) and/or to reduce the switching frequency (i.e., higher efficiency). Moreover, the converter provides inherent current sharing between the two inductors, natural balance of the voltage across the series-capacitor and lower conduction losses. The converter operation is validated with a 100W and 48-Vto-5/3.3/2.5/1.8V prototype that achieves a peak efficiency of 95.8% and a full load efficiency of 91.1%.