High step-down ratio DC-DC converter with multi-leg transformer

In this paper, a multi-leg transformer structure is proposed to realize high effective turns ratio for high step-down ratio dc-dc converters. By adding an extra degree of freedom, the proposed transformer structure can reduce the number of winding layers and the cost of a planar transformer. The proposed core and winding structures effectively reduce the number of primary turns, which results in balanced core loss and winding loss, reduced dc winding loss, and reduced fringing effect loss. To meet the core height and volume requirements, different structures are compared and high efficiency and high power density are achieved. Several design considerations are discussed, and extra trace copper loss is minimized by careful layout design. A 200 kHz 400 V-to-2 V 120 W forward converter prototype is built to verify the proposed transformer design.