The operating principles, theoretical analysis, and design methodology of a high-efficiency step-up converter are presented. The integrated boost-flyback converter (IBFC) uses coupled-inductor techniques to achieve high step-up voltage with low duty ratio, and thus the slope compensation circuit is disregarded. The voltage gain and efficiency at steady state are derived using the principles of inductor volt-second balance, capacitor charge balance and the small-ripple approximation for continuous-conduction mode. Finally, a 35 W, 12 V DC input, 48 V DC output, fsw= 40 kHz IBFC has been implemented in the laboratory to validate the theoretical analysis. A design procedure is expounded, and design guidelines for selecting critical components are also presented. It is shown that high voltage gain with high efficiency can be achieved by the IBFC system.