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This paper proposes a CLL resonant dc-dc converter as an option for offline applications. This topology can achieve zero-voltage switching from zero load to a full load and zero-current switching for output rectifiers and makes the implementation of a secondary rectifier easy. This paper also presents a novel methodology for designing CLL resonant converters based on efficiency and holdup time requirements. An optimal transformer structure is proposed, which uses a current-type synchronous rectifier (SR) drive scheme. An 800-kHz 250-W CLL resonant converter prototype is built to verify the proposed circuit, design method, transformer structure, and SR drive scheme.