Abstract: LLC converters have been widely used in recent years because of their soft switching advantages. Traditional LLC converters use variable frequency control. It is difficult to achieve a wide voltage input when the switching frequency range is constrained. As a result, LLC converter has a short hold-up time after power failure, and is limited in applications such as UPS. In response to this kind of problem, many domestic and foreign research results of wide-voltage input range LLC converters that can be used for hold-up emerge. The problems of the narrow range of the traditional LLC gain are analyzed, the research results from the control strategy and circuit structure are summarized, and the advantages and disadvantages of them are analyzed.