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This paper presents a three-level isolated LLC resonant converter using a novel modulation strategy for wide-voltage range operation. With the proposed modulation strategy, the DC gain of the converter can be changed between Vout and ½Vout. The output voltage can be then adjusted within the two resulting voltage ranges by either varying the switching frequency of the converter or utilizing a post regulator. In the last option, the three-level converter works as an electronic transformer with variable turns ratio. Therefore, the converter can be designed to operate with a narrow operation condition maintaining high efficiency despite of wide-voltage range variations. A hybrid gate driver which simplifies the implementation of the proposed modulation method is also presented. The novel modulation method is analysed and verified on a 20kW, LLC three-level resonant prototype operating at 110 kHz.