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LLC resonant converter has been widely used in dc-dc converters. In this paper, optimum dead-time and maximum switching frequency of a wide-adjustable-range LLC resonant converter are investigated for realizing the zero voltage switching (ZVS) operation even under the worst-case conditions. Analyses demonstrate that these parameters depend on the converter inductance ratio and ratio of the converter resonant capacitor and the effective capacitance appeared in parallel with the drain-sources of the power MOSFETs. The necessary dead time for realizing the ZVS operation can be minimized by choosing the normalized maximum switching frequency, properly. Using the dead-time optimum value, soft switching is achieved for all power devices even under the worst-case conditions. Developed prototype of the converter has been tested under different loads (0-3 A dc) and input voltage conditions (320-370 V dc) to achieve a wide-adjustable-range output voltage (40-165 V dc). This dc-dc converter is used as an ion implanter arc power supply. The calculated optimum dead-time and maximum switching frequency are approximately equal to 184 ns and 205.7 kHz, for realizing the ZVS operation at the worst-case conditions. These parameters in the prototype are approximately equal to 195 ns and 203.5 kHz, respectively.