LLC series resonant converter is the most attractive topology for medium power application due to its high efficiency and wide input range. Despite these advantages, it is difficult to optimally design to satisfy hold up time requirement, and it reduces efficiency of the converter and power density is degraded because of bulky magnetizing components. To solve these drawbacks, LLC series resonant converter with auxiliary hold-up time compensation circuit is proposed. By using auxiliary hold up time compensation circuit, the LLC converter gets higher gain when hold-up of output voltage is required, and the efficiency of the converter can be improved by the optimal design in normal operation mode. In this paper, operational principle of the proposed circuit is presented and experimental results with prototype are given to confirm the validity of the proposed circuit.