The two-stage 48 V-12 V-1.8 V voltage regulator module (VRM) structure is gaining more and more attention in high-end server applications. The LLC converter is the preferred choice for the first conversion stage because it provides the isolated 12-V output efficiently, which is then converted to 1.8 V by the multi-phase Buck converter. Since the CPU works in sleep-mode most of the time, the light-load efficiency is very important for energy saving. This paper proposes to change the primary side of the LLC converter from the full-bridge (FB) configuration into the half-bridge (HB) configuration dynamically in the light-load condition, so that the output of the LLC dc transformer (DCX) can be changed from 12 to 6 V. This will increase the overall light-load efficiency significantly due to the reduced core loss of the LLC DCX and the reduced switching loss of the multiphase Buck converter. To achieve a fast transition of the bus voltage between 12 and 6 V, the capacitance for the intermediate bus is properly selected and the optimal trajectory control for transition between the FB configuration and HB configuration is proposed. A high-efficiency high-density 48-12 /6 V LLC DCX is designed, and experiments on the two-stage VRM demonstrate that there is a fast transient response and a more than 10% light-load efficiency improvement.