Dynamic bus voltage control for light load efficiency improvement of two-stage voltage regulator

The two-stage 48V-12V-1.8V 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 12V output efficiently, which is then converted to 1.8V 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 configuration into the half-bridge configuration dynamically in the light load condition, so that the output of the LLC DC Transformer (DCX) can be changed from 12V to 6V. 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 multi-phase Buck converter. To achieve a fast transition of the bus voltage between 12V and 6V, the capacitance for the intermediate bus is properly selected and the Optimal Trajectory Control (OTC) for transition between the full-bridge and half-bridge is proposed. A high-efficiency, high-density 48V-12V/6V LLC DCX is designed, and experimental results on the two-stage VRM demonstrate that there is a fast transient response and a more than 10% light load efficiency improvement.