Dynamic Bus Voltage Control for Light Load Efficiency Improvement of Two-stage Voltage Regulator

摘要

the 2-stage 48V-12V-1.8V VRM structure is gaining more and more attention in the high-end server application. The LLC converter is the preferred choice for the first conversion stage. It provides the isolated 12V output. The multi-phase VR, then, takes 12V and converts it to 1.8V. Since the CPU works at the sleep-mode in 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 full-bridge configuration into half-bridge configuration dynamically in the light load condition, so that the output of LLC DCX can be changed from 12V to 6V, which increases the overall light load efficiency significantly due to the reduced core loss of the LLC DCX and the reduced switching loss of the multiphase VR. To achieve fast transition of bus voltage between 12V and 6V, the capacitance for intermediate bus is properly selected and Optimal Trajectory Control (OTC) for transition between full-bridge and half-bridge is proposed. Experimental results demonstrate the fast transient response and more than 10% light load efficiency improvement.