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

Fig. 1. (a) Different configurations for heavy load and light load, full-bridge for heavy load
The two-stage 48V-12V-1.8V VRM structure is gaining more and more attention for use in high-end server applications because of its significant improvements of overall efficiency. The LLC converter is the preferred choice for the first conversion stage, and provides isolated 12V output. The multi-phase VRM then takes 12V and converts it to 1.8V. This second-stage conversion has to provide precision regulation with fast dynamic responses for the CPU and related loads. The CPUs are constantly operating between sleep mode and the wake-up mode, and most of the time they are in the sleep-mode stage. Therefore, the efficiency in sleep mode is critical to reducing the total electricity usage in the server. However, the light-load efficiency is quite a challenge for the two-stage structure. The major losses in wake-up mode are the conduction losses for the LLC converter and the VRM, while the major losses in sleep mode are the core loss for the LLC converter and the switching loss for the VRM.

This paper proposes to change the structure of the primary side from a full-bridge structure to a half-bridge structure dynamically in light-load conditions, so that the output of the 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 reduced switching loss of the multi-phase VR. To achieve the full-bridge operation shown in Fig. 1(a), the Q3 switch needs to be synchronized with Q1, and the Q4 switch needs to be synchronized with Q2. To achieve the half-bridge operation shown in Fig. 1(b), the Q3 switch needs to kept on all the time, and the Q4 switch needs to be kept off all the time. However, the control of the transient between the full-bridge and half-bridge is very challenging due the large dc bias in the resonant capacitor voltage, as shown in Fig. 2. Transition from one steady state to the other involves large energy changes. The fast dynamic changes can only be achieved using state trajectory control; in other terms, it controls the instantaneous energy of the tank. Details about how to control the transient is presented in the full paper.

Along with the concept proposed above, a two-stage 48V-12V/6V-1.8V structure with dynamic bus voltage control for LLC DCX is proposed. To achieve fast transition of the bus voltage between 12V and 6V, the minimum capacitance for the intermediate bus is investigated, and optimal trajectory control for transition between the full-bridge and half-bridge is proposed and verified by experiment. Greater than 10% light-load efficiency improvements are achieved, as shown in Fig. 3.

Fig. 1. (b) Half-bridge for light load
Fig. 2. Steady state-trajectory of LLC converter

Fig. 3. Light load efficiency improvement
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