Design Optimization of Unregulated LLC Converter with Integrated Magnetics for Two-Stage 48V VRM
modules (VRMs) have been used in telecom applications for years. A recent study indicates 48 V VRMs, instead of 12 V VRMs, are deemed a more efficient and cost-effective architecture for data cen- ter applications.
Two-stage, 48 V VRM architecture can help achieve the re- quired efficiency and power density for the target application with a quicker integration to the existing data centers. As shown in Fig. 1, the first stage is an unregulated dc-dc (DCX) converter stepping down the input voltage to an intermediate bus voltage, followed by the mature technology of a multi-phase buck converter. The soft- switching capabilities of the first stage combined with the simplicity and scalability of the second stage promote this solution as a suitable candidate for future data centers to achieve the required efficiency and power density. One key element to achieve these goals is the first stage unregulated converter design.
In this work, an LLC converter with a matrix transformer structure is proposed for the first-stage converter. Two DCX con- verters are designed with different transformation ratios to evaluate the overall efficiency of the two stages with different intermediate bus voltages. With GaN devices and integrated magnetic structure, the two converter prototypes shown in Fig. 2 are designed, and they achieve very high efficiency (> 98%) and power density (> 1200 W/ in3<\sup>). The efficiency of the two stages is then evaluated with the con- clusion that 6 V for the intermediate bus will help achieve higher efficiency for the two-stage architecture.