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High efficiency Sigma VRs (2007)
Year: 2007
To achieve high efficiency with low cost, the sigma VR structure is proposed, as shown in Fig. 1. It composes Buck converter and high efficiency DC/DC transformers. The inputs of two converters are in series, while the outputs are in parallel. The power ratio is equal to the input voltage ratio for the two converters. As a result, the power ratio can be controlled by n. For example, if n=8, Vo = 1.2V, then Vin1 = 9.6V, Vin2=2.4V, Vin1/Vin2 = 4:1. That means high efficiency DCX handles 80% output power, while Buck only needs to provide 20% output power.
By taking advantage of CPES inductor-less topology, more than 90% efficiency can be achieved by DC/DC transformer. Since the sigma converter efficiency is dominated by DCX efficiency, it can achieve around 90% efficiency at 600kHz frequency, which is 4% higher than state-of-the-art multi-phase Buck VR for server applications.
Fig. 1. Sigma VR structure.
Fig. 2. Efficiency comparison between sigma VR and multi-phase Buck VR (600kHz).
