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High-Efficiency "Sigma" Voltage Regulator

Year: 2008
Fig. 1. Sigma VR structure.
High efficiency is generally required from Voltage Regulators (VRs) that supply power to microprocessors. For example, efficiency exceeding 90% is desired for laptop VRs to extend the battery life. High efficiency is also expected for server VRs to save energy and electricity bills since there are many microprocessors in server systems, especially in server farms. It is very hard for the existing technology, represented by multi-phase buck VR, to meet these future efficiency requirements.

To achieve higher efficiency with low cost, the sigma VR structure composed of a buck converter and high efficiency dc-dc transformer (DCX) is proposed. The inputs of the 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 the example shown in the figure below with n=8, DCX handles 80% of the output power, while buck only needs to provide only 20%, so that the overall efficiency is dominated by the DCX efficiency.

By taking advantage of the CPES inductor-less DCX topology, the sigma converter operating at switching frequency of 600 kHz can achieve around 90% efficiency, or about 4% higher than state-of-the-art multi-phase buck VR for server applications.

Fig. 2. Efficiency comparison.

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