High-Efficiency "Sigma" Voltage Regulator
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.