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Small-Signal Analysis of the Sigma Converter

Image of circuit representation of the sigma converter.
Fig. 1. Circuit representation of the sigma converter.
For the new generation of server voltage regulators (VRs), Intel is employing the use of 48V to 1V boards to improve efficiency. Conventionally, a two-stage solution is used for higher efficiency than a single-stage solution. For 48V to 1V VRs, a quasi-parallel topology, the sigma converter, is shown to have high efficiency and high power density. In order to design the sigma converter for VR applications, the large-signal performance as well as the small-signal behavior of the sigma converter must be studied. In this paper, the small-signal model of the sigma converter is derived and examined for voltage regulator (VR) applications.

The sigma converter is composed of a fixed-frequency LLC running at the resonant frequency and a buck converter, with the inputs of the converters connected in series and the outputs of the converters connected in parallel, as shown in Fig. 1. The small-signal model of the sigma converter can be obtained by connecting the models of its components in the quasi-parallel con-figuration shown in Fig. 2. For the fixed-frequency LLC running at resonant frequency, a trans-former with turns ratio n:1 and an equivalent inductance is used. For the buck converter, the three-terminal switch model is used.

The output of the VR is required by Intel to follow a specific load-line during steady-state for server applications. In order to achieve an accurate load-line, a constant output impedance is needed. Fig. 3 shows the ideal output impedance of a design with parameters of Vin1=40V, Vin2=8V, Vo=1V, fo=fs_LLC=1MHz, fBuck=2MHz, n=40, Cr=110nF, Lr=192nH, Lm=22μH, Cin1=400nF, Cin2=2.1μF, Co=862μF, and RL= 12.5 mΩ. From the Bode plot, two double-poles and a double-zero are observed. To achieve a constant output impedance with the sigma converter, the con-verter components must be carefully designed before control designs are considered.


Image of small-signal model of the sigma converter.
Fig. 2. Small-signal model of the sigma converter.
Image of output impedance of the ideal sigma converter.
Fig. 3. Output impedance of the ideal sigma converter.
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