Modeling and Control of Quasi-Parallel Sigma DC–DC Converter for Single-Stage 48-V Voltage Regulator

With the improvement of data center processing performance, the voltage regulator module (VRM) structure of 48-V bus is being proposed more and more to improve the system efficiency and to reduce the cost of data center. For a 48–1.8-V VRM, a quasi-parallel configuration, the sigma converter, is shown to have a peak efficiency of 95.2% and a power density of 700 W/in3 [16]. In order to accurately compensate the design of the sigma converter to meet the VRM requirements, a decoupling method is proposed in this article to construct a small-signal model of a quasi-parallel structure according to the interaction characteristics of duty cycle, output voltage, and input floating-point voltage during sigma operation. Based on this decoupling model, precise analytical expressions for various transfer functions are derived, such as control-to-output and output/input impedance, whose Bode plots are highly consistent with those in SIMPLIS simulation. In order to make the derived transfer function easy for compensation design, the control-to-output transfer function is simplified to standard zero-pole form. A voltage mode control design is provided and verified through the prototype verification board of sigma structure. Tests show that the compensation design based on the derived transfer function can achieve voltage regulation under different loads.