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Fast-Transient Control Scheme for 48V-PoL Four-Phase Series Capacitor Buck Converter

Circuit diagram
Fig. 1. Four-phase SCB converter with COT control
  This work presents the transient performance of four-phase series capacitor buck (SCB) converter with the proposed improved constant on-time (COT) control scheme for VRM application. The four-phase SCB converter can either work in four-phase interleaving or two-phase interleaving operation. Unlike conventional multiphase buck converter, the SCB converter only allow phase overlap between non-subsequent phases. Due to the phase overlap restriction, it is revealed that the two-phase operation has a better load step-up transient performance, whereas the four-phase operation has a better load step-down transient performance.

  In order to achieve a good transient performance at both load step-up and load step-down transient, a control strategy to use four-phase operation at steady-state and allowing phase-overlap between non-subsequent phase at load step-up transient is proposed. A modification both in the phase manager and clock generator is done to enable phase overlap between non-subsequent phases at load step-up transient. With the proposed control scheme, the load step-up transient performance is improved while maintaining a good load step-down transient performance of four-phase operation.
Simlation results
Fig. 2. Load step-up transient simulation results: (a) four-phase interleaving operation with the proposed control scheme (b) conventional four-phase interleaving operation

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