Analysis of Switched-capacitor DC-DC Converters in Soft-charging Operation

This paper formulates a formal analysis method for switched-capacitor (SC) dc-dc converters in soft-charging operation. Through soft-charging, the charging/discharging loss of a SC converter can be minimized or even eliminated by allowing the output voltage to vary to a greater extent. Usually a soft-charging SC converter is followed by a magnetic converter for ripple reduction and better regulation. For any given two-phase switched-capacitor topology, the proposed method can be used to determine whether soft-charging is at all possible and if so, the required capacitor sizes to achieve soft-charging. The proposed method also gives the resultant output voltage ripple due to soft-charging operation. A number of different topologies are analyzed, including Dickson, Series-parallel, Ladder, Fibonacci and Doubler configurations. The analysis gives insights regarding the expected improvement in performance when these topologies are cascaded by a magnetic converter. Through comparison to simulated results and existing work, the validity of the proposed method is confirmed.