ANALYSIS, OPTIMIZATION, AND PRACTICAL DESIGN OF A SPLIT-PHASE DICKSON SWITCHED CAPACITOR CONVERTER FOR HIGH CURRENT APPLICATIONS

Abstract

The Switched-Capacitor Converter (SCC) topology has been gaining attention from researchers because of their advantages of higher power density, better switch utilization, and low component stress. However, SCCs have a fundamental downside in which capacitor charge redistribution causes significant current spikes. This can be solved using as split-phase control, which addresses charge redistribution in the Dickson SCC by controlling the charging and discharging of the flying capacitors, such that the equivalent branch voltages line up when switching occurs. In adapting the split-phase Dickson SCC topology for high-current applications, two significant sensitivities of split-phase control were identified. Differences between the actual values of the flying capacitors results in split-phase control not fully compensating for charge redistribution, due to the capacitors’ uneven charge and discharge rates. Furthermore, input voltage ripple tends to cascade into the circuit, leading to voltage imbalance within the circuit. To minimize capacitor mismatch and factoring in the problem of ceramic capacitor de-rating, the flying capacitors of the split-phase Dickson SCC were designed by referring to capacitor de-rating curves supplied by component suppliers. However, potential inaccuracies in the curves, combined with typical manufacturing tolerances of ±20%, led to the development of an in-circuit flying capacitor measurement method to allow for possible tuning of the flying capacitors. The sensitivity of the Dickson SCC to input voltage ripple was initially bypassed by using a very large input capacitor to suppress input voltage ripple. However, this resulted in additional losses due to the equivalent resistance of the electrolytic capacitor that was used. As such, an input voltage compensation method, which adjusts the duty cycles of the ‘split-phase’ switches to compensate for the additional voltage difference resulting from input voltage ripple interference, was developed. The research presented in this thesis provides in-depth analyses of both sensitivities of the split-phase Dickson converter, as well as a discussion on their impacts on the design of components for the high-current split-phase Dickson SCC. Test results of a 48 V to 12 V, 35 A split-phase Dickson SCC prototype are also presented and discussed.