Capacitive Divider Based Passive Start-up Methods for Flying Capacitor Step-down DC-DC Converter Topologies

摘要

This paper introduces two methods for limiting the voltage stress during start-up across switches of multi-level flying capacitor (ML-FC) step-down dc-dc converters. For a general N-level converter, the presented methods reduce the voltage stress to (N-1) times lower value than that of a conventional buck, allowing lower voltage rating transistors with smaller specific on-resistances to be used. These methods require no active control of switches on initial start-up and rely on segmentation of the input filter capacitor or the utilization of flying capacitors as part of voltage dividers. The speed of these schemes is limited only by the size of the flying capacitor, parasitics in the conduction pathway, and the quality of the start-up diode and the low side switch body diodes. The methods have been verified on a 3-level buck 24V-to-5V, 20W prototype, with input voltage rise times of less than 1μs showing the effectiveness of the start-up circuits.