Design and Analysis of a High Step-Down Ratio Capacitively-Isolated Flying Capacitor Multilevel Resonant Converter

High-performance, high step-down ratio isolated dc-dc converters are typical components in grid-tied rectifiers for applications such as data center power delivery and consumer electronics charging. Although transformers are often utilized to achieve galvanic isolation, capacitive isolation offers an alternative approach. This work proposes and analyzes a capacitively-isolated resonant dc-dc converter employing a flying capacitor multilevel (FCML) bridge leg to achieve a high step-down ratio with zero voltage switching. The FCML bridge leg is of particular use in this application as the topology inherently provides both voltage conversion and excites the resonant tank with a frequency which is a multiple of the switching frequency. Limits on the maximum permissible isolation capacitance are identified through analysis of the constraint of touch current. A 380V to 47V hardware prototype utilizing class Y2 safety capacitors is constructed and tested to a maximum output power of 680W.