Small Signal Modeling of a Four-Level Flying Capacitor Multilevel Totem-Pole PFC Converter

Abstract

In this paper, the small signal model for a four-level flying capacitor multilevel (FCML) totem-pole PFC converter is presented. In contrast to conventional PFC converters, the state space equations for the FCML PFC converter in a complete switching cycle change over the line cycle. If the standard state space averaging technique is applied, it will only evaluate a single combination of state space equations corresponding to only one segment of the line cycle. Since the four-level FCML PFC converter consists of three different segments in one half-line cycle, this technique is not applicable to derive a comprehensive small signal model of the converter that is required for regulation and transient stability. Moreover, the effects of flying capacitors on the FCML PFC dynamics are nulled out using the average model due to their natural balancing capability. In this work, the Fourier analysis of time-interval modulated switched network is used to determine the closed form small-signal control to output frequency response and verify its accuracy with the experimental results. The dynamic characteristic of the FCML converter is also evaluated for the variations in the converter passive elements. Finally, a hardware prototype is designed, fabricated, and tested for ac input 120 Vac, 400-V dc output, and 1-kW power rating demonstrating peak efficiency of 98.48%, power factor 0.995 and THD of 4.26% to observe the system behavior under load step changes.