Dynamic Level Selection for Full Range ZVS in Flying Capacitor Multi-Level Converters

Abstract

This paper presents a control technique for flying capacitor multi-level (FCML) converters to achieve zero-voltage switching (ZVS) across the full range of duty cycles, with application in high power density and high efficiency power converters. Previous works have used variable frequency control to enable ZVS at specific duty cycles in FCML converters, but have not been able to use these methods to allow ZVS across the full range. This work uses dynamic level selection and variable frequency control to increase inductor current ripple at duty cycle ranges for which ZVS was previously unattainable. An experimental 5-level FCML prototype has been built using GaN devices on a single-sided PCB to demonstrate this control technique. We demonstrate 4-level and 5-level operation with ZVS at duty cycles that are not possible with 5-level operation alone, as well as a dynamic level transition with active capacitor voltage balancing.

Dynamic Level Changing for Full Range ZVS in Flying Capacitor Multi-Level Converters

“…This work extends our previous conference publication [11], with key additions, including the consideration of resonant operation in determining frequency limits, a method for determining the parameters for active balancing during a level transition, and additional experimental results. The remainder of this manuscript is organized as follows: Section II reviews the basics of FCML operation; Section III derives the fundamental characteristics of FCML converters that prevent ZVS operation at specific duty cycles and proposes dynamic level selection to overcome these challenges; Section IV describes the active balancing method for level transitions and presents a method to determine parameters for active balancing for shortest settling time; Section V demonstrates the method of dynamic level selection for a wide duty cycle range in hardware; and Section VI summarizes the contribution of the paper.…”

“…4) Capacitor de-rating for C f ly,k (t) and C out (t), based on V C,k (t − 1) and V Cout (t − 1) 5) Capacitor current, ±∆i L (t), depending on the subperiod 6) Change in flying capacitor voltage, (11), where dt is the length of time of the sub-period (area under the capacitor current curve)…”

Multimode Modulation With ZVS for a Single-Phase Single-Stage Inverter

“…The modes described above were obtained from a theoretical point of view regarding the control of the power flow among the three ports, however, considerations such as ZVS and limits in switching frequency must be addressed for practical implementation [28]. The proposed modulation introduces two additional switching states when the inductor current crosses 0 A.…”