A Floating Voltage Source-Based Turn-Off Snubber for a Non-Isolated Multilevel SiC DC-DC Converter

This article presents a turn-off snubber for a high-power, nonisolated, multilevel dc-dc converter (MLC) implemented with SiC MOSFETs. Although SiC devices are known to reduce switching losses, a snubber is essential to mitigate the high dv/dt ratio on the switch’s drain–source voltage, which results from the surge in current when the SiC switch is turned off. This work proposes a floating-voltage source (FVS)-based snubber, presenting a single-circuit solution that provides soft turn-off (ringing suppression) capabilities for multiple power switches in MLC. Moreover, it ensures that this soft turn-off feature is consistently effective across all duty-cycle (D) ratios of the MLC without any constraint. This work also presents two approaches for realizing the FVS-based snubber: dissipative and energy-regenerative methods. In this article, a prototype of a 7-kW three-level flying-capacitor buck (TLFCB) converter with a 537 V_DC input is developed. This prototype successfully demonstrates the effectiveness of the proposed snubber across D ranging from 0.2 to 0.8.

I. Introduction

NONISOLATED multilevel dc-dc converters (MLCs) are commonly employed to relieve switch voltage stress and minimize filter volume in high-voltage, high-power applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. In addition to adopting advanced MLC topologies, recent advancements in these applications have led to a significant shift towards exploiting the advantages of SiC devices instead of traditional Si technology, with the goal of further minimizing power loss [11], [12], [13]. Widely proven to be more effective, SiC devices exhibit lower on-resistance than Si devices under equivalent breakdown voltage conditions [14]. Moreover, the lower switching losses and significantly reduced diode reverse recovery losses in SiC-based MLCs could potentially decrease the necessity for soft-switching schemes, which are mandated in traditional converters implemented with Si MOSFETs and IGBTs [15], [16], [17], [18]. However, the SiC MOSFET’s large current capacity and low parasitic capacitances are likely to cause a high turn-off dv/dt ratio on the switch’s drain–source voltage, resulting in over-voltage spikes, ringing, and gate driver malfunctions [19], [20], [21]. To suppress such fatal faults, thereby ensuring reliability in high-power applications, the SiC-based MLC inescapably necessitates the use of a turn-off snubber. Of course, the operational requirements of the turn-off snubber may limit the MLC’s maximum available switching frequency.