Design of GaN-Based Multilevel Switched Capacitor Converters-Benefits and Challenges

With the significant reduction in board space occupied by the smaller GaN transistors, topologies that require a greater number of active devices as a tradeoff for reduced passive size, the main barrier to higher power density, have become attractive. Switched capacitor multilevel converters are good examples of topologies that can effectively reduce or eliminate passive components. Two GaN-based proof-of-concept designs, based on the three-level buck converter, one for a low-voltage (LV) 48-V server application and the other as a candidate for a higher voltage (HV) 400-V power factor correction circuit application, are discussed in this article. As a result of using lower figure-of-merit devices, significant efficiency gains are observed for the LV and HV converters developed in this article compared to a two-level topology. Smaller passive size (mainly inductors) also provides a significant increase in power density for the LV converter. A startup scheme is discussed, which eliminates the need for extraneous control loops or high-voltage-rated devices for the top switch. Finally, the thermal benefit of the three-level converter is demonstrated when compared to a two-level system for the 48-V server application.