An Overview of Hybrid DC-DC Converters: From Seeds to Leaves

With the surging demands for higher current at sub-1-V supply level in high-performance digital systems, high-efficiency and high-current-density power converters are essential for system integration. Higher voltage supply buses are emerging for high-current applications to reduce the IR losses on the power delivery networks. Thus, there is a wide voltage gap between the power bus and the digital supply rails at the point of load (PoL). Meanwhile, battery-powered portable or wearable devices favor extremely high-power-density solutions, calling for novel power conversion topologies, which have been the hottest topic in the power management IC area in the past decade. This article reviews the switched-capacitor-inductor (SCI) hybrid dc-dc buck converters from the topology "seeds" to their "leaves." Here, we define six seeds, they are: 1) three-level buck; 2) double-step down buck; 3) inductor-first buck; 4) always-dual-path buck; 5) buck-buck; and 6) multiple-output hybrid buck. We try to analyze and summarize their pros and cons, and to derive the evolution of the hybrid dc-dc converters, with milestone examples. Then, we share our observations, design intuitions, and suggestions to help the researchers and engineers to pick up and design a new SCI hybrid dc-dc converter.