# A High-Conversion Ratio Multiphase Converter Realized with Generic Modular Cells
## 1 Introduction  
With the development of cloud computing and data centers, the demand for efficient, high-power conversion has significantly grown up [1, 2, 3, 4, 5]. In recent years, DC distribution systems have been widely used to replace the traditional AC systems in data centers due to their higher efficiency. The increased popularity of using a 48 V DC power distribution network in today's computing industry, where multiple CPUs and DRAMs are part of the main consumer, has reinforced the need for DC-DC Point of Load (PoL) systems capable of carrying the conversion of 48 V-to-1.x V. The majority of modern supply methods are using either a two-stage conversion in series such as 48 V-to-12 V and then 12 V-to-1.x V [6, 7, 8, 9, 10, 11, 12, 13, 14, 15], or a direct-conversion approach 48 to 1.x V that could be magnetically isolated or not [16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34]. Recently, step-down conversion solutions based on unregulated switched-capacitor converters (SCC) as a first stage, followed by an inductor based regulated stage, i.e., derivatives of buck converters, have been introduced [6, 8, 10, 35]. Multiphase interleaved buck converters are generally used for the point of load stage [12, 36, 37], achieving fast dynamic response and delivering a high current to the load, while reducing the output filter volume and obtaining a high efficiency over wide load range. In a two-step high-conversion approach, the objective of the first-stage converter is to maintain a high efficiency and loosely hit the operation region of the second stage [16, 17]. Even though the first andthe second stages are efficient on their own, the overall conversion efficiency after both conversions is insufficient for many competitive applications, due to the multiplication of the two efficiencies. In addition, bulky transformers or heat sinks [10; 24; 25] are not desirable for applications that require both power density and large conversion ratios. The development of GaN-based FETs enables the use of high-density single-stage conventional buck converters for the direct-conversion approach. Yet, applying a single-stage buck converter or using multilevel buck converters for the full 48 V-to-1.x V conversion remains impractical due to the time resolution challenges, associated with narrow \"on\" times and very high switching frequencies that require PWM generation hardware with precision on the order of ten picoseconds in both the duty cycle and jitter of the signal [31; 38]. In addition, duty cycles on the order of 1/48 are still very challenging in terms of RMS losses in the circuit. Some prior art suggests using advanced control methods to improve the efficiency of the conventional buck converters [39; 40; 41; 42; 43; 44; 45; 46]. Other solutions consist of combinations of the buck topology with flying capacitor topology, offering several advantages over the two-stage conversion solutions [12; 21], such as extended conversion ratio, wide dynamic range, and the integration of multilevel and multiphase architecture features in a single conversion stage, which reduce inductor voltage swing and lower the switching loss. One derivative of the multiphase buck converter with extended-duty cycles--known as the series-capacitor buck converter--performs better than a buck converter and stands out as an attractive solution [23; 26; 27]. This converter can produce fast dynamic response to load changes, along with symmetrical operation and natural current sharing between phases, and reduce transistors' voltage stress. However, previously documented topologies with extended-duty ratio are somewhat limited for application in high-power cases, where a very high conversion ratio and high currents are involved. One solution to higher voltage gain is presented in [34] using a three-phase series-capacitor buck converter. However, it lacks the current extension and architecture generalization.