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Open Access Publications from the University of California

Scholarly Works (15 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2024)

Hybrid switched-capacitor (SC) converters have seen increased use in applications that demand both high efficiency and high power density. These converters exhibit the traditional benefits of pure SC converters, such as efficient utilization of switches and the use of energy-dense capacitors, along with lossless capacitor charge transfer due to the use of one or more augmenting inductors. Increased performance can also be obtained by using more complex control schemes than the traditional two-phase control common with pure switched-capacitor converters.

Split-phase control, one such modified control scheme, is used to ensure full soft-charging of all flying capacitors in several hybrid switched-capacitor topologies belonging to the Dickson-derived class of converters. Here, capacitors are inserted into the switch-capacitor network in a staggered manner to ensure that they do not over- or under-charge in each phase, which would result in lossy hard-charging transitions. However, the time at which to insert these capacitors can change based on operating condition, component tolerance, and phase-ordering. This work will present an analysis of these effects, as well as describe specific control schemes and active-tuning methods that can ensure full soft-charging operation.

In addition, other control schemes such as multi-resonant operation, can be utilized to achieve high-performance designs. Multi-resonant hybrid switched-capacitor converters operate with multiple operating phases per switching period, and can achieve the same conversion ratio as standard two-phase hybrid switched-capacitor converters with a fewer number of switches and capacitors, allowing for higher efficiency and power density design. One such topology, the cascaded series-parallel (CaSP) converter, will be analyzed, and several high-performance hardware prototypes designed for 48 V data center dc-dc power delivery will be presented.

    Cover page: Split-Phase and Multi-Resonant Operation of Hybrid Switched-Capacitor Converters
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    Resonant switched-capacitor (SC) converters are becoming increasingly attractive for high performance applications due to their efficient utilization of switches and use of high energy density capacitors. Multiphase SC converters can offer further improvements in performance, as they can achieve the same conversion ratio as traditional two-phase SC converters with fewer capacitors and switches, making them ideal for large conversion ratio applications. This paper presents theoretical analysis and experimental results for a resonant multi-phase SC converter comprising a cascaded series-parallel topology derived from the conventiona1 4-to-1 series-parallel converter. The proposed converter can achieve a 6-to-1 conversion ratio with the same number of switches and capacitors as the 4-to-1 series-parallel converter. A 48-to-8 V prototype designed for data center applications was built and tested with 40 A maximum output current. The prototype achieved 99.0% peak efficiency (98.5% including gate drive loss) and 2230 W/in 3 power density, demonstrating one of the best in-class performances.
      Cover page: A High Performance 48-to-8 V Multi-Resonant Switched-Capacitor Converter for Data Center ApplicationsCreative Commons 'BY-NC-ND' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2020)
      Compared with conventional switched-capacitor (SC) converters with two operating phases, multi-phase SC converters can achieve the same conversion ratio with fewer capacitors and switches. This feature makes multi-phase SC converters an attractive candidate for applications with large conversion ratios. To further improve the performance by enabling soft-charging and soft-switching operations, SC converters can be transformed to resonant switched-capacitor (ReSC) converters with augmented inductor(s). This work presents an 8-to-1 multiphase ReSC converter design that features the least-possible number of switches and capacitors for the achieved gain. Both theoretical analysis and experimental results from a practical implementation are provided to demonstrate the benefits of the multi-phase resonant approach. A 48-to-6 V converter prototype with 40 A output current for data center applications was built and tested. The prototype achieved 98.6% peak efficiency (98.0% including gate drive loss) and 1675 W/in3 power density, achieving one of the best overall in-class performances.
        Cover page: A 48-to-6 V Multi-Resonant-Doubler Switched-Capacitor Converter for Data Center ApplicationsCreative Commons 'BY-NC-ND' version 4.0 license
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2022)
        Conventional design of coupled inductors is focused on a specific converter topology, and the size advantage of coupled inductors over uncoupled inductors is rarely analyzed systematically. This work models the core size for generalized two-phase gapped coupled inductors and provides an effective way to compare the core size for different duty ratios (corresponding to different topologies) and different coupling coefficients. Both the analytical model and hardware prototypes are provided to demonstrate a significant size reduction by using tightly-coupled inductors in hybrid switched-capacitor converters with a large duty ratio. The proposed coupled inductors are applied in a Multi-Level Binary (MLB) Point-of-Load (PoL) converter, achieving 94.4% peak system efficiency and 474 W/in3 power density with 48-to-1.8 V conversion and 60 A output. Both the efficiency and power density are higher than the conventional MLB-PoL converter using discrete inductors.
          Cover page: Core Size Scaling Law of Two-Phase Coupled Inductors – Demonstration in a 48-to-1.8 V Hybrid Switched-Capacitor MLB-PoL ConverterCreative Commons 'BY-NC-ND' version 4.0 license
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2021)
          Switched-capacitor (SC) converter topologies can achieve high efficiency and high power density due to their efficient switch utilization and use of high energy density capacitors. The use of resonant inductors can further improve efficiency by enabling soft-charging and soft-switching operations. However, the number of components (switches and capacitors) increases proportionally with the conversion ratio, increasing circuit complexity and potentially reducing performance. Because of this, SC converters with multiple operating states per switching cycle are attractive for high-step down applications as they can achieve the same conversion ratio as two-phase SC converters with fewer components. This work proposes and explores a new multi-phase resonant cascaded series-parallel (CaSP) topology, comprising a cascaded 2-to-1 SC stage with a 4-to-1 series-parallel stage. A 48-to-6 V prototype design for data center applications was built and tested, and achieved 98.6% peak efficiency (98.1% including gate drive loss), 95.3% full-load efficiency (95.2% including gate drive loss) at 70 A output, and 2140 W/in3 power density by box volume.
            Cover page: A High Performance 48-to-6 V Multi-Resonant Cascaded Series-Parallel (CaSP) Switched-Capacitor ConverterCreative Commons 'BY-NC-ND' version 4.0 license
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2021)
            A 4-to-1 cascaded resonant switched-capacitor (ReSC) converter, comprising two cascaded 2-to-1 ReSC converters, has been demonstrated to have high efficiency and high power density in previous literature. This work explores the approaches to further improve the performance of the cascaded ReSC converter, including merging two phases of the first stage into one phase and reducing the intermediate decoupling capacitor. The impact of the intermediate capacitor on the interaction between two stages is studied. With the optimization of circuit structure, active and passive component selection, layout, etc., this converter achieves a power density of 4068 W/in3 under 48 V input and 60 A output. The converter also has high efficiency for the entire load range, achieving 99.0% peak efficiency and 97.9% full-load efficiency with gate drive loss included. Both power density and efficiency are superior to the state-of-the-art.
              Cover page: A 48-to-12 V Cascaded Resonant Switched-Capacitor Converter Achieving 4068 W/in3 Power Density and 99.0% Peak EfficiencyCreative Commons 'BY-NC-ND' version 4.0 license
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2021)
              The rapid growth in data-center electricity consumption has created an imperative need for more energy-efficient solutions to data center power delivery. This work proposes a multi-phase hybrid switched-capacitor (SC) converter for direct 48 V to point-of-load (PoL) conversion with high performance. The proposed topology can be viewed as a 6to-1 cascaded series-parallel (CaSP) converter merged with a three-phase interleaved buck converter with automatic current balancing. Due to multi-phase operation, the CaSP stage is able to achieve a higher conversion ratio compared to twophase SC converters with the same number of components as well as lower turn-on voltage for switching devices. Moreover, the proposed CaSP-PoL topology benefits from the frequency multiplication effect similar to that in flying capacitor multilevel converters, which allows for lower frequency operation of higher voltage rated switches and doubled frequency operation of buck inductors, contributing to a further reduction in switching loss and inductor size. A 48 V-to-PoL hardware prototype was built in vertical assembly structure for compact packaging and tested up to 90 A output current, achieving 94.1% peak efficiency (93.2% including gate drive loss) and 702 W/in 3 power density (by box volume) at 2.0 V output voltage.
                Cover page: A Multi-Phase Cascaded Series-Parallel (CaSP) Hybrid Converter for Direct 48 V to Point-of-Load ApplicationsCreative Commons 'BY-NC-ND' version 4.0 license
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2023)
                Hybrid switched-capacitor (SC) converters have gained popularity due to their efficient switch utilization and use of energy-dense capacitors, which allows them to achieve high efficiency and power density even at large conversion ratios. The Dickson converter is one such popular hybrid SC converter, as it can achieve the theoretical minimum switch stress rating for a given operating condition. However, unlike other hybrid SC topologies that can automatically achieve full soft-charging through the addition of one or more augmenting inductors, certain Dickson variants also require the use of special switching schemes to fully soft-charge all flying capacitors. This technique, denoted as split-phase switching, inserts extra sub-phases into the control scheme so that flying capacitors are disconnected or connected at staggered times. Traditionally, split-phase timing has been calculated analytically, sometimes using imprecise models. This paper instead proposes an active control technique for detecting hard-charging events on the flying capacitors, such that the split-phase timing automatically converges on soft-charging operation. The technique is validated on an 8-to-1 resonant single-inductor Dickson hardware prototype. While this method is demonstrated on a resonant fixed-ratio converter in this work, the technique can also be applied to regulating split-phase applications, as well as used to detect hard-charging events in general.
                  Cover page: An Active Split-Phase Control Technique for Hybrid Switched-Capacitor Converters Using Capacitor Voltage Discontinuity DetectionCreative Commons 'BY-NC-ND' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2020)
                  There is an increasing need for more efficient power conversion from 48 Vdc to point-of-load (PoL) applications in datacenters. This work presents a new hybrid converter with Multi-Level Binary (MLB) voltages on the flying capacitors that is well-suited for PoL applications with very high conversion ratios. The proposed MLB-PoL converter can be viewed as an 8-to-1 multi-phase doubler switched-capacitor (SC) converter merged with a two-phase interleaved buck converter. Compared to other two-phase hybrid SC topologies, multi-phase operation can help achieve higher conversion ratio in the SC stage with an equal or fewer number of components, and thus reduce the switch andinductor stress of the following buck stage. In addition, the output inductors in the proposed topology benefit from a frequency multiplication effect similar to that of the flying capacitor multilevel (FCML) converter. This can help further reduce the inductor size without increasing the switching frequency. A 48 V to 2.5-1.0 V converter prototype with 65 A output current was built and tested. At 48 V to 2 V, the prototype achieved 95.1% peak efficiency (94.3% including gate drive loss) and 395 W/in3 power density.
                    Cover page: MLB-PoL: A High Performance Hybrid Converter for Direct 48 V to Point-of-Load ApplicationsCreative Commons 'BY-NC-ND' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2021)
                    Proper utilization of GaN devices generally necessitates ultra-low inductance printed circuit board (PCB) layout in the main commutation loop or switching cell of a power converter. The flying c apacitor m ultilevel ( FCML) t opology in particular contains many of these switching cells, thus design optimization becomes critical. A novel asymmetrical cell layout design with sub-1 nH commutation loop inductance is proposed and characterized for the FCML by utilizing a transient- and impedance-based measurement technique in conjunction with 3D field simulation. To validate its efficacy, this switching cell design is demonstrated within a prototype high performance step-down intermediate bus converter supply (vout = 48 V) with wide input voltage range (vin = 48 to 340 V).
                      Cover page: Low-Inductance Asymmetrical Hybrid GaN HEMT Switching Cell Design for the FCML Converter in High Step-Down ApplicationsCreative Commons 'BY-NC-ND' version 4.0 license
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