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1. Natural Balancing of Flying Capacitor Multilevel Converters at Nominal Conversion Ratios

   https://doi.org/10.1109/COMPEL.2019.8769724  

   Xia, Ziyu; Dobbins, Benjamin L.; Stauth, Jason T. (June 2019, 2019 20th Workshop on Control and Modeling for Power Electronics (COMPEL))

   This work explores the mechanisms and limitations of natural voltage balancing in flying capacitor multilevel (FCML) DC-DC converters. A simple discrete-time state space model is used to explore the fundamental conditions that will lead to (or prevent) natural balance of flying capacitor voltages, along with the balancing dynamics. The treatment is used to highlight straightforward ways to alleviate problems with natural imbalance by adjusting the switching scheme. The model is compared against circuit simulations and the proposed switching scheme is verified in a hardware prototype. 

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2. A Modulation Scheme for Differential-Mode ZVS Resonant-Switched-Capacitor Rectifier

   https://doi.org/10.1109/TPEL.2024.3477388  

   Rad, Sadegh Esmaeili; Gupta, Shantanu; Mazumder, Sudip K (January 2025, IEEE Transactions on Power Electronics)

   This article proposes a predictive modulation scheme for a differential mode resonant switched capacitor rectifier (DMRSCR) to achieve high efficiency and power factor correction (PFC) for wide voltage gain. The modulation scheme ensures extensive zero-voltage switching (ZVS) turn-ON on all the switches under varying sinusoidal input voltage without requiring additional circuits or sensors. Four key control parameters, namely, phase shift ratio, duty cycle ratios, and switching frequency, are controlled for the converter to maintain ZVS turn-ON, PFC, output voltage regulation, and reduced resonant inductor current ripple. The article outlines a detailed DMRSCR model to deduce the dependency of the four control and converter design parameters on the converter operation. Based on the model, a complete converter design process is provided. A DMRSCR prototype rated at 1.1 kW was built using the underscored design methodology to validate the proposed modulation scheme, reaching a peak efficiency of 98.27% 

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3. A Two-Stage Cascaded Hybrid Switched-Capacitor DC-DC Converter with 96.9% Peak Efficiency Tolerating 0.6V/μs Input Slew Rate During Startup

   https://doi.org/10.1109/ISSCC42613.2021.9365763  

   Xia, Ziyu; Stauth, Jason T. (February 2021, IEEE International Solid- State Circuits Conference (ISSCC))

   null (Ed.)

   Efficient high-conversion-ratio power delivery is needed for many portable computing applications which require sub-volt supply rails but operate from batteries or USB power sources. In such applications, the power management unit should have a small volume, area, and height while providing fast transient response. Past work has shown favorable performance of hybrid switched-capacitor (SC) converters to reduce the size of needed inductor(s), which can soft-charge high-density SC networks while supporting efficient voltage regulation [1-5]. However, the hybrid approach has its own challenges including balancing the voltage of the flying capacitor and achieving safe but fast startup. Rapid supply transients, including startup, can cause voltage stress on power switches if flying capacitors are not quickly regulated. Past approaches such as precharge networks [3] or fast balancing control [5] have startup times that are on the order of milliseconds. This paper presents a two-stage cascaded hybrid SC converter that features a fast transient response with automatic flying capacitor balancing for low-voltage applications (i.e., 5V:0.4 to 1.2V from a USB interface). The converter is nearly standalone and all gate drive supplies are generated internally. Measured results show a peak efficiency of 96.9%, <; 36mV under/overshoot for 1A/μs load transients, and self-startup time on the order of 10μs (over 100× faster than previous works). 

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4. A Control Scheme Based on Lyapunov Function for Cascaded H-Bridge Multilevel Active Rectifiers

   https://doi.org/10.1109/APEC39645.2020.9124234  

   Jean-Pierre, Garry; Altin, Necmi; Shafei, Ahmad El; Nasiri, Adel (March 2020, 2020 IEEE Applied Power Electronics Conference and Exposition (APEC))

   null (Ed.)

   The cascaded H-bridge multilevel active rectifier is a prominent converter configuration. It presents compelling advantages, including high adjustability for a number of applications, such as in solid-state transformers, traction applications, medium and high power motor drives and battery chargers. However, when the H-bridge is operating under an unbalanced load and asymmetrical voltage conditions, it becomes important to design advanced control strategies to maintain the stability of the system. In this study, a Lyapunov-function based control method is proposed for controlling the single-phase cascaded H-bridge active rectifier to achieve global asymptotic stability. A capacitor voltage feedback is added to the conventional Lyapunov-function based stabilizing control method to minimize the resonance of the LCL filter. Additionally, a Proportional-Resonant (PR) control approach is adopted to obtain the reference current signal. This increases the robustness of the current control scheme. A DC voltage balancing control procedure is also employed to prevent the unbalanced DC voltage conditions among the H-bridges. The DC voltage is controlled via a PI controller. The capability of the control approach is verified with simulation and experimental studies. 

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5. A 100kW Switched-Tank Converter for Electric Vehicle Application

   https://doi.org/10.1109/APEC.2019.8721841  

   Li, Yanchao; Ni, Ze; Liu, Chengkun; Wei, Mengxuan; Cao, Dong (March 2019, 2019 IEEE Applied Power Electronics Conference and Exposition (APEC))

   This paper presents a 100kW one-cell switched-tank converter (STC) for electric vehicle (EV) application. A new evaluation method that evaluates different converter topologies has been proposed in this paper to show the advantages of the STC over the boost converter and 3-level flying capacitor multilevel (FCML) converter. Both non-interleaved (1-phase) and interleaved (2-phase and 3-phase) operation of the STC have been analyzed. The analytical study shows that it is difficult to achieve the optimum design of the passive components such as input and output capacitors in 1-phase converter because of the high RMS current flowing through them. This means the passive components need to be over-designed in order to meet the current stress requirement. For instance, the designed capacitance of input capacitor is several times of the required value, which leads to bulky capacitor size. Therefore, this paper evaluates the potentials of using 2-phase and 3-phase interleaved operation to address this issue. Two operation modes, zero-voltage switching (ZVS) mode and zero-current switching (ZCS) mode, are evaluated to show the ZCS operation mode is more suitable for the presented converter with interleaved operation. By using the interleaving concept, the predicted 100kW 3-phase interleaved converter can achieve 60% size reduction based on the 1-phase converter design. And the predicted power density of the 3-phase interleaved STC can achieve 115kW/L power density. Simulation results are provided to validate the theoretical analysis. Both 1-phase and 3-phase 100kW prototypes under developing are shown in this paper. 

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