Flying Capacitor Multilevel Converter Research Articles

Modeling and Analysis of Shutdown Dynamics in Flying Capacitor Multilevel Converters

This work explores the dynamic behavior of the flying capacitor multilevel (FCML) converter during unplanned shutdown. A model for a general N-level FCML converter is developed, which captures capacitor non-linearities, component leakage paths, and body diode behavior. This work highlights how switch voltage ratings may be exceeded during unplanned shutdown, and proposes several mitigation strategies. Using a ten-level FCML converter hardware prototype, the time-domain behavior of the model is verified, and a successful hardware mitigation strategy is demonstrated which ensures safe and rapid converter shutdown.

Design and Flight Qualification of a Flying Capacitor Multilevel Converter for Electric Aircraft Applications

Multilevel switched-capacitor converters, such as the flying capacitor multilevel (FCML) converter, offer high energy density and efficiency, over a wide voltage range, making them suitable for applications in electrified aircraft. However, concerns of reliability due to the increased number of active and passive components have limited the use of high-level count converters in aircraft applications. This work addresses the challenges of designing a high-level count converter with the goal of extremely high efficiency and power density, with maintained reliability and flight readiness. We present detailed component selection and design techniques to achieve this goal, along with a high-performance 28.2 kW/kg, 99.52% efficient high-voltage dc-dc step-up power converter for experimental validation. Finally, we demonstrate successful completion of critical flight qualification testing of the hardware prototype, including thermal, shock and vibration.

A Novel Carrier Based Precharging Technique for Single-Phase Odd-Level Flying Capacitor Multilevel Converters (FCMCs)

Flying Capacitor Multilevel Converter(FCMC) needs all its flying-capacitors charged to the appropriate voltage levels during stat-up. This paper presents a novel carrier based precharging technique for single-phase odd-level FCMCs. A new set of modified phase-shifted carrier signals are derived for precharging. These carrier signals when compared to zero modulation signal, ensure the synthesisation of sufficient number of zero output voltage switching modes (zero-modes) of the converter. The proposed precharging technique is detailed using a five-level FCMC and then generalized to higher odd-level FCMCs. State-space averaging models are derived to validate the proposed technique mathematically. Extensive simulation and experimental results are provided to validate the functionality of the proposed technique.

A new flying capacitor multilevel converter topology with reduction of power electronic components

High power capacity and reliability are characteristics of multilevel inverters. The using a collection of DC sources can produce a terminal voltage that is very close to sinusoidal. The power quality can be improved by adding more levels, but this makes the control system more complicated and expensive. The number of power components in a multilevel inverter has been studied for decades. So, research needs to be done on multilevel inverter configurations to find ways to add levels with fewer power switches than with traditional topologies and those that have already been proposed. In this research, a new power-efficient arrangement of a flying-capacitor inverter is introduced. In order to illustrate the suggested topology, a seven-level multilevel inverter is constructed and demonstrated in a simplified form. Fewer power components, including power switches, capacitors, and gate driver circuits, are required in this topology than in other topologies described in the recent literature, which is one of its main advantages. The improvement mentioned above can be seen in the way this topology works, which is shown by the characteristics of the circuit. MATLAB/Simulink R2021a is used to simulate and verify the circuit to ensure the proposed topology is correct.

Improved Variable Switching Frequency Control for Capacitor Voltage Ripple Regulation in Multilevel Flying Capacitor Converter

For multi-level flying capacitor (FC) converter in dc-ac applications, variable switching frequency (VSF) control is an attractive solution to regulate the FC voltage ripple compared with the constant switching frequency (CSF) control. However, it is shown that the existing VSF control presents a higher switching loss in the three-phase operation when the identical switching frequency is applied to three phase-legs to reduce the complexity of VSF control implementation. To reduce the device switching losses in three-phase operation, this letter proposes a new strategy for the switching frequency selection, which allows each phase-leg to select its switching frequency individually. The control complexity of the proposed method is also reduced and is very similar to CSF control. The experiment result is also shown to verify the proposed variable switching frequency control strategy.

Detection and Identification of Power Device Failures Using Discrete Fourier Transform for Fault-Tolerant Operation of Flying Capacitor Multilevel Converters

For a flying capacitor multilevel converter (FCMC), prompt detection of power switch failures is crucial for fault-tolerant operation. This article presents a new technology for identifying and locating faulty cells in FCMC. Mathematical derivation points out that different fault types and locations will present different high-frequency harmonics. The new technology extracts high-frequency harmonics in the switch node voltage and looks up a preestablished table to identify the fault type and location. The discrete Fourier transform (DFT) can be easily performed to analyze the harmonic using the built-in IP core in field-programmable gate array (FPGA). For an FCMC with any number of levels, the new approach requires only one voltage sensor and can accurately locate the faulty cell after one carrier cycle when the fault occurs. Both simulation and experimental results validate the proposed concept on a five-level 100-kHz GaN FET-based FCMC prototype. Furthermore, we have experimentally demonstrated the fault-tolerant operation of the FCMC after a switch short and open fault is detected and identified by the new technique.

Analysis of Capacitor Voltage Imbalance in Hybrid Converters and Inherently Balanced Operation Using Symmetric Architecture

This letter investigates the origin of the flying capacitor voltage imbalance in hybrid converters. An intuitive voltage–charge relationship is established to give a general explanation of the flying capacitor voltage balance in hybrid converters. The relationship is applied to devise a relatively simple and intuitive method to identify the behavior and performance difference of capacitor voltage and inductor current balancing performance in hybrid converters for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V_{\text{out}}&lt; \frac{V_{\text{in}}}{N}$</tex-math></inline-formula> cases. Conventional hybrid converters with an even number of inductor charging intervals are shown to be susceptible to flying capacitor voltage imbalance, while flying capacitors in hybrid converters with inductors having odd charging intervals have inherently balanced operations. As a direct result of the analysis, a new symmetric architecture and the operation of three-level buck converters are introduced to achieve an inherent balance of flying capacitor voltages, which can be applied to other flying capacitor multilevel converters. Hardware prototypes are implemented and measured for the verification of the analytical analysis and the new symmetric operation

Decentralized Control for Balancing the Cell Voltages of a High Conversion Ratio Flying Capacitor Multilevel Converter

This article presents a decentralized control strategy for the balancing of the cell voltages of a flying capacitor multilevel converter, implementing a large number of cells to address a high conversion ratio. The originality of this article is that no reference is required for regulating the capacitor voltages leading to an auto-balance capability. It is composed of several local cell-voltage balancing controllers and a global load-current regulator. For theoretical analysis purposes, a change of state variables is proposed, leading to a new model of the converter. Then, the study of the decentralized balancing method is performed based on a modal analysis of the system. A decoupling of the output current regulation and the cell-voltage balancing loops is proposed, and then observed. The synthesis of the controllers, guaranteeing the stability of the system within a large bandwidth for each mode, is obtained. To address the fault-tolerance requirement, this decentralized control strategy provides an opportunity for auto-reconfiguration, allowing changes in the number of active cells during operation. Simulations of the separated modal responses show the stability and bandwidth of the control loops for dc/dc 500 to 16 V high conversion ratio applications. Experimental results, performed on a 5-cell multilevel converter, demonstrate the validity of the proposed control method.

A Six-Level Flying Capacitor Multilevel Converter for Single-Phase Buck-Type Power Factor Correction

This article investigates behavior of flying capacitor multilevel (FCML) converters in single-phase buck-type power factor correction (PFC) applications. Recent developments in FCML converters using gallium nitride (GaN) transistors are leveraged to improve power density of a single-phase 240 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{V}_{\text{RMS}}$</tex-math></inline-formula> ac to direct 48 V dc conversion stage in data center power delivery applications. Here, we experimentally demonstrate this concept in a digitally controlled six-level FCML converter hardware prototype. The experimental prototype can deliver 4.5 A average output current at 48 V, resulting in 216 W output power. A key contribution of this article is experimental demonstration of an FCML buck converter in a single-phase PFC application where the flying capacitor voltages follow fractions of the rectified input voltage by swinging at twice-line frequency.

Model Predictive Control Using Artificial Neural Network for Power Converters

There has been an increasing interest in using model predictive control (MPC) for power electronic applications. However, the exponential increase in computational complexity and demand of computing resources hinders the practical adoption of this highly promising control technique. In this article, a new MPC approach using an artificial neural network (termed ANN-MPC) is proposed to overcome these barriers. A power converter with a virtual MPC controller is first designed and operated under a circuit simulation or power hardware-in-the-loop simulation environment. An artificial neural network (ANN) is then trained offline with the input and output data of the virtual MPC controller. Next, an actual FPGA-based MPC controller is designed using the trained ANN instead of relying on heavy-duty mathematical computation to control the actual operation of the power converter in real time. The ANN-MPC approach can significantly reduce the computing need and allow the use of more accurate high-order system models due to the simple mathematical expression of ANN. Furthermore, the ANN-MPC approach can retain the robustness for system parameter uncertainties by flexibly setting the input elements. The basic concept, ANN structure, offline training method, and online operation of ANN-MPC are described in detail. The computing resource requirement of the ANN-MPC and conventional MPC are analyzed and compared. The ANN-MPC concept is validated by both simulation and experimental results on two kW-class flying capacitor multilevel converters. It is demonstrated that the FPGA-based ANN-MPC controller can significantly reduce the FPGA resource requirement (e.g., 2.11 times fewer slice LUTs and 2.06 times fewer DSPs) while offering a control performance same as the conventional MPC.

Investigation of Capacitor Voltage Balancing in Practical Implementations of Flying Capacitor Multilevel Converters

Capacitor voltage natural balancing is an attractive feature of flying capacitor multilevel (FCML) converters. However, with the commonly used phase-shifted pulsewidth modulation, the capacitor voltages still can deviate, and active balancing is often required. Although the natural balancing mechanism and its dynamics have been extensively studied in existing literature, some sources that are responsible for capacitor imbalance in engineering practice are still unclear. This article experimentally investigates the origins of the voltage imbalance in practical implementations of such converters. It presents the corresponding circuit analysis as well as solutions that improve balancing. It is shown that the source impedance and the input capacitance can greatly deteriorate capacitor balancing. Moreover, we also demonstrate in theory and with experiments that an FCML converter with an even number of levels inherently has stronger immunity to such disturbance than that with an odd number of levels. It is also found that the gate signal propagation delay mismatch in half-bridge gate drivers can lead to capacitor imbalance, and this problem is addressed by an alternative gate drive power supply design. Finally, the variations of <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> -state resistance among different switches are found to have a relatively small impact on capacitor voltage balancing.

Dynamic Level Changing for Full Range ZVS in Flying Capacitor Multi-Level Converters

This paper presents a control technique for flying capacitor multi-level (FCML) converters to achieve zero-voltage switching (ZVS) across the full range of duty cycles, with application in high power density and high efficiency power converters. Previous works have used variable frequency control to enable ZVS at specific duty cycles in FCML converters, but have not been able to use these methods to enable ZVS across the full range. This work uses dynamic level selection and variable frequency control to increase inductor current ripple at duty cycle ranges for which ZVS was previously unattainable. An experimental 5-level FCML prototype has been built using GaN devices on a single-sided PCB to demonstrate this control technique. We demonstrate 4-level and 5-level operation with ZVS at duty cycles that are not possible with 5-level operation alone, as well as a dynamic level transition with active capacitor voltage balancing.

Five Level Flying-Capacitor Multilevel Converter Using Dynamic Voltage Restorer (DVR)

This paper deals with dynamic voltage restorer (DVR) controlled by a five-level flying-capacitor multi level converter. To decrease the power-quality disturbances in distribution system, such as voltage imbalances, harmonic voltages, and voltage sags. The organisation of this paper has been divided into three parts; the first one eliminates the modulation high-frequency harmonics using filter increase the transient response. The second one deal with the load voltage; and the third is flying capacitors charged with balanced voltages. The MATLAB simulation results effectively for five level flying capacitor multilevel converters charged with balanced voltage regulation.

Theoretical Analysis and Experimental Validation of Flying-Capacitor Multilevel Converters Under Short-Circuit Fault Conditions

Addressing the increasing demand for high-efficiency and high-power-density converters, the flying-capacitor multilevel converter has shown itself as a promising topology. A key advantage of this topology is the reduced voltage rating of the switches, though also makes it vulnerable to device failure during short-circuit conditions. Despite large interest in fault-tolerant operation of these converters, alongside detailed descriptions of flying-capacitor balancing, little research has focused on the converter short-circuit fault analysis, which may cause a switch failure if not properly designed for. Therefore, this article presents a comprehensive model describing the large-signal short-circuit switching behavior of a general N-level flying-capacitor multilevel converter. Highly simplified models used to predict the evolution of the switch current and voltage stress during the fault are proposed, targeted at practicing engineers for conservative design guidelines. These models are used to determine the critical time for remedial action of the converter before reaching some predefined maximum conditions. A 2-to-10-level fully configurable flying-capacitor multilevel converter and a fault circuit hardware prototype are used to experimentally perform different short-circuit tests that show a good match to the measured behavior.

Capacitor and input voltage estimation scheme for Flying Capacitor Multilevel Converters modelled as a Petri Net

This paper presents an estimation method for the capacitor and input voltages in Flying Capacitor Multilevel Converters (FCMC). The scheme uses only the output current and voltage measurements. Furthermore, the estimation scheme is represented as a Petri Net, allowing a compact and formal approach to represent the estimation process. This scheme simplifies the needed hardware by using only one voltage sensor, simplifying its associated electronics and communication channels, allowing its real-time implementation in systems with a high number of levels. Simulation and experimental results obtained with a nine-level FCMC current-controlled chopper are presented. The converter control loops use the estimates. Different operating conditions are used to evaluate the system, verifying that the estimation scheme can successfully replace the (n-1) needed voltage sensors with only one.

Open Circuit Switch Fault Detection in Flying Capacitor and Cascaded H-Bridge Multilevel Converters

This article proposes a simple and fast active power device open circuit fault detection and localization technique. The proposed technique applies to the family of flying capacitor multilevel converters (FCMCs), including dc/dc FCMCs, single or multi-phase H-bridge FCMCs, and cascaded H-bridge multilevel converters. The localization technique needs to sense the voltage and direction of current at the output terminals of converters to detect and localize the fault. A circuit with multiple window detectors is utilized to track the output voltage level and current direction with high bandwidth. The algorithm compares the measured and expected terminal voltage, considering the commanded switch states and the terminal current direction. Once potential fault locations are identified, healthy switches are filtered out from the set of potential faulty switches over successive switching transitions. The proposed technique is experimentally validated on a single phase H-bridge FCMC. The switch state commands and pulsewidth modulation (PWM) is handled by a DSP while the fault detection and localization algorithm is implemented on a separate field programmable gate array (FPGA). The asynchronous operation of the DSP and FPGA is discussed.

Estimation of the capacitor voltages in flying capacitor multi‐level converters

Flying capacitor multi-level converters use charged capacitors as a critical element. For proper operation, the capacitor voltages must be known and regulated. Direct measurement is straightforward, but when the number of required measurements is high, this is a complicated approach. This paper introduces an estimation method for n capacitor voltages. The scheme is based on a system of ( n + 1 ) equations, which is defined in a way that it incorporates information of the actual capacitor voltages and open-loop estimates of the capacitor voltages. The solution of the system gives the best estimates of the capacitor voltages in the sense of least square errors. The method requires only output voltage and output current sensors and its computational burden is low, saving cost and design efforts, and simplifying the hardware system. The performance of the proposed scheme is evaluated experimentally in a 9-level flying capacitor chopper, where the estimates are used by the voltage balancing strategy and by the output current controller.

A Single-Objective Modulated Model Predictive Control for a Multilevel Flying Capacitor Converter in a DC Microgrid

This paper presents a single-objective modulated model predictive control for a bi-directional DC-DC flying capacitor (FC) converter in a microgrid. The presence of FC facilitates the converter to integrate a low-voltage battery to a high-voltage DC bus at reduced voltage stress on its power switches. The converter in such a configuration demands a multi-objective controller to accomplish DC bus and FC voltage regulations, and bi-directional power flow. The proposed controller realizes these multiple control objectives by determining optimum duty ratio for the power switches using a single-objective cost function based on battery current. In doing so, the converter realizes its multiple control objectives without weighting factors in the cost function and operates its power switches at a fixed switching frequency. The proposed controller also eliminates an additional control loop by utilizing an improved dynamic reference model to generate appropriate battery current reference for the DC bus voltage regulation and bi-directional power flow. Finally, the proposed system is validated experimentally under step-response of DC bus voltage, load, PV power, and system parameter variations, and compared with a finite control set model predictive control to prove its effectiveness.

Design, Operation, and Loss Characterization of a 1-kW GaN-Based Three-Level Converter at Cryogenic Temperatures

This article investigates the potential for high power density, high-efficiency power conversion using GaN-based flying capacitor multilevel power converters at low temperature. Specific focus is given to the application of hybrid aircraft powered by liquefied natural gas. Building off of prior characterization of GaN switches over temperature, this article outlines the impact of near-cryogenic operation on the losses of a complete 1-kW inverter. Specific focus has been given to the characterization of the losses in ceramic capacitors and inductors at near-cryogenic operation. A 1-kW GaN-based three-level power converter was designed and successfully tested from room temperature down to $-$ 140 $^{\circ }$ C. Along with the first demonstration of a flying capacitor multilevel converter and associated components at such low temperature, a breakdown of losses over temperature is included to facilitate a better understanding of the opportunities and challenges introduced by this article.

A Symmetric Modified Multilevel Ladder PMIC for Battery-Connected Applications

This article describes a symmetric modified multilevel ladder (SMML) converter that is capable of directly converting down from Li-ion battery voltage ranges to system-on-chip (SoC)-compatible voltage ranges while using low-voltage power MOSFETs in the power stage. Compared to conventional flying-capacitor multilevel converters, the SMML converter features reduced conduction losses, naturally balanced flying capacitors, and internal generation of all the supplies required for drivers and level shifters. A prototype shows that the converter achieves a peak efficiency of 90% and a peak power density of 0.52 W/mm2 while providing up to 16.7 $\times $ conversion ratios at still acceptable efficiencies.