Flying Capacitor Converter Research Articles

Two Stages FCS-MPC for a Flying Capacitor Converter With Improved Switching Performance

Two Stages FCS-MPC for a Flying Capacitor Converter With Improved Switching Performance

A Reconstructed Nearest Level Modulation Method With ABC Coordinate-Based Deadbeat Predictive Control for the Four-Level Single Flying Capacitor Converter

A Reconstructed Nearest Level Modulation Method With ABC Coordinate-Based Deadbeat Predictive Control for the Four-Level Single Flying Capacitor Converter

Investigation of Soft-Switching QSW Technique in DC/DC SiC-Based Flying Capacitor Converter With Q2L Control

This paper investigates the soft-switching quasi-square-wave (QSW) technique in DC/DC SiC-based flying capacitor converter with quasi-two-level (Q2L) control establishing design guidelines for constructing high-efficiency soft-switched bidirectional converters with low-voltage power transistors. The general idea behind the soft-switching in the proposed system is based on near-CRM (critical conduction mode) operation using TCM (triangular current mode) and Q2L control with the addition of auxiliary transistor capacitances. The soft-switching process is based on a resonance between the capacitances in parallel to each transistor, and the main inductor - ZVS at turn-on is ensured through proper modulation pattern according to TCM-Q2L control, and the turn-off soft-switching mechanism is assured through the novelty in the form of adding auxiliary capacitance in parallel to each of the SiC power MOSFETs according to QSW operation. Thus, the converter can reach very high efficiency (peak at 99.5%), maintaining soft-switching for a wide operating range, using only small auxiliary SMD capacitors and a low-volume flying capacitor, leading to high power density. The presented study is based on experimental tests using a 1.5 kV model at up to 15 kW and includes the impact of various auxiliary capacitance values and optimal capacitor value selection for maximizing the converter efficiency.

Robust Single-Loop Control Strategy for Four-Level Flying-Capacitor Converter Based on Switched System Theory

For the four-level flying capacitor converter (FFCC), it is required to simultaneously control the output voltage and balance the flying-capacitor voltages. Existing control methods for the FFCC always employ multiple control loops, where for every controlled voltage variable an independent control loop is to be designed. However, such a control structure of multiple loops leads to a complex coupling effect among these loops. Due to the coupling, it is difficult to achieve a coordinated design of a large number of control parameters and demonstrate the control system stability. To simplify the control design and the implementation, this article proposes a single-loop control strategy by using the switched system theory. All the voltages are driven towards their desired values with a single control loop, instead of using several individual controllers. Thus, the issue of devoting efforts to dealing with the couplings among multiple control loops is bypassed. Moreover, the system's large-signal stability is ensured in a straightforward manner, thereby simplifying the stability analysis. In the design process, the system parameter uncertainties are also considered. Comparative experiments demonstrate the good transient performance and the strong robustness of the proposed control strategy.

Accelerated Model Predictive Control Based on a Greedy Algorithm With Round-Robin Priority for N-Level Single-Phase Flying Capacitor Converter in Solid-State Transformer

Accelerated Model Predictive Control Based on a Greedy Algorithm With Round-Robin Priority for <i>N</i>-Level Single-Phase Flying Capacitor Converter in Solid-State Transformer

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.

Switching Control Strategy for DC–DC Converters Based on Polynomial Lyapunov Function and Sum-of-Squares Approach

The introduction of switched system theory opens new perspectives for the control design of dc/dc converters. Regarding the classical two-level converters, two switching control design methodologies have been developed based on the common quadratic Lyapunov function (CQLF) or piecewise quadratic Lyapunov function (PQLF). However, the CQLF-based design method is not applicable for the multilevel dc/dc converter (MC). As for the PQLF-based design method, it encounters the problem that the control design is very complex, involving a nonlinear optimization problem. To further advance the application of the switching system theory in power converters, a design framework based on the polynomial Lyapunov function and the sum-of-squares approach is proposed in this article. The proposed design method generalizes the CQLF-based one. Moreover, compared with the PQLF-based design method, the mathematical design procedure can be significantly simplified. Experimental validations on an MC, i.e., the three-level flying capacitor converter and a two-level Buck-Boost converter illustrate the versatility and the effectiveness of the proposed design framework. Moreover, comparative tests show the good control performance of the designed switching control strategy. The currently presented works are preliminary but could constitute an interesting direction for developing control solutions of power converters.

フライングキャパシタ変換器をベースとした非絶縁形DC-DC変換器の電流制御法比較

This paper presents a bidirectional non-isolated dc-dc converter based on three-level flying-capacitor converters. The converter uses multiple auxiliary converters, each of which is based on cascaded chopper cells and an inductor, which enables active control of the inductor current. This paper compares the following two current control methods: conventional sinusoidal-wave current control method and proposed trapezoidal-wave current control method, in terms of ac-current peak values, dc-current values, current root-mean-square (RMS) values, voltage ripples in dc-capacitor voltages, current ripples in dc currents, and converter efficiency. The validity of the comparison and trapezoidal-wave current control method is verified in a simulation using MATLAB and experimentally using a 2-kW downscaled model.

Validation of the Quasi-Two-Level Operation for a Flying Capacitor Converter in Medium-Voltage Applications

Standard medium-voltage converters are operated at low switching frequencies using bulky passive components. One concept to change this involves the quasi-two-level operation (Q2O) of multilevel converters that use fast-switching semiconductors to minimize the need for passive components. The flying capacitor converter (FCC) uses SiC semiconductors and operates with Q2O to minimize passive components. In this paper, two different quasi-two-level algorithms are analyzed. A medium-voltage prototype was built and low-voltage and medium-voltage measurements were used to validate the concept. A particular focus is on the overvoltage, the dv/dt behavior of the converter, as well as the dynamic behavior.

A Split-Inductor Flying Capacitor Converter for Medium-Voltage Application

This article presents a split-inductor flying capacitor (SIFC) converter for medium-voltage applications to improve reliability and eliminate the shoot-through issue. Reliability and lower total harmonic distortion (THD), mainly caused by the pulsewidth modulation (PWM) dead time, are the vital aspects while designing higher power density converters. The dead time in the PWM can be reduced/removed in the proposed converter. This will increase the voltage/current gain and enhance the quality of the voltage/current waveforms even at lower modulation index and light loads than the traditional flying capacitor (FC) and neutral point clamped (NPC) converters. Furthermore, an improved phase-shifted PWM (IPS-PWM) is used to maintain the flying capacitor’s nominal reference voltage. A comparison is made between SIFC and five different existing multilevel topologies. The proposed converter has higher voltage/current gain, lower THD over conventional FC and NPC converters, and higher efficiency than the other three topologies. Finally, the SIFC converter’s robustness with IPS-PWM is validated using experimental results in different operating conditions such as sudden load changes, modulation index, output frequency, and power factor (unity, lagging, and leading).

Vector Analysis Based Multiobjective-Modulated Model Predictive Control for Four-Switching-State Multilevel Converters

The article proposes a new vector analysis based model predictive control (VAMPC) method for multilevel converters with four different switch configurations. It is based on track of an error vector using fundamental state vectors, and can achieve deadbeat control of four switching states multilevel converters with multiple control objectives. Compared with conventional finite control set model predictive control (FCS-MPC) method, VAMPC can achieve excellent spectrum characteristics in the case of a slight increase in computational burden. The VAMPC method is experimentally validated on a 100-kHz GaN-based three-level flying capacitor (FC) converter. Compared with the FCS-MPC, VAMPC leads to significant reduction in steady-state control errors. For example, the inductor current total harmonic distortion of the FC converter is now reduced by more than 60%.

멀티레벨 플라잉 커패시터 컨버터의 플라잉 커패시터 초기충전 방안에 관한 연구

This paper proposes the flying capacitor(FC) initial charging method of the multilevel FC converter by using simple RC circuit. In the proposed initial charging circuit, the energy for charging FC is delivered from DC-side DC power supply during the start-up of the FC converter. The initial charging circuit works separately from the main FC converter and dose not disturb the main FC converter operation. The voltage stress of the main converter switches is limited the allowable level during initial charging operation. The effectiveness of the proposed FC initial charging method is verified by carrying on the PSIM simulation.

Bidirectional H8 AC–DC Topology Combining Advantages of Both Diode-Clamped and Flying-Capacitor Three-Level Converters

A three-level bidirectional ac–dc converter with H8 topology is proposed to combine the advantages of both neutral-point clamped and flying-capacitor (FC) three-level converters for medium-voltage (MV) ac–dc solid-state transformer (SST) applications. The innovative H8 converter features self-balanced capacitor voltage, common-mode voltage elimination, and small capacitance of the FCs. The proposed carrier-based modulation enables the H8 converter to operate under both active and reactive power conditions. A 10-kVA ac–dc converter prototype with 1.7-kV SiC MOSFETs and 3.3-kV SiC diodes verifies the feasibility and advantages of the proposed topology and the modulation method. The capacitance of the FCs in the H8 converter is reduced by 45 times compared to that of the FC converters. The H8 converter efficiency is tested up to 99% at 2-kV dc voltage. The proposed H8 converter is suitable for the next-generation PV inverter, grid-forming inverter, MV fast charger, and SST applications.

Thirteen-Level Multilevel Inverter Structure Having Single DC Source and Reduced Device Count

This article proposes a single dc source-based hybrid three-phase thirteen-level inverter suitable for medium voltage applications. The multilevel structure is composed of a three-level T-type neutral point clamped (NPC), half-bridge, and three-level Flying capacitor converter with optimal switch count and high dc bus voltage utilization as high as 0.63 times the dc-link voltage. This requires a special attention to a novel capacitor balancing method using redundant switching states in order to operate at any power factor under grid-connected mode. A unified common-mode voltage is injected in the carrier based pulse width modulation (PWM) technique to maintain the voltage balance in dc-link capacitors as well as floating capacitors. Simulation results are presented under different operating conditions that include unity power factor operation with passive load as well as grid-connected inverter mode. The performance of the proposed multilevel inverter in the grid-connected mode is also validated with experimental results using a lab prototype.

A Modular Multilevel DC–DC Converter With Flying Capacitor Converter Like Properties

This article proposes a novel modular multilevel dc-dc converter with high power-density potential based on a cascaded string of voltage source submodules (VSMs) merged with a current source submodule (CSM) that is configured to rapidly invert the polarity of the current in the VSM string. The proposed hybrid converter combines the modularity advantages of the modular multilevel converter (MMC) and the high effective frequency advantages of the flying capacitor converter. Unlike other hybrid nonisolated MMC structures the proposed converter features a novel CSM stage enabling a near 50% reduction in string current, and the input source and output load to be commonly grounded. In addition, the proposed operational approach enables self-balancing of the VSMs and a simple overall control structure. This article introduces the main operating principles of this novel topology termed the current shaping modular multilevel forward converter along with a proposed control design. A peak efficiency of 97.1% is measured from a laboratory-scale prototype at 950-137V and 1.45 kW.

Weighting Factor Free Model Predictive Control for a Flying Capacitor Converter in a DC Microgrid

This paper presents a single-objective model predictive control for a bidirectional DC-DC flying capacitor (FC) converter, which integrates a battery to the microgrid. The presence of multiple FCs facilitates the converter to interface a low-voltage battery to a high-voltage DC bus at reduced voltage stress on its power switches. However, such converter requires a controller that fulfils multiple objectives, namely DC bus and FC voltage regulations, and bidirectional power flow. The key feature of the proposed controller is that it utilizes a model predictive control with a single-objective cost function based on battery current and a redundant state selection scheme to attain these multiple objectives. Adopting such a strategy not only reduces computational burden but also eliminates the need for weighting factors and requirement of complex numerical models for its estimation. Another notable aspect is that the controller employs an improved dynamic reference model to generate appropriate battery current reference for the DC bus voltage regulation, without necessitating a secondary control loop and additional current sensors. Finally, experimental results of the proposed system for a step-response of the DC bus voltage, varying PV power, and loads are validated and compared against finite control set model predictive control.

Medium Voltage Flying Capacitor DC–DC Converter With High-Frequency TCM-Q2L Control

This article presents a novel control method for a multilevel dc–dc flying capacitor converter for applications in medium voltage range. Quasi-two-level modulation provides the possibility to obtain low flying capacitance even below 1 μF, whereas the triangular current mode controller part enables to achieve zero voltage switching at turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> for a wide operating range, lowering power losses, and providing the possibility to operate at higher frequencies. The proposed novel control converges the advantages of these both methods leading to very high efficiency with high power density. Performance of the flying capacitor converter with the proposed control method is illustrated with a model-based Saber simulation at first, and then, an experimental model operating at wide frequency range (40 ÷ 250 kHz) and in medium voltage range was designed and constructed. A series of laboratory tests at up to 1.5 kV dc and 10 kW confirmed the noteworthy characteristics of the converter. The voltage between the power devices is balanced and the efficiency achieved reached as high as 99.1%. Therefore, the proposed converter can be competitively employed against classical two-level, as well as three-level and series connection-based counterparts.

Dual-Stage Control Strategy for a Flying Capacitor Converter Based on Model Predictive and Linear Controllers

This article proposes a novel dual-stage control strategy for a flying capacitor converter. During transients, the proposed control scheme applies finite-control-set model predictive control to drive the system close to the desired reference, including all the known nonlinearities of the system in the converter model. When the converter state is in a neighborhood of the reference, the dual-stage controller switches to a pulsewidth modulation based linear controller with an integral action. In this case, the linear controller is reformulated in its feedback form. Thus, the internal linear controller states can be updated based on the actual input applied by the predictive controller. This allows the dual-stage controller to achieve a smooth transition when commuting between controllers and a zero-steady-state error even when load parameter errors are present. Experimental results are provided to verify the effectiveness of the proposed dual-stage controller.

Performance Comparison of Five-Level Active Neutral Point Converter Based on Phase Disposition-PWM and Alternate Phase Opposition Disposition-PWM

The work in this paper presents the performance analysis of the reduced component count converter which is the 5-Level Active Neutral Point Converter (5LANPC). This 5-level converter has been configured by stacking the traditional 3-Level Neutral Point Converter with the Flying Capacitor converter. Two types of control algorithms were considered and compared to explore the performance of the 5LANPC. The first algorithm was based on the Phase-Disposition-Pulse Width Modulation (PD-PWM), while the second one was based on the Alternate Phase Opposition Disposition-Pulse Width Modulation (APOD-PWM). These algorithms are used to determine the required voltage level and according to the required level the state of the switches is selected through a simplified voltage balance algorithm. This voltage balance algorithm deals with the redundant switching states to maintain the voltages of the 5LANPC capacitors at a specified level. The comparison between these two modulation strategies was performed by simulation based on MATLAB/Simulink package. Simulation results showed compelling outcomes involving the two techniques concerning the voltage and current characteristics, as well as the equilibrium in the capacitor voltages. By comparing the simulation results, it was found that the performance of the system is relatively better using the PD-PWM strategy.

Single-Phase PFC Rectifier With Integrated Flying Capacitor Power Pulsation Buffer

Single-phase Power Factor Correction (PFC) rectifiers with sinusoidal grid currents are inherently subject to an input power fluctuating at twice the mains frequency. In order to potentially mitigate bulky, heavy and failure-prone electrolytic dc-link capacitors, active Power Pulsation Buffer (PPB) concepts are proposed in the literature. For converter systems employing Flying Capacitor (FC) multilevel bridge-legs, the FCs can be utilized as a twice-mains frequency energy storage, i.e., as an integrated active PPB without the need for additional power components. Such ac-dc-stage-integrated FC PPBs capable of buffering the complete input power variation are known in literature which, however, require sophisticated control strategies with varying switching frequency and discontinuous conduction mode. This paper presents a novel ac-dc-stage-integrated FC-PPB approach which is compatible with standard PFC control concepts and enables a significant reduction in dc-link voltage variation. First, a control concept cycling the FC voltage in a wide range without interfering with the grid current controller is derived step by step and verified by means of circuit simulations. Design guidelines for the calculation of suitable FC capacitance values are presented and the limits in buffering capability are discussed. The concept is then experimentally verified with a 2.2kW three-level FC single-phase PFC rectifier employing 600V GaN power semiconductors where the dc-link voltage variation is reduced by 28% compared to conventional operation. Last, the applicability of the ac-dc-stage-integrated FC-PPB concept to other FC converter topologies is discussed.