Flying Capacitor Research Articles

Stability Properties of the 3-Level Flying Capacitor Buck Converter Under Peak or Valley Current Programmed Control

This paper investigates basic stability properties of the three-level flying-capacitor Buck converter when operated under current-mode control (CMC). The proposed analysis is developed for both peak CMC (P-CMC) and valley CMC (V-CMC), and addresses both the static instability of the inductor current and the flying capacitor (FC) voltage runaway phenomenon. Conditions for avoiding current subharmonic oscillations are derived first, along with the expressions of the minimal compensation ramp, which guarantee current stability throughout the entire operating range. As for the stability of the FC voltage, P-CMC results to be inherently unstable unless the converter operates with a relatively large peak-to-peak inductor-current ripple, a feature not yet recognized in the literature. However, V-CMC results to be inherently stable once the static instability of the current is eliminated with an external compensating ramp—a property that has been highlighted in the literature but never formally proven. The proposed analysis is verified both using a simulation model and experimentally using a 3.3-V, 500-mA, 500-kHz prototype.

Modeling and Hierarchical Structure Based Model Predictive Control of Cascaded Flying Capacitor Bridge Multilevel Converter for Active Front-End Rectifier in Solid-State Transformer

This paper introduces modeling and control of a cascaded flying capacitor bridge (CFCB) multilevel converter for a solid-state transformer. The finite control set-model predictive control (FCS-MPC) is easy and simple to implement for many applications. However, the computational complexity increases with the increase in the number of switches. Furthermore, as the number of control variables increases, the weighting factors also increase. Thus, it is difficult to apply to a system with a large number of switches and variables to be controlled similar to a CFCB multilevel converter. In this paper, the proposed method divides variables such as current, dc-link voltage, and flying capacitor voltage into layers. The proposed method reduces the number of states to be considered in the control, which shortens the computational time and simplifies expansion of the flying capacitor bridge. In addition, since weighting factors are not used, there is no tradeoff between current quality and voltage quality, and there is no difficulty in selecting weighting factors. Algorithms that include a method for compensating for the distortion caused by the delay of the digital control system are also described. The effectiveness of the proposed method is verified via experiments using a two-cell CFCB multilevel converter.

Sensorless Stabilization Technique for Peak Current Mode Controlled Three-Level Flying-Capacitor Converters

This article describes a sensorless stabilization technique for peak current mode control of three-level flying-capacitor (3 ${\rm LFC}$ ) buck converters. The technique eliminates the well-known instability associated with the flying-capacitor (FC) voltage runaway, leading to stable and balanced FC voltage. In the proposed technique, the FC voltage imbalance is detected by measuring the duty cycle mismatch $D_{A}-D_{B}$ between two groups of switches of the 3 ${\rm LFC}$ topology, an information already available to the controller without any additional sensing. The resulting compensation leads to a compact solution, which can be easily embedded into an analog or digital integrated controller. Two distinct approaches are developed in this article to implement the required correction on $D_{A}-D_{B}$ , namely a sensorless peak offsetting modulation (PO-MOD) and a sensorless interleaving time modulation (IT-MOD). Operation, implementation challenges, and performances of both are investigated. The proposed solutions are verified through computer simulations and experimentally using a 16.5-to-3.3 V, 500 mA, 500 kHz 3 ${\rm LFC}$ prototype.

A Novel Seven-Level Active Neutral-Point-Clamped Converter With Reduced Active Switching Devices and DC-Link Voltage

This paper presents a novel seven-level inverter topology for medium-voltage high-power applications. It consists of eight active switches and two inner flying capacitor (FC) units forming a similar structure as in a conventional active neutral-point-clamped (ANPC) inverter. This unique arrangement reduces the number of active and passive components. A simple modulation technique reduces cost and complexity in the control system design without compromising reactive power capability. In addition, compared to major conventional seven-level inverter topologies, such as the neutral point clamped, FC, cascaded H-bridge, and ANPC topologies, the new topology reduces the dc-link voltage requirement by 50%. This recued dc-link voltage makes the new topology appealing for various industrial applications. Experimental results from a 2.2-kVA prototype are presented to support the theoretical analysis presented in this paper. The prototype demonstrates a conversion efficiency of around 97.2% ± 1% for a wide load range.

Model Predictive Control for a Hybrid Multilevel Converter With Different Voltage Ratios

This paper proposes a model predictive control (MPC) strategy for a new hybrid multilevel converter (HMC) with different capacitor voltage ratios. The HMC provides redundant switching states, making it easy to balance the flying capacitor (FC) voltages. A discrete-time model of the HMC is built, where all the control objectives are formulated with the switching functions. The optimum switching state will be selected by evaluating all redundant switching states to minimize the cost functions. Compared with multicarrier pulsewidth modulation methods, the MPC does not require complex modulations, and the proposed HMCs can be operated with different voltage ratios to generate higher voltage levels without increasing the number of switches and FCs by using the MPC. Experimental results validate the effectiveness of the proposed control strategy for the HMCs under various operating conditions. The sensitivity of parameters is also studied with large load variations.

Fundamental Circuit Topology of Duo-Active-Neutral-Point-Clamped, Duo-Neutral-Point-Clamped, and Duo-Neutral-Point-Piloted Multilevel Converters

Multilevel voltage-source converters are well-suited for power conversion applications demanding higher power density, reliability, efficiency, and power quality. An unremitting and persistent research for developing advanced multilevel converter topologies with improved characteristics, performance, modulation techniques, and control methods continues. This paper proposes duo-neutral-point-clamped (D-NPC), duo-active-neutral-point-clamped (D-ANPC), and duo-neutral-point-piloted (D-NPP) multilevel voltage-sourced converter topologies. The D-NPC, D-ANPC, and D-NPP converters phase-leg is realized by adding low-frequency semiconductor power switches to their structures. This results in a substantial reduction in the number of the high-frequency pulsewidth-modulation insulated-gate bipolar transistors and clamping passive devices including diodes as well as flying-capacitors (FCs). Moreover, a drastic abatement in the total voltage rating and total stored energy of the FCs within the D-ANPC topology is achieved compared to the classic ANPC configuration. The experimental results are provided for D-NPC, D-ANPC, and D-NPP converters to validate the feasibility of their topology and modulation method for control of the multilevel converters.

Flying Capacitor Resonant Pole Inverter with Direct Inductor Current Feedback

Industrial applications, such as semiconductor manufacturing equipment, require power amplifiers that provide high power with high precision and bandwidth. The Flying Capacitor Resonant Pole Inverter (FC RPI) provides a multilevel configuration with high switching frequencies and Zero-Voltage Switching (ZVS) across the entire operating range. However, the charge-based modulation scheme that is applied to ensure ZVS depends heavily on correct measurement of the zero-crossings of the filter inductor current. The delay incorporated in the measurement chain results in significant distortion of the output current, which deteriorates the performance of the end application. This research proposes to apply direct current feedback of the per-period average filter inductor current, measured using a high bandwidth Anisotropic Magneto-Resistive (AMR) sensor, to correct the distortion introduced in the output current. The simulation results of the complete converter and control configuration indicate a significant improvement in performance: 9dB increase in Spurious Free Dynamic Range (SFDR) and 16dB decrease in Total Harmonic Distortion (THD).

Comprehensive Analysis of Single-Phase Full-Bridge Asymmetrical Flying Capacitor Inverters

This paper presents a comprehensive analysis of single-phase full-bridge asymmetrical flying capacitor inverters under distinct capacitor voltage values. The asymmetrical operation is explored in order to increase the number of output levels for the same number of semiconductors. A formation law for the flying capacitors’ voltages is proposed according to the desired number of output levels. The voltage regulation of the flying capacitors is carried out using the charge regulation analysis. The modulation strategy is also verified to overcome the inverter unbalance problem. Simulation and experimental results are included to demonstrate the performance of the proposed structures.

A New Single DC Source Six-Level Flying Capacitor Based Converter With Wide Operating Range

This paper presents a new six-level flying capacitor based (FC-based) multilevel converter with one dc source and the capability of operating in all power factors and modulation indexes. Multilevel converters with one dc voltage source are attractive in many applications as they do not need rather expensive and bulky multiwinding input transformer connection at the dc side. On the other hand, not all classic multilevel converters with one dc source can produce any desirable number of output voltage levels at all power factors and/or modulation indexes. In this paper, a hybrid structure is proposed in which six voltage levels can be realized at the ac terminals. The modulation technique and the control strategy for the FC voltage balancing are presented. To show the advantages of the proposed converter, different performance criteria, such as switch count and rating, the size of capacitors, switching frequency, and power losses, are compared with other existing six-level topologies. The results indicate that the proposed structure is superior to other six-level converters from different standpoints. Simulation results are used to further evaluate the performance of the proposed converter. A laboratory-type experimental setup is used to validate the theoretical results.

Voltage Jump Suppression and Capacitor Voltage Fluctuation Analysis for a Four-Level Hybrid Flying Capacitor T-Type Converter

In low and medium voltage power conversion systems, multilevel converters are becoming more and more attractive due to improved power density. However, the complexity of topology and control is a big challenge for the application of multilevel converters. In this paper, a four-level (4L) hybrid flying capacitor (FC) T-type converter has been researched in detail. The topological advantage of the converter is displayed in comparison to existing four-level converters. According to the feature of the topology, the operating status has been analyzed and the reason for the voltage jump is researched in detail during the dead-time period. A strategy to reduce voltage jump by adjusting the switching states has be presented. The FC voltages can be balanced by selecting the appropriate switching states. The relationships between the fluctuations of FC voltages and the modulation index and power factor (PF) have been analyzed by simulation results. The performance of the 4L converter has been investigated in MATLAB/Simulink as well as on a down-scaled laboratory prototype.

Three-Phase Three-Level Flying Capacitor PV Generation System with an Embedded Ripple Correlation Control MPPT Algorithm

This paper presents the implementation of a maximum power point tracking (MPPT) algorithm in a multilevel three-phase photovoltaic (PV) system using the ripple correlation control (RCC) method. Basically, RCC adopts the inherent oscillations of PV current and voltage as perturbation, and it has been predominantly used for single-phase configurations, where the oscillations correspond to the 2nd order harmonics. The implementation of an RCC-MPPT algorithm in a three-phase system has not been presented yet in the literature. In this paper, the considered three-phase three-level converter is a three-level flying capacitor (FC) inverter. The proffered RCC method uses the 3rd harmonic components of PV current and voltage for the estimation of the voltage derivative of the power dPpv/dVpv (or current, dIpv/dVpv), compelling the PV array to operate at or very close to the maximum power point. The analysis and calculation of the low-frequency PV current and voltage ripple harmonic components in the three-phase flying capacitor inverter is presented first, with reference to centered carrier-based three-level PWM. The whole grid-connected PV generation scheme has been implemented by MATLAB/Simulink, and detailed numerical simulations verified the effectiveness of the control method in both steady-state and dynamic conditions, emulating different sun irradiance transients.

An Improved Active-Neutral-Point-Clamped Converter With New Modulation Method for Ground Power Unit Application

This paper proposes a novel 11-level improved active-neutral-point-clamped (11L-IANPC) converter for ground power unit (GPU) application. This hybrid topology consists of the 5L-ANPC and the proposed low-voltage submodule. The proposed 11L-IANPC converter possesses only one flying capacitor (FC) and reduced number of high-frequency switches. Hence, voltage diversity and ratings, stored energy, and the number of costly and bulky FCs are significantly decreased. Moreover, operating the augmented submodule at the low voltage and power leads to doubling the number of output voltage levels, whereas the low extra cost and size are imposed on the proposed converter. In addition, suggested sensorless switching method leads to self-balancing of FC and significantly amends the output voltage frequency spectrum. These improvements cause significant reduction in control complexity of the proposed 11L-IANPC converter, remarkable decrease in size of the bulky and costly output LC filter, and improving the dynamic and steady-state performance of the proposed static GPU. The simulation and experimental results validate the feasibility of the proposed converter as well as its suggested modulation and control method.

Performance analysis and design optimisation of 3-⌀ Packed U Cell inverter for industrial drive applications

Nowadays, the importance for multilevel inverters is getting more and more in the field of medium and high power applications. This paper discusses Packed U Cell (PUC) multilevel inverter which is the most advanced topology. The figure of merit of this topology is reduction in the number of switches as level increases. Hence, it reduces the cost implementation besides topology complexity compared to other existing topologies such as neutral-point clamping (NPC), flying capacitor (FC), cascaded H-bridge (CHB) and hybrid cascaded H-bridge (HCHB). Furthermore, there is no need of transformers in this whole concept. Hence, it avoids the bulky installations. The 3-⌀ induction motor is fed with 7, 15 and 31 levels Packed U Cell (PUC) inverter topologies individually. The stator current, speed response and electromagnetic torque is shown for each level. The performance of induction motor is analysed in terms of % torque ripples and % total harmonic distortion (%THD). The whole simulations are carried out by using MATLAB/Simulink version R2012b.

Performance analysis and design optimisation of 3-⌀ Packed U Cell inverter for industrial drive applications

Nowadays, the importance for multilevel inverters is getting more and more in the field of medium and high power applications. This paper discusses Packed U Cell (PUC) multilevel inverter which is the most advanced topology. The figure of merit of this topology is reduction in the number of switches as level increases. Hence, it reduces the cost implementation besides topology complexity compared to other existing topologies such as neutral-point clamping (NPC), flying capacitor (FC), cascaded H-bridge (CHB) and hybrid cascaded H-bridge (HCHB). Furthermore, there is no need of transformers in this whole concept. Hence, it avoids the bulky installations. The 3-⌀ induction motor is fed with 7, 15 and 31 levels Packed U Cell (PUC) inverter topologies individually. The stator current, speed response and electromagnetic torque is shown for each level. The performance of induction motor is analysed in terms of % torque ripples and % total harmonic distortion (%THD). The whole simulations are carried out by using MATLAB/Simulink version R2012b.

Voltage Balancing Control of Hybrid Stacked Multicell Converters Based on Modified Phase-Shifted PWM

The hybrid stacked multicell converter (HSMC) is a competitive converter in the low-voltage high-efficiency applications. Proper modulation and voltage balancing control are important for the operation of the HSMC, which may ensure the HSMC to obtain high-quality output and low power losses. A hybrid phase-shifted PWM (PS-PWM) with the modified reference signal is developed for nine-level HSMC (9L-HSMC), and the neutral point voltage can be naturally balanced in a fundamental cycle. Based on the hybrid PS-PWM, an active voltage balancing control is put forward for the flying capacitor (FC). The voltage balancing of FC is implemented by slightly modifying the reference signals to adjust the duty cycles of the high-frequency switches symmetrically. This method does not need complex calculations, and it can be easily extended to higher level HSMC and other FC-based configurations. The simulations are carried out to validate the merits of low power losses and superior power quality. A downscale experimental platform of a 9L-HSMC is a setup to verify the effectiveness of the proposed control strategy under both the steady state and the transient states.

A Survey of Multilevel Voltage Source Inverter Topologies, Controls, and Applications

Multi-level converters are every day attracting research interest due to it tremendous positive contributions they are making in the power industries. The converter has put hope in the minds of power electronic engineers that a time will come when it will break a record by providing an efficient means of utilizing the abundant renewable energy resources.<strong> </strong>The paper presents a review of multilevel voltage source converters that are widely being used in engineering applications. It reports the technological advancements in converter topologies of Flying Capacitor (FC), Neutral Point (NPC) /Diode Clamped, and Cascaded H-Bridge (CHB) with their respective advantages and disadvantages. Recent customized/hybrid topologies of the three-phase multilevel inverter with reduced component count and switching combination are reported. The paper also reviewed different modulation techniques such as the multilevel converter carrier base PWM, Space Vector Modulation techniques (SVM), and Selective Harmonic Elimination method (SHE-PWM). Finally, various multilevel converters areas of application were highlighted. This review will expose the reader to the latest developments made in the multi-level topologies, modulation techniques, and applications.

A Novel STATCOM Based on Flying Capacitor Modular Multilevel Converter

A new static synchronous compensator (STATCOM) based on Flying Capacitor modular multilevel converter (DCM2C) is proposed in this thesis. In this converter topology the capacitor voltage is clamped by using a low power rating diode in each sub-module (SM). The quantity of voltage sensors is significantly reduced and is free from the number of voltage levels. Furthermore the voltage balancing control method becomes very simple and the capacitor voltage balance speed is fast. Based on the structure of MMC the DCM2C-STATCOM has the capability of Var compensation and negative sequence current compensation. The topology characteristics and compensation control method of DCM2C-STATCOM are investigated in this thesis. That the capacitor voltage of the proposed DCM2C-STATCOM can be well balanced and the Var and negative-sequence current compensation are effective.

Development of DC to Single-Phase AC Voltage Source Inverter With Active Power Decoupling Based on Flying Capacitor DC/DC Converter

In the present, a power decoupling method without additional component is proposed for a dc to single-phase ac converter, which consists of a flying capacitor dc/dc converter (FCC) and the voltage source inverter (VSI). In particular, a small flying capacitor in the FCC is used for both a boost operation and a double-line-frequency power ripple reduction. Thus, the dc-link capacitor value can be minimized in order to avoid the use of a large electrolytic capacitor. In addition, component design, of, e.g., the boost inductor and the flying capacitor, is clarified when the proposed control is applied. Experiments were carried out using a 1.5-kW prototype in order to verify the validity of the proposed control. The experimental results revealed that the use of the proposed control reduced the dc-link voltage ripple by 74.5%, and the total harmonic distortion (THD) of the inverter output current was less than 5%. Moreover, a maximum system efficiency of 95.4% was achieved at a load of 1.1 kW. Finally, the high power density design is evaluated by the Pareto front optimization. The power densities of three power decoupling topologies, such as a boost topology, a buck topology, and the proposed topology are compared. As a result, the proposed topology achieves the highest power density (5.3 kW/dm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) among the topologies considered herein.

Multicarrier-Based PWM Strategies With Complete Voltage Balance Control for NNPC Inverters

Compared with multilevel space vector modulation, level-shifted multicarrier sinusoidal pulsewidth modulation (SPWM) for nested neutral point-clamped (NNPC) inverters requires much less calculations and implementation efforts, but the existing conventional SPWM cannot suppress the severe flying-capacitor (FC) voltage ripples under low output-frequency conditions. In order to solve this problem, a multicarrier-based PWM strategy with complete voltage balance control (VBC) capacity is proposed in this paper. For NNPC inverters operating under low output-frequency conditions, two preprocessing steps, the zero-sequence injection and the modulation-signal rearrangement, are proposed before the comparisons between level-shifted carriers and modulating signals. These preprocessing steps make the average currents flowing through FCs zero, which is beneficial to greatly reduce the FC voltage ripples under low output-frequency conditions. Together with additional VBC, the proposed modulation can assure the FC voltages balanced under the entire output-frequency conditions. Principles and implementation processes of proposed modulation techniques and VBC methodsare elaborated in detail and compared with conventional modulation methods. Both simulation results from a 5-MVA system and experimental results based on a 5-kVA platform are presented to verify the effectiveness.

Leakage Current Suppression of Three-Phase Flying Capacitor PV Inverter With New Carrier Modulation and Logic Function

Flying capacitor photovoltaic (PV) inverters have been widely discussed in the literature. However, the relevant leakage current issues have not received much attention. In this paper, the modulation techniques for a transformerless three-phase flying capacitor PV inverter are investigated for the leakage current suppression. First, the theoretical analysis of the system common mode model and leakage current is presented. Second, three kinds of conventional carrier-based modulation techniques are evaluated, and results indicate that they fail to effectively suppress the leakage current. Third, a new carrier modulation and logic function is presented. It is able to maintain the constant common mode voltage, and consequently, the leakage current can be significantly reduced. Finally, the experimental tests are conducted on a transformerless three-phase flying capacitor PV inverter to verify the effectiveness of the proposed method.