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Summary: We propose an equalizer using two capacitors along with a resistor and a cell access network to address the weakness of low equalization power for the flying‐capacitor based equalizer, which integrates the merits of simplicity and reliability in the application of cell‐to‐cell battery equalization. The equalization process is implemented by alternately charging the capacitors from high state of charge (SoC) cell and discharging the capacitors to low SoC cell, where the capacitors are run in parallel in charging and in series in discharging, and the charging or discharging duration is determined by ξ, the multiple of the time constant of the circuit. A protection is also designed to suppress the peak current occurred when initializing the equalizer due to the zero‐boot voltage of the capacitor. Theoretical analysis shows that the equalization power, which is governed by the lumped resistance of equalization path and ξ, can range from less than 1 W to greater than 6 W. The battery discharging time increases about 5.14% under constant current of 0.5 C in the experimental test. Additional advantage of our equalizer is its operating frequency as low as several Hz, which benefits hardware implementations. The equalizer can attain high equalization power through simple capacitors and a regulating resistor, which is suitable to be used for industrial applications. [ABSTRACT FROM AUTHOR]

Integrated Motor Drives (IMDs) are gaining popularity in industrial Variable Speed Drive (VSD) applications, thanks to their more compact realization and simpler installation. However, mission profiles of, e.g., servo applications, demand overload torques of two to three times the nominal value during several seconds, which is thermally challenging for the power electronics. Accordingly, high efficiency and power density of the inverter are of paramount importance for motor integration. Multi-Level Flying Capacitor inverters (ML-FCis) benefit from a reduced output filter volume and improved switching and on-state performance of low-voltage devices for increasing number of levels $N$ , whereas the PCB overhead and gate drive volume increases. In this paper, the most power dense solution between a seven-level (7L) FCi with Si semiconductors and a three-level (3L) FCi with GaN semiconductors is evaluated. Thereby, a straightforward design procedure allows to dimension both FCis for 99% efficiency at nominal operation, while providing a high short-term overload torque (three times the nominal torque) also for low inverter output frequencies. Due to its slightly higher power density and greatly reduced complexity, a phase module of the 3L GaN FCi is realized as an IMD hardware demonstrator. A transient thermal model is employed to specify the feasible overload operating range, considering the limited heat spreading in the baseplate and parameter variations, e.g., from manufacturing tolerances. The experimental analysis of the demonstrator verifies an efficiency of 98.94% and the practically required overload capability.

In order to extend motor operation range, a self-voltage-boosting AC motor drive is proposed in this paper. Compared with the traditional frontend DC-DC converters, the proposed topology reuses the power switches inherently involved within the three-phase inverter without using any additional active components. By modifying the time distribution of the two zero vectors in traditional seven-segment space vector pulse width modulation (SVPWM) technology, the inductor current and the flying capacitor voltage can be adjusted. The proposed topology has the advantage of a wider operating range without extra cost. Moreover, the performance of the motor drive is not affected. Experimental results demonstrate the correctness of the proposed topology.

ObjectivesFor a marine 690 Vac bidirectional converter based on a three-level flying capacitor, in order to make it start stably and transition to its normal working state, the flying capacitor must be pre-charged. This paper proposes a pre-charging strategy based on single-tube control which is suitable for three-level flying capacitors.MethodsThe on-off of the three-level outer tube is controlled on the basis of the flying capacitor, and efficient and fast pre-charging control is performed.ResultsWhen this strategy is applied to a 690 Vac bidirectional converter, the pre-charging of the flying capacitor is completed within 1 second in Buck mode and less than 5 seconds in Boost mode. After pre-charging, the voltage of the flying capacitor remains at the value of half of the voltage of the high voltage side, and the voltage of the input side capacitor remains at the value of the input side voltage.ConclusionsAfter experimental verification, the proposed pre-charging strategy based on controlling the outer tube of the three-level flying capacitor can be applied to a 690 Vac bidirectional converter device without the need for additional pre-charging power devices, which not only simplifies the topology but also ensures the reliable and fast pre-charging of flying capacitors.

This paper presents a literature survey of more than twenty years on AC/DC multilevel converters with reduced switch count, in which some power switches can be removed letting only diodes. This simplification reduces the number of gate drivers, cost and complexity of the system. In this work, the considered topologies are derived from the conventional multilevel configurations on literature: Neutral Point Clamped, Cascaded H-Bridge, Flying Capacitor, Modular Multilevel Converter and T-Type. This article is divided in five sections, each one with papers with topologies derived from the five multilevel configurations previously mentioned. In each section, there are up to a maximum of four subsections, considering the main focus of each paper and contribution: derived and hybrid topologies, control techniques, analysis and hardware and design. At the end of each section, a summary table concerning the referenced papers is shown containing comparison information among the presented references in terms of: number of components per phase of the studied converter, applied modulation technique, applied control technique, suggested application given by the authors, converter’s output power considered and the respective efficiency considering this point of operation. With this extensive bibliography work, this paper serves as a very good reference to researchers interested in the topic to initiate their work or to update their studies. Also, the applications suggested by the authors give a direction of which topologies fit best different applications.

With the development of space exploration, high-power and high-voltage power systems are essential for future spacecraft applications. Because of the effects of space radiation such as single event burnout (SEB), the rated voltage of power devices in converters for a spacecraft power system is limited to a level much lower than that for traditional ground applications. Thus, multi-level DC/DC converters are good choices for high-voltage applications in spacecraft. In this paper, a fault-tolerant strategy is proposed for a three-level flying capacitor DC/DC converter to increase the reliability with minimal cost. There is no extra hardware needed for the proposed strategy; the fault tolerance of the converter is only achieved by changing the software control strategy. A stage analysis of the proposed strategy is provided in detail for different fault locations and ratios between the input and output voltage. Finally, a simulation model and prototype are built to verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

To use fewer components to construct a multilevel inverter (MLI) with more levels and high quality of the output voltage, a hybrid MLI is developed by combining a double-mode switched-capacitor (SC) unit with a T-type unit fed by flying capacitors (FCs). The SC unit is used to boost the amplitude of the output voltage to twice the dc input voltage, while the T-type unit is responsible for increasing the number of output levels. A total of ten transistors, four capacitors, and two diodes are used to generate 17 different levels. As two of the four capacitors are charged by a single dc source directly and the other two operate as FCs, there is no capacitor voltage ripple accumulation (CVRA) problem, resulting in high-quality output voltage. In addition to topology, a voltage balance controller of the FCs is developed to cooperate with the APOD-PWM strategy. Compared with other single-phase SC-based MLIs, the proposed inverter has a lower cost per level. Finally, to verify the feasibility of the proposed inverter, both the simulation model and experimental prototype are established. The results show that the proposed inverter has high-quality output voltage waveform and good dynamic response. The measured efficiency is above 96.5 for a wide range of loads, and the maximum value is up to 98.36.

The emergence of wide bandgap power devices has brought the attention back to the flying capacitor (FC) multilevel inverters with a large number of stages, in an effort to increase the power density by minimizing the passive components. The main challenge that such systems face, particularly the ones based on high-frequency Gallium-Nitride devices and small-value ceramic capacitors, relate to the stringent requirements for precise and fast capacitor balancing. Conventional natural balancing techniques exhibit poor settling times, while most improved natural balancing methods are not easily scalable to more than five levels. The alternative of active balancing normally requires one isolated sensor per FC which increases the overall system cost and footprint, or a single ac-side sensor that is more compact but calls for sophisticated PWMs that again are not available for multiple levels. In this paper we introduce a generalized pulse width modulation (PWM) strategy based on the phase-shift and carrier swapping principles for an arbitrary number of levels. We provide an easy and intuitive method for the extraction of the PWM pattern, the switching states, and their sequence. Simulations were carried out in Matlab/Simulink and experimental tests were conducted on a single-phase 7-level GaN inverter prototype. Not only is the extended PWM advantageous in natural balancing, but it also provides the right zero switching states for ac-side FC sensing in active balancing.

The bidirectional dc-dc converter is a critical component for extending the use of renewable energy and improving the efficiency of high-power electronic systems. This paper presents the analysis of the stray inductance of a commutation loop and the commutation behavior of IGBT devices in a flying capacitor three-level bidirectional DC-DC converter (FC-3L-BDC) phase-leg power electronic building block (PEBB). An FC-3L-BDC phase-leg PEBB was designed as an example, which can be used to build 400 kW to MW-grade light rail train chargers, battery energy storage interface converters, or metro regenerative braking energy recovery converters with a single PEBB or several PEBBs interleaved parallel. In order to optimize the stray inductance of commutation paths and realize snubberless operation, a five-layer laminated bus bar was carefully designed, and the stray inductance of the bus bar was extracted by three-dimensional finite element analysis simulation. To obtain higher accuracy, the stray inductances of IGBT devices and capacitors were extracted from the test instead of their datasheets. Then, the accuracy of the commutation loop stray inductance analysis method was verified by practical experiments. The impact of the stray inductance of the commutation loop on the commutation behavior of IGBT devices was analyzed, and the switching characteristics of IGBT devices were measured under maximum DC-link voltage and entire current rating range at the temperatures of −40 °C, 25 °C, and 150 °C, respectively, finding that neither the excessive turn-off overvoltage of IGBTs nor the snappy reverse recovery of FWDs was observed. [ABSTRACT FROM AUTHOR]

In this work, a multilevel 42-sided polygonal space vector structure for suppression of lower order harmonics for open-end induction motor (OEIM) drive applications is proposed. The proposed power circuit topology consists of two inverters feeding an OEIM from either side. The main inverter fed with a single dc-link providing active power for motoring operation is switched at low switching frequency. The secondary inverter fed with a capacitive supply is switched at high frequency to suppress lower order harmonics upto 39th order, up to the base speed of operation allowing maximum utilization of the dc-link. The advantages of lower order harmonic suppression in motor phase voltage, for polygonal space vector structures, are combined with multilevel inverter topology. This offers various advantages like multilevel operation with low inverter switching for the main power delivery inverter and harmonic suppression throughout the modulation range. The low switching frequency primary inverter generates harmonics of the order $6*k \pm 1$ , where $k = 1, 2, 3,{\ldots }$. The harmonics are suppressed by the capacitor-fed secondary inverter, which supplies only reactive power for suppression of lower order harmonics in motor phase voltage. Use of a single dc-link facilitates four-quadrant operation of the inverter. The proposed scheme is validated for steady-state and dynamic performance by experimental results. [ABSTRACT FROM AUTHOR]

This article presents a fully integrated multi-phase (MP) buck converter for on-chip power management units. A peak-and-valley differential sensing (PVDS) scheme based on a flying capacitor is proposed to evenly balance the multiple inductor currents without significant power overhead even under mismatches in inductances, parasitic series resistances, and control-signal skews among phases. In addition, dynamic re-allocation of on-chip capacitors enables an even more optimal frequency response and output ripple based on the number of activated phases. It also improves the chip area efficiency, resulting in a higher power density. The proposed DLL-based MP clock generation provides high granularity in the phase-shedding control to achieve high efficiency over a wide load range. The proposed fully integrated buck converter using six bond-wire inductors (1 nH) was fabricated in a 28-nm CMOS process. The chip running at a frequency of 400 MHz/phase can finely adjust the number of active phases by an integer step ranging from 1 to 6 through sensing a load current. A maximum power density of 1.23 W/mm2 and a peak efficiency of 83.7% were measured. This work achieved an ultra-fast dynamic voltage scaling (DVS) rate of 75 mV/ns. [ABSTRACT FROM AUTHOR]

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Flying capacitors (FCs) are very important in multilevel inverter systems when it comes to synthesizing additional voltage levels for various medium-voltage industrial applications, including renewable energy, motor drives, and power transmission. In particular, they are responsible for forming the five-level output pole voltages in five-level hybrid active neutral-point-clamped (ANPC) topologies. This is only possible when the FCs reach their reference values (i.e., 25% of the DC-link value). When a five-level hybrid ANPC inverter operates in the high-frequency (HF) range, the output three-phase current waveforms are less distorted when compared to the low-frequency (LF) performance. Instantaneously achieving a five-level output pole voltage during HF operation is a challenging task. This is due to the large capacitance of some existing types of FCs. In this study, a novel analysis of the effects of a FC on the performance of a five-level hybrid ANPC inverter is presented with simulation and experimental validations. Film capacitors significantly enhanced system performance when compared to electrolytic capacitors due to their many advantageous features, including the ability to charge and discharge quickly during HF operation. In addition, the total harmonic distortion of the output pole voltage is significantly suppressed. Consequently, the size of the required filter can be reduced. Therefore, the proposed system is highly desirable for various industrial applications. [ABSTRACT FROM AUTHOR]

In an era where high-frequency flying capacitor (FC) multilevel inverters (MLIs) are increasingly gaining attention in energy conversion systems that push the boundaries of power density, the need for a compact, fast, and accurate FC voltage monitoring system is also increasing. In this article, we designed and developed a new FC measurement system, based on precise sampling of the inverter switching node voltage, through a bidirectional clamping circuit. The deviation of FC voltages from their nominal values is extracted by solving a set of linear equations. With a single sensor per phase and no isolation requirements, as opposed to dozens of sensors in traditional FC monitoring, our approach results in significantly lower cost, complexity, and circuit size. Detailed device-level simulations in LTspice and system-scale simulations in MATLAB validate the accuracy and speed of the proposed measurement system and the balancing strategy in steady-state, abrupt load change, and imbalance conditions. Experiments carried out in a three-phase gallium nitride five-level inverter prototype reveal a gain in precision and bandwidth that is more than 30 times that of conventional methods, at a fraction of their cost and footprint. The recorded performance renders the developed sensor an ideal solution for fast MLIs based on wide-bandgap technology. [ABSTRACT FROM AUTHOR]

Due to their numerous advantages, multilevel inverters are commonly utilized in high and medium voltage applications. Design and simulation of Six-Switch Five-Level Active Neutral Point Clamped Inverter (6S-5L-ANPC) associated with LC filter is presented, the inverter can produce pure sinusoidal voltage, the proposed modulation can satisfy the regulation of the flying capacitor voltage, the behavior of 6S-5L-ANPC inverter is validated through simulation, limitation in the present design of the inverter is discussed. However, the converter is not able to maintain the FC voltage balancing under modulation index variation. To overcome this limitation, a PID Controller based in output voltage control is proposed. The aim of control is: i) create a sinusoidal voltage at the converter output with amplitude and frequency set by the reference signal, ii) guarantee the self-balancing of the FC capacitor under variable load and reference voltage variation. The controller is tested under various operating situations in simulation. The validity of this strategy is demonstrated using simulation results. [ABSTRACT FROM AUTHOR]

This article presents a three-switch (3S) boost converter with hysteresis current control applied in MiniLED panels. It uses only three power switches to achieve high conversion ratio (=7.5) and low inductor current (20% lower than conventional boost converters). Meanwhile, the charging time of the flying capacitor will increase as the duty cycle increases. It enables the proposed 3S boost converter to have a high drive current (=1.2 A). In addition, the right half-plane (RHP) zero of the proposed topology is related to the switching frequency. By dynamically adjusting the position of RHP zero during transients, the recovery time and output voltage drop can be reduced at the same time. When the load increases from 120 mA to 1.2 A, the recovery time can be accelerated to 42 $\mu \text{s}$ , and the maximum efficiency can reach 97.4%. [ABSTRACT FROM AUTHOR]

The study aims to assess the impact of violation of the assumption about normality of the investment portfolio returns on its risk measures. The article is focused on the Value at Risk (VaR) metric required by major regulatory authorities for bank risk assessment. Using historical share prices of several Russian companies it is shown that the assumption about returns normality is not supported by statistical tests. It is also shown that the empirical distribution of the assets returns is described by Johnson’s distribution. The Kolmogorov-Smirnov test supports the obtained results. The tests proposed by the authors allow estimating the loss in accuracy in parameters calibration of the autoregressive model, obtained by using the maximum likelihood method when the asset returns have non-gaussian distribution. It was found that the loss in the accuracy lies in the range [22%, 26%] for absolute returns and in the range [33%, 38%] for relative returns depending on the autoregression parameter which varies in the range [–0.9, 0.9]. The error of ten-day VaR estimation was calculated for 1% (99%) and 5% (95%) significance levels. At a significance level of 5% (95%) the VaR metric obtained under the assumption that the asset returns have normal distribution is lower than the true value by 7% (6%) for absolute returns and 4% (13%) for relative returns, which indicates strong underestimation of the portfolio risk. At a significance level of 1% the metric is conservative exceeding the true value by 12.5%.

Multiport converters (MPCs) that integrate multiple converters into a single unit have been proposed to reduce the converter count in electric vehicle power systems where multiple batteries are employed. However, conventional MPCs comprising a dual active bridge (DAB) converter and an interleaved PWM converter require feedback control loops and current sensors to balance the inductor currents. Furthermore, the transformer utilization decreases as the duty cycle of the interleaved PWM converter drifts away from 0.5. This paper proposes a novel MPC that integrates a DAB converter and an interleaved PWM converter with an automatic current balancing capability and improved transformer utilization. A 1‐kW prototype with a half‐bridge or full‐bridge circuit on the high‐voltage side was built for the experimental verification, and loss analysis was performed. The results demonstrated the automatic current balancing capability, improved transformer utilization, and flexible power exchange among three ports. [ABSTRACT FROM AUTHOR]

Grid-connected inverters (GCI) are commonly used in PV system applications to generate a regulated AC current to feed into the grid. Transformerless inverters are the most advanced inverters that are used in industry, which provide efficiency with smaller size and lower cost. This paper proposes a grid-connected single-phase transformerless inverter with the technology of common ground and the virtual DC bus concept. In this topology, the grid neutral is connected directly to the PV ground, which generates a constant common mode voltage (CMV), thus leading to the elimination of the leakage current caused by the PV array's parasitic capacitance. The proposed inverter has a buck–boost circuit with a flying capacitor to generate the DC bus for a negative power cycle, four switches, and two diodes. A unipolar sinusoidal pulse width modulation (SPWM) technique is used which reduces the output filter requirements. In addition, only one switch carries the load current during the active states of both the negative and positive power cycle, thus minimizing the conduction losses. One more advantage presented in the proposed inverter is its ability to charge the flying capacitor during all operation states due to the existence of the buck–boost circuit. Design and theoretical calculations were conducted in this paper to optimize the losses. Moreover, the PSIM simulation was used to validate the proposed topology inverter, verify the performance by showing leakage current elimination, and achieve unipolar voltage in the output bus. The simulation results show a peak efficiency of 98.57% for a 2 kW inverter, which agrees with the theoretical calculations. [ABSTRACT FROM AUTHOR]

This article presents novel control strategies for a seven-level (7L) T-type active neutral-point-clamped (T-ANPC) converter based on the level-shifted pulse width modulation (LS-PWM) technique. Compared with classical 7L topologies, such as NPC, flying capacitor converter, and cascaded H-bridge converter, the 7L T-ANPC converter can reduce the device requirements while reserving the advantages of high redundancy and low conduction losses. Furthermore, the internal switching state redundancy provides freedom to balance the flying capacitor (FC) voltage and realize the fault-tolerant operation flexibly. The control strategy with modified LS-PWM technique is developed for the T-ANPC to obtain desired outputs under normal and switch open fault modes. The proposed control strategy considers the complete balancing of dc-link neutral-point voltage and FC voltages. The feasibility of the proposed 7L T-ANPC and control strategy is evaluated by simulations and experimental tests under steady-state and several typical transient operating conditions. [ABSTRACT FROM AUTHOR]

The multilevel inverter is the most advanced power conversion system, whose main advantage is to improve power quality and efficiency. Cascaded H‐bridge (CHB) multilevel inverter is a kind of inverter that can be selected in the field of motor drive systems, electric vehicles and photovoltaic grid‐connected power generations. However, the CHB multilevel inverters require a great deal of independent direct current (dc) power supplies, which makes the realization of these inverters very complicated in practical applications. In this article, model predictive control for CHB inverter with flying capacitors is proposed to improve the above drawbacks. Simulation and experimental results show that voltage balance of the flying capacitor can be maintained and simultaneously the output voltage of the inverter normally. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

The operation of the LM27762 bipolar DC–DC converter chip was simulated using WEBENCH software developed by Texas Instruments. The main attention is paid to the influence of the load current on the negative voltage ripples at the output of the charge pumping system and the chip output. The results of the calculations are compared with the results of the experiments. It is shown that the model of operation of the microcircuit created by the manufacturer is approximate. When the microcircuit operates in burst mode, which is realized at relatively low load currents, the discrepancies in the number of discharge – charge cycles, voltage poles at the output of the pumping system, and in the time intervals between bursts are very significant. The model does not allow obtaining data on the ripples of the negative voltage at the chip output. Experiments have shown that the very limited data on output voltage ripples given in the microchip documentation is underestimated. They significantly depend on the input and output voltage and load currents. The discrepancy between the experimental results and the simulation results obtained using the model of the manufacturer LM27762 showed that the actual perfection test of the microcircuits should be based on three factors: experiment, simulation and joint analysis of the obtained results. The simulation results of the operation of the LM27762 circuit board of bipolar DC–DC converter using the WEBENCH Power Designer software developed by the Texas Instruments manufacturing company are presented. The calculations are made at the company's website. For the experimental studies the ready-for-use test board of LM27762EVM test module, provided by the manufacturer of the LM27762 circuit board, is used. The use of the test board, simulator, and software developed by the manufacturer of the circuit board ensures to have no problem while comparing the simulation and experimental results, which can be associated with the use of circuit components with differing characteristics, or with the board layout. The technical documentation for the circuit board does not have information about the characteristics of the keys and capacitors used in the circuit board. Unfortunately, a description of the developed model is not available either. The information mainly concerns the influence of load currents on negative voltage ripples at the charge pump system output and at circuit board output. The simulation has shown that already at a load current of 40 mA, the control system starts operating with a permanent charge pump frequency, while, according to the results of the experiments, the control system continues to work in the pulse train mode even at a higher current of 50 mA. The pulse train for such current consists of 15 charge transfer pulses, the interval between the trains is 3 μs, and the increase in negative voltage per pulse train is also about 75 mV. The comparison made of simulation results and the results of the experiment have shown that the model of operation of the LM27762 circuit board, created by the manufacturer, is approximate. It has been established that owing to a large number of assumptions in the developed circuit board model, when it operates in the pulse train mode, which is realized at relatively low load currents, the discrepancies are rather significant in the number of discharge–charge cycles, the pulsation amplitude at the charge pump system output, and the time intervals between the pulse trains. The simulation of operation of the circuit board does not give data on the negative voltage pulses at its output. The experiments have shown that the data provided in the documentation on the circuit board output voltage pulses, though quite limited, are underestimated. They largely depend on the input and output voltage and load currents. In general, the comparison of the results of simulation of the operation of the charge pump system of the LM27762 circuit board, when it operates in the pulse train mode, and the corresponding experimental results shows that the coincidence of the calculation results and those of the experiment in terms of quality can be considered satisfactory. The discrepancy between the experimental and simulation results, obtained using the manufacturer’s LM27762 model has proved that the true testing of the quality of circuit boards requires three factors to be taken into consideration: experiment, simulation and joint analysis of the obtained results.

A 4-level flying capacitor converter (FCC) operation is considered on a base of discrete state-space model. A transition matrix is obtained for a pulse width modulation (PWM) period for large normalised voltage command values [1/3, 1). The transition matrix elements are expanded into power series by small parameters. The matrix eigenvalues are presented in the form of power series as well. Six separate transients are constructed for six possible initial FCC states on a PWM period. Inductor current and capacitors’ voltage transients are found for the voltage source power-up as the arithmetic average of the six separate transients. Finally, the discrete solutions are replaced by equivalent continuous ones. Simple and accurate formulas for inductor current and capacitors’ voltage transients demonstrate good agreement with simulation results.

A 4-level flying capacitor converter (FCC) operation is considered on a base of discrete state-space model. A transition matrix is obtained for a pulse width modulation (PWM) period for small normalised voltage command values [0, 1/3]. The transition matrix elements are expanded into power series by small parameters. The matrix eigenvalues are presented in the form of power series as well. Six separate transients are constructed for six possible initial FCC states on a PWM period. Inductor current and capacitors’ voltage transients are found for the voltage source power-up as the arithmetic average of the six separate transients. Finally, the discrete solutions are replaced by equivalent continuous ones. Simple and accurate formulas for inductor current and capacitors’ voltage transients demonstrate good agreement with simulation results.

Interleaved converters are a typical solution to enhance a current capacity and to reduce current ripples of an input or output port. Conventional interleaved LLC resonant converters need additional switches and passive components with a feedback control loop for active current balancing, increasing the circuit complexity and cost. This paper proposes a novel three-phase interleaved LLC resonant converter with an automatic capacitive current balancing capability. Two flying capacitors are added to a conventional interleaved LLC resonant converter in order to realize not only the automatic capacitive current balancing but also the reduced switch voltage stresses and asymmetric resonant operations that contribute to reducing rms currents as well as copper losses of transformers. The detailed operation analysis, including the capacitive current balancing mechanism and gain characteristics derivation, are performed. The experimental verification using a 1-kW prototype demonstrated that the automatic capacitive current balancing could be achieved despite the significant mismatch in transformers' parameters.

The paper presents the results of studies of the operation of the inverting DC–DC converter with charge pump and LDO, which are part of the combined bipolar secondary power supply LM27762, at the near to maximum input voltage of 5.5 V and an output voltage of –4.9 V. The ripple voltages were measured at various load currents from 10 to 250 mA at the positive pole of the flying capacitor, at the output of the charge pump system and at the output of the microcircuit. It was shown for the first time on the basis of the obtained information that at low load currents up to about 107 mA the charge pump system operates in the burst mode, and at currents greater than 109 mA – in the charge pump mode with a constant frequency. The results do not confirm the information in the documentation on the microcircuit that, at themaximum input voltage of 5.5 V, the charge pump can enter the PWM mode in hot conditions. When working in burst mode, the presence of LDO in the LM27762 chip reduces the ripples of the negative voltage at the output. However, they significantly exceed the values given in the documentation on the chip. During switching to the constant–frequency mode, the level of negative voltage ripples at the output of the microcircuit decreases sharply, but it increases with further increase of the load current and exceeds the values given in the documentation.

The paper considers a discrete state-space model for transients in a three-level flying capacitor DC–DC converter. A transition matrix is obtained for a pulse width modulation (PWM) period. The matrix elements are expanded into a power series using a selected small parameter. The matrix eigenvalues that determine the natural balancing dynamics transients are presented in the form of power series as well. Four separate transients are constructed based on four possible PWM period initial states (topologies). Inductor current and capacitor voltage transients are found for the voltage source power-up as the arithmetic average of the four separate transients. The discrete solutions are replaced by continuous ones. The resulting transients that are elementary functions of the circuit parameters, PWM period, and voltage reference demonstrate good agreement with the simulation results.

This paper presents a new switched-ladder structure for multilevel converter which consists of several bidirectional and unidirectional switches along with DC voltage sources. The values of DC sources in the proposed topology are determined based on a new mathematical algorithm. The proposed multilevel converter is an extended structure, which can produce any levels at output voltage waveform. To prove the merits of the proposed structure, the proposed converter is compared with other similar structures. According to comparison results, it is shown that the presented structure requires the least numbers of DC sources, IGBTs, drivers and on-state switches. Also, the value of voltage rating of the switches is analyzed. The experimental results are provided for the proposed 17-level converter to prove the performance of suggested structure. [ABSTRACT FROM AUTHOR]

This paper presents a study of the nonlinear dynamic behavior a flying capacitor four-level three-cell DC-DC buck converter. Its stability analysis is performed and its stability boundaries is determined in the multi-dimensional paramertic space. First, the switched model of the converter is presented. Then, a discrete-time controller for the converter is proposed. The controller is is responsible for both balancing the flying capacitor voltages from one hand and for output current regulation. Simulation results from the switched model of the converter under the proposed controller are presented. The results show that the system may undergo bifurcation phenomena and period doubling route to chaos when some system parameters are varied. One-dimensional bifurcation diagrams are computed and used to explore the possible dynamical behavior of the system. By using Floquet theory and Filippov method to derive the monodromy matrix, the bifurcation behavior observed in the converter is accurately predicted. Based on justified and realistic approximations of the system state variables waveforms, simple and accurate expressions for these steady-state values and the monodromy matrix are derived and validated. The simple expression of the steady-state operation and the monodromy matrix allow to analytically predict the onset of instability in the system and the stability region in the parametric space is determined. Numerical simulations from the exact switched model validate the theoretical predictions. [ABSTRACT FROM AUTHOR]

This article presents a simple yet effective capacitor voltage balancing method for a five-level nested neutral point clamped (NNPC) converter. The NNPC has been recently developed for medium-voltage high-power applications and has interesting features. This hybrid topology has three flying capacitors in each leg. Like other flying capacitor-based topologies, it is necessary to regulate and balance the voltage of the flying capacitors in an NNPC converter to certain values. In this article, a new approach is proposed to accomplish this task. The proposed regulation technique employs line current direction and voltage deviation of flying capacitors to achieve regulation. The charging/discharging status of flying capacitors with respect to different switching states is used to define a priority index. This index is eventually used to select the proper switching state. A detailed investigation is presented to determine the best switching state for each level of the output voltage. The proposed method does not need any cost function and is very intuitive and simple to implement. Using simulations, the amplitude of the flying capacitor voltage ripple is measured, which shows reduction under all operating conditions. The feasibility and performance of the proposed method are confirmed by using a laboratory-type experimental setup. [ABSTRACT FROM AUTHOR]

In a two-stage single-phase photovoltaic (PV) grid-connected inverter, the second harmonic current (SHC) in the PV panel will affect the maximum power point tracking operation and degrade the energy harvesting efficiency. In order to solve this problem, a flying capacitor clamped (FCC) boost three-level (TL) converter is adopted in this article, and the flying capacitor is employed to compensate the SHC for removing a large electrolytic capacitor. The propagation mechanism of the SHC is analyzed from the perspective of impedance, and a virtual impedance in parallel with the intermediate dc bus is introduced, achieving a perfect SHC compensation performance. The implementation of the virtual parallel impedance is illustrated, and the key parameters design of the FCC boost TL converter is given. A 3-kW two-stage single-phase PV grid-connected inverter is fabricated and tested, and the experimental results verify the validity of the theoretical analysis and the proposed control scheme. [ABSTRACT FROM AUTHOR]

This letter proposes a loop-free autocalibration technique for self-balancing of flying capacitors in a three-level dc–dc buck converter. The proposed converter utilizes a dual-path power stage with balancing switches that adaptively short flying capacitors at de-energizing phases. Thus, the dual-path three-level converter ensures that flying capacitor voltages are self-balanced at half the input voltage without using additional feedback calibration loops, leading to robust operation and competitive efficiencies. The prototype was fabricated in the 0.13-μm CMOS BCD process and adopted two flying capacitors of 4.7 μF each and an inductor of 10 μH with a dc resistance of 128 mΩ. When an 8-V input was converted to a 3-V output voltage with a load current of 400 mA, the measured efficiency was 88.6% at 800 kHz switching while flying capacitors are automatically balanced. [ABSTRACT FROM AUTHOR]

This article proposes a symmetrical hybrid multilevel converter topology with a constant DC source and a fuel cell source (FCS) for enhancing the voltage levels and minimization of total harmonic distortions (THD). The proposed topology is employed using a modified phase shifted pulse width modulation (PS‐PWM) in order to control the power electronic switches. In this hybrid topology, the first converter operated through low voltage with high frequency and the second converter through high voltage with fundamental frequency for improving the voltage levels. The THD for the hybrid multilevel converter with respect to current and voltage is 1.97% and 15.10%, respectively. The results show that the proposed topology performs better with a constant DC source compared to the fuel cell source. Furthermore, the less component count of the proposed topology is compared with the existing topologies. Comparisons are made to validate the results of the proposed topology with different sources and the existing topologies. Highlights: A symmetrical hybrid multilevel converter for different sources is proposed.Enhanced voltage levels with less component count are proposed.The minimization of THD with constant DC source is proposed.A modified PS‐PWM controller for effective voltage balancing is discussed. [ABSTRACT FROM AUTHOR]

Abstract—A hybrid switched capacitor multi-level inverter topology is proposed in this paper. The topology has a basic unit and an extendable "n" module. The number of levels synthesized and the voltage gain delivered is based on number of such "n" modules connected in series. For "k" number of "n" modules, 4k + 9 levels can be obtained in the terminal voltage with a voltage boost of 1:(k + 2). Also, the topology has self-balancing ability which is mathematically validated. A 13-level prototype of hybrid switched capacitor multi-level inverter (HSC-MLI) is developed and its performance is analyzed in simulation and experimental environments. The HSC-MLI is compared with recent switched capacitor multi-level inverter topologies in the aspects like number of capacitors, number of diodes, number of drivers, number of levels synthesized and voltage gain and the results are presented. The simulations are done in MATLAB environment and the experimental prototype is fabricated using IGBTs 1HW15N120E1. [ABSTRACT FROM AUTHOR]

Grid-connected inverters (GCI) are commonly used in PV system applications to generate a regulated AC current to feed into the grid. Transformerless inverters are the most advanced inverters that are used in industry, which provide efficiency with smaller size and lower cost. This paper proposes a grid-connected single-phase transformerless inverter with the technology of common ground and the virtual DC bus concept. In this topology, the grid neutral is connected directly to the PV ground, which generates a constant common mode voltage (CMV), thus leading to the elimination of the leakage current caused by the PV array’s parasitic capacitance. The proposed inverter has a buck–boost circuit with a flying capacitor to generate the DC bus for a negative power cycle, four switches, and two diodes. A unipolar sinusoidal pulse width modulation (SPWM) technique is used which reduces the output filter requirements. In addition, only one switch carries the load current during the active states of both the negative and positive power cycle, thus minimizing the conduction losses. One more advantage presented in the proposed inverter is its ability to charge the flying capacitor during all operation states due to the existence of the buck–boost circuit. Design and theoretical calculations were conducted in this paper to optimize the losses. Moreover, the PSIM simulation was used to validate the proposed topology inverter, verify the performance by showing leakage current elimination, and achieve unipolar voltage in the output bus. The simulation results show a peak efficiency of 98.57% for a 2 kW inverter, which agrees with the theoretical calculations.

The algorithm of operation of MCP1253 charge pump converter in buck mode is studied for the first time by detailed measurements of ripples of output voltage and flying capacitor voltage. Some experimental results for MCP1253 operation in regulation buck mode at combinations of input voltage 5.1 V and output voltage 3.5 V, input voltage 5.1 V and output voltage 4.8 V and input voltage 2.2 V and output voltage 1.7 V for the range of load current from 2 to 100 mA are presented as examples. Realized investigations revealed that algorithm of MCP1253 operation in buck mode, described by its manufacturer, is not defined the all different cases of its use. The algorithm depends on the ratio between input and output voltages, their values and load currents.

The three-level full-bridge (TLFB) dc−dc converter has been widely used in high-voltage, high-power applications. In the experimental test, the neutral-point voltage deviation appears; moreover, it has also been found out that if the flying capacitors are introduced, the input capacitor voltages could realize self-balance even if no active balancing solutions are adopted. In the previous publications, the role of flying capacitors is generally considered to extend the range of soft switching; however, the self-balance ability has not been mentioned or investigated in any publications thus far. In order to fill this gap, this paper provides the detailed mode operation analysis of the TLFB converter and reveals the cause of the imbalance. In addition, the mechanism of the self-balance ability provided by the flying capacitors is explained in detail, which gives a deep insight into the converter. At last, the influence factor of the voltage error in steady state has been analyzed, and the specific expression of the voltage error is also derived. The feasibility of the theoretical analysis is verified by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

Multilevel inverters (MLIs) are playing a pivotal role in the power sector with potential applications, such as interfacing renewable energy sources with the grid and several industrial drive applications. MLIs with a smaller number of switching devices are more promising due to their compact size, reduced cost, and higher efficiency compared with their traditional counterparts. This paper, therefore, presents a new three-phase seven-level inverter. This topology is a combination of two cascade-connected two-level voltage-source inverters (VSIs) and H-bridge cells with flying capacitors (FCs). This paper presents the operating principle and the balancing technique for the dc-link capacitors and FCs. The generation of various output voltage levels and the limitation of the sinusoidal pulsewidth modulation control for FC voltage balancing is also presented. The number of components in the proposed circuit configuration and their voltage ratings are considerably lower compared with the recently proposed topologies. The behavior of the proposed circuit configuration is first assessed with simulation studies and is then tested with a laboratory prototype. The simulation and experimental results validate the effectiveness of the proposed topology and the voltage balancing technique. [ABSTRACT FROM AUTHOR]

The operating principle of the clamp circuits like clamping diodes and flying capacitor in three-level LLC converter is different from other three-level converters, because the mean value of the resonant tank current is always zero and pulse-frequency modulation control is generally used instead of pulsewidth modulation control. And the converter with only the clamping diodes or flying capacitor is difficult to realize voltage balance. In this paper, the characteristics of the three-level LLC converter with single clamp circuit are first analyzed. Then, the operation principle of the hybrid-clamped circuit under the unbalanced voltages on input capacitors is described with the mode analysis, based on which the mechanism for voltage autobalance is elaborated and the effects of two clamp circuits are derived. Then, the design of the clamping diodes and the flying capacitor is also involved. Finally, a three-level LLC converter prototype is built and the experimental results verify the validity of the theoretical analysis and design. [ABSTRACT FROM AUTHOR]