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Power converters and electric motor drives when tightly regulated behave as constant power loads. These loads are different from resistive loads and have destabilizing negative impedance characteristics, which impact a system's stability. A boost converter is intrinsically nonlinear and is a nonminimum phase system at the output voltage with respect to the control input. The linear approximation of this boost converter loaded with a constant power load has a zero and poles in the right half of the $s$ -plane, making the system unstable and very difficult to control. Control techniques that employ some form of system inversion cannot be implemented for a nonminimum phase system. This paper describes a technique to modify the nonminimum phase boost converter to a minimum phase for a constant power load, further implementing the input–output linearization technique to stabilize the system. This paper also provides a methodological analysis of the problem followed by the proposed solution. Furthermore, it verifies the analysis of the proposed solution through simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes a new high-efficiency photovoltaic (PV) converter for grid connection through a high-leg delta transformer. The converter is composed of a symmetrically connected boost converter and three half-bridge inverters. One of the three half-bridge inverters is connected to the boost converter, and the others are directly connected to the PV terminals. As a result, this circuit configuration enables to reduce the power losses in both boost converter and inverters. This paper also proposes a new cooperative control method between the symmetrically connected boost converter and inverter. The control method can reduce the average switching frequency to 75% of that in a conventional one, resulting in a great reduction in the switching power loss. Experimental results confirm that the proposed circuit configuration makes it possible to improve its European efficiency from 91.6% to 94.5%. Moreover, system performance is evaluated on the assumption of maximum power point tracking operation. [ABSTRACT FROM AUTHOR]

This paper discusses dynamic behavior of a dual-active-bridge (DAB) dc–dc converter. Conventional phase-shift control methods for the DAB converter may cause dc offsets in both inductor current and transformer magnetizing current in transient states. The dc offset in the inductor current would introduce an excessive peak current through the switching devices. The dc offset in the magnetizing current may induce magnetic-flux saturation. Conventional methods simultaneously turn on and off the diagonal switches in each H-bridge converter and produce a square-wave voltage with a 50% duty ratio. In contrast, the proposed method in this paper independently controls each switch to modify the duty ratio in transient states. This paper clearly derives the requirements of each switch to eliminate the dc offsets in both currents with a settling time shorter than half the switching period. Experimental results using a 5-kW 20-kHz system verify the validity of the proposed control method, which is effective not only in a single step change, but also in a continuous change in the phase-shift reference. [ABSTRACT FROM AUTHOR]

This paper documents a new three-switch, isolated boost dc–dc converter. The major features of the proposed converter are as follows: 1) continuous input current; 2) reduced one active switch, one additional diode, and one additional capacitor; 3) unchanged primary and secondary voltage waveforms of the transformer when the duty cycle is changed; and 4) removal of the snubber circuit. This paper presents the operating principles, analysis, parameter design guidelines, and simulation results for the proposed converter. To verify the performance of the proposed converter, a 400 W prototype was constructed with a 40–60 V dc input. A PID controller was used to maintain the dc output voltage at 400 V. The simulation and experimental results matched those of the theoretical analysis. [ABSTRACT FROM AUTHOR]

An extended-duty-ratio (EDR) boost converter is studied extensively in this paper for high voltage gain applications with a wide input (and/or output) voltage range. The EDR is a unique combination of an interleaved, multiphase boost converter and switched capacitor configuration that achieves high voltage gain with significantly lower switch voltage stress and switching losses compared to conventional high-gain solutions. Most of the switches in the multiphase EDR experience only a fixed fraction of the output voltage ($1/M, 2/M$ , etc., where $M$ is the number of phases). Through extensive analysis over a wide operating range, it is shown here that the EDR boost converter has inherent current sharing among the phases only in a limited range of duty ratio— $(M-1)/M \leq D \leq 1$. As the duty ratio reduces beyond this range as required in wide input voltage applications, inherent current sharing property is lost. In this paper, techniques to ensure current sharing under all operating zones without requiring current sensors are presented. Instead of having equal duty ratio for each phase, it is adjusted for each phase according to the operating region of the converter. Extensive analysis is presented to derive the required duty ratio changes for the different phases. The proposed concept is validated with experimental results from a 250 W, 3-phase EDR boost, and GaN-based hardware prototype. [ABSTRACT FROM AUTHOR]

Conventional single-phase power-factor-correction (PFC) rectifiers with active power decoupling capability typically require more than three active switches in their circuits. By exploring the concept of power-buffer cell, a new single-stage PFC rectifier with two active switches, one inductor and one small power-buffering capacitor is reported in this paper. The proposed converter can achieve high-power factor, wide output voltage range, and power decoupling function without using electrolytic capacitor. Additionally, an automatic power decoupling control scheme that is simple and easy to implement is proposed in this paper. The operating principle, control method, and design considerations of the proposed rectifier are also provided. A 100-W prototype with ac input voltage of 110 Vrms and a regulated dc output voltage ranging from 30 to 100 V has been successfully designed and practically tested. The experimental results show that with only a 15 μF power-buffering film capacitor, the proposed converter can achieve an input power factor of over 0.98, peak efficiency of 93.9%, and output voltage ripple of less than 3%, at 100-W output power. [ABSTRACT FROM PUBLISHER]

This paper proposes a ripple injection method in a two-stage intermediate bus architecture to overcome the incremental negative-impedance effect in the intermediate bus converter (IBC) induced by the constant power load, which is essentially a tightly regulated point of load (PoL) converter. The proposed method uses the voltage ripple of the intermediate bus in the feedback path for the closed-loop PoL converter while operating the IBC in open loop. The small-signal model is derived and verified using SIMPLIS simulation. The voltage ripple is shown to carry the inductor current information of the PoL converter. Thus, the proposed scheme is expected to retain the superior line regulation of current mode control for the PoL converter even without sensing the inductor current. Load current feed forward can improve the transient response for the PoL converter. Moreover, the load current information is also used to generate the appropriate duty ratio for the IBC during transient recovery. A simple PI controller is used for the PoL converter, which reduces the design complexity using the proposed control scheme. A hardware prototype is made, and the superior performance using the proposed control scheme is experimentally verified. [ABSTRACT FROM AUTHOR]

This paper proposes a high-efficiency three-level (TL) dc–dc converter with reduced circulating current and rectifier voltage stress by using coupled inductor in the rectifier circuit. Because of decreased circulating current, the conduction loss in the primary circuit is reduced. In addition, since the rectifier diodes with low forward voltage drop can be employed due to the small voltage stress, the conduction loss of rectifier diodes can be reduced. Therefore, the efficiency can be increased due to the reduction of the conduction losses in the primary circuit and rectifier diodes. The characteristics of the proposed converter are analyzed and compared with other TL converters. The validity of the proposed converter is verified by the experimental results of the prototype with 540–600 VDC input and 750 W (48 V/15.625 A) output. [ABSTRACT FROM AUTHOR]

This paper proposes a constrained modulated model-predictive control (M $^{2}$ PC) scheme for an LC-filtered voltage-source converter (VSC). To tackle the coupling effects of the state variables in a second-order LC filter, a dual-objective cost function (CF) is used to explicitly track both capacitor voltage and inductor current references, which can achieve an improved voltage quality. To handle the state and control input constraints of VSCs, a constrained M $^{2}$ PC scheme is proposed with an “online post-correction” constraint-handling technique. First, the unconstrained optimal voltage vector (OVV) is derived. It is generated by seeking the minimum analytical solution of the CF offline, simplifying the online implementation. Then, an “online post-correction” strategy is employed by reconsidering the constraints to correct the precalculated OVV online, which guarantees the future states within the allowed range. Finally, the corrected OVV is synthesized by the space vector modulation, resulting in a fixed switching frequency and low harmonics. Compared with the typical constrained model-predictive control, the presented scheme has the advantages of improved steady-state performance, flexible constraint-handling ability, and lower computational cost. Additionally, design procedures for weighting factor selection in the CF are given. Comparative experiments are investigated to verify the presented control strategy. [ABSTRACT FROM AUTHOR]

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]

The pulse-width modulated (PWM) switching rectification that can achieve a high power factor (PF) for increasing the energy transfer efficiency between an LC resonator and a rectifier and voltage step-up and -down conversion is proposed for a wireless power transfer (WPT) receiver. The proposed method can emulate the switching rectifier as a resistive load by using an inductor and integrated phase synchronizers. Additionally, similar to a switched-inductor converter that controls the duty cycle ratio (D), the proposed PWM rectifier can control the output voltage VOUT when the input is a rectified, wirelessly coupled voltage instead of a constant voltage. Thus, unlike a conventional PWM switching rectifier for ac mains, an additional voltage conditioning circuit would not be needed after the proposed rectifier for WPT. The proposed PWM switching rectification is implemented in the AMS 0.18 μm 1.8 V/5 V CMOS process. PF = 1 is measured, indicating the most efficient energy transfer, compared to only 0.55–0.65 in a peak detection rectifier. Additionally, 88.2% of peak power conversion efficiency of the switching rectifier is achieved, and the maximum output power is 80.3 mW at 500 kHz of the WPT frequency. Moreover, the measured voltage conversion ratios ranging between 0.73× and 2× are demonstrated in this paper. [ABSTRACT FROM AUTHOR]

The right-half-plane (RHP) zero in a continuous conduction mode boost converter results in a significantly restricted closed-loop bandwidth (BW) for higher voltage gain and/or load current conditions. A compensating ramp is used for current-mode control (CMC), and a higher ramp slope degrades the BW. Variable-frequency digital CMC offers inherent current-loop stability and real-time tuning scope for higher closed-loop BW. However, the challenges are to select the sampling frequency and sampling instant of the output voltage with discontinuous ripple due to the effective series resistance of the output capacitor, and its impact on stability. This paper shows that an event-based current-mode constant off-time digital modulator achieves superior stability and performance in a boost converter along with the reduced RHP zero effect over other digital CMC techniques. Using a discrete-time framework, the fast-scale stability conditions and small-signal models are analytically derived for various digital CMC techniques, which are validated using SIMPLIS simulation. A boost converter prototype is tested, and the analytical predictions are verified experimentally. Further, the analysis is extended to a non-inverting buck–boost converter. [ABSTRACT FROM AUTHOR]

Small-signal stability of ac power systems is able to be assessed by measuring their terminal impedance, and a transformerless series injector is very attractive for wideband measurement of load impedances. A typical implementation of the transformerless series injector is paralleled H-bridge (PHB) converters, where dc voltage imbalance will occur when system current magnitude is larger than a critical point, and thus the operation region of the series injector is seriously restricted. To address this issue, this paper presents a deep analysis on the mechanism behind dc voltage imbalance for PHB-based series injector, and it is revealed that a positive feedback exists in the dc voltage balancing loop under high system current magnitude. Then an enhanced control scheme is proposed to balance dc voltages in full system current range, where a reactive component is injected in the output voltage while reactive circuiting current is introduced among H-bridge converters for redistributing active power. Furthermore, the system behavior with the proposed scheme is comprehensively analyzed and the reference design for the injected reactive component magnitude in the output voltage is offered. Finally, the imbalance mechanism and the proposed scheme are validated by simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper analyzes the possibility of controlling the output voltage of a standalone single-phase inverter by directly controlling the output filter capacitor current without using a dedicated output voltage sensor. The plant modeling and controller design are presented. The proposed method depends on having the value of the output filter capacitance. A method to estimate the output filter capacitance is also presented. Rigorous analysis is done to show that the proposed sensorless scheme is largely insensitive to parameter variations and ensure that the output voltage is within specified regulations at utility level. It is also demonstrated in this paper that compared to the conventional voltage control scheme the proposed control scheme ensures an improved total harmonic distortion of the output voltage waveform. Experimental results presented validate the proposed scheme. [ABSTRACT FROM AUTHOR]

In this paper, the Z-source converter is introduced to power factor correction (PFC) applications. The concept is demonstrated through a wireless power transfer (WPT) system for electric vehicle battery charging, namely Z-source resonant converter (ZSRC). Due to the Z-source network (ZSN), the ZSRC inherently performs PFC and regulate the system output voltage simultaneously, without adding extra semiconductor devices and control circuitry to the conventional WPT system such as conventional PFC converters do. In other words, the ZSN can be categorized as a family of the single-stage PFC converters. In addition, the ZSN is suitable for high-power applications since it is immune to shoot-through states, which increases reliability and adds a boost feature to the system. The ZSRC-based WPT system operating principle is described and analyzed in this paper. Simulations and experimental results based on a 1-kW prototype with 20-cm air gap between the system primary and secondary sides are presented to validate the analysis and demonstrate the effectiveness of the ZSN in the PFC of the WPT system. [ABSTRACT FROM PUBLISHER]

An LLC resonant converter would have light-load voltage regulation problems due to the effect of parasitic capacitances in a high-frequency range. In this paper, an energy feedback control method is proposed to improve the light-load regulation capacity by using synchronous rectifiers (SRs). The SR bridges are controlled ahead of the primary inverter bridges to deliver the energy from load to source, reducing the output voltage independent of the load. Then the desired voltage can be achieved even in no-load conditions by regulating the operation time of the energy feedback mode. The gain, voltage ripple, and efficiency performances of the proposed control method are discussed based on the precise analysis and an accurate model of the converter. Moreover, the light-load operations of the LLC resonant converter are analyzed deriving the critical load condition. Finally, the proposed control method is verified by the experiments of a 12 V/8 A LLC resonant converter prototype. [ABSTRACT FROM AUTHOR]

A 1-φ hybrid nine-level inverter (H9LI) topology has been proposed in this paper. The proposed H9LI topology uses a simple phase disposition pulsewidth modulation strategy to generate nine-voltage levels in the output. The main advantage of this topology is that it has a low switch count (ten switches) compared to the existing nine-level inverter topologies. To regulate the voltage across auxiliary capacitor, two innovative control techniques are proposed, which are integrated with the inverter modulation technique itself. Hence, it does not require any extra voltage balancing circuits to maintain the voltage across the auxiliary capacitor and input dc capacitors. A major advantage of these control techniques is that they eliminate the sensing of the coupled inductor current. Another significant advantage of H9LI is that loss distribution among all the power switches is more uniform compared to existing nine-level inverters. Due to low part count and absence of extra voltage balancing circuits, the H9LI achieves higher efficiency (η ≍ 94.5%) and lower cost. Furthermore, the requirement of filter size reduces due to the presence of coupled inductor in H9LI. The proposed 1-φ grid connected H9LI is verified through MATLAB/Simulink simulations and validated through experiments on a laboratory prototype of 400-VA rating. [ABSTRACT FROM AUTHOR]

A distributed dc grid connected photovoltaic (PV) generation configuration based on hybrid-connected three-port converters (TPCs) and its control strategy are proposed in this paper. Distributed maximum power point tracking (MPPT) and autonomous voltage sharing control are achieved with the proposed configuration and control strategy. The proposed system consists of multiple modular pulsewidth modulation plus phase-shift (PPS) controlled TPCs, which feature soft switching and low voltage stress. The input port of each TPC is connected to an independent PV energy source to achieve individual MPPT, and the output ports of these TPCs are connected in series to interface with a high-voltage (HV) dc bus, while the bidirectional ports are in parallel to build a low-voltage (LV) dc bus. The mismatch power of input sources can be transferred through the LV dc bus among these TPCs, and the power and voltage balancing at the HV output side can be realized. By regulating the voltage reference of a bidirectional port in a linear relationship with the output voltage, output voltage sharing is realized with only the module's own voltage and current being sensed. Fully modular design is achieved. System stability with the proposed control strategy is revealed by using the Routh–Hurwitz criterion. Simulation and experimental results are provided to verify the effectiveness and advantages of the proposed configuration and control strategy. [ABSTRACT FROM AUTHOR]

High step-down voltage conversion is necessary to bridge the voltage gap between main power converters and low-voltage auxiliary electronics in power conversion systems. Switched capacitor converters (SCCs) are an attractive candidate as a high step-down converter, but their poor voltage regulation capability may limit their applications. Pulsewidth modulation (PWM) SCCs with adjustable high step-down voltage conversion are proposed in this paper. The proposed PWM SCCs can be derived from traditional SCCs by replacing an energy transfer capacitor with a switched-capacitor-inductor (SCL) cell that comprises inductors, capacitors, and diodes to realize PWM-controllable voltage conversion. The voltage step-down ratio of the proposed PWM SCCs is not only PWM controllable but also adjustable with structures of SCCs and SCL cells. Two representative PWM SCCs were taken as examples to perform the operational analysis. The prototypes of both representative PWM SCCs were built and tested to demonstrate the proposed concept. [ABSTRACT FROM AUTHOR]

In this paper, a novel soft-switching secondary-side modulated dc–dc structure with extended zero voltage switching (ZVS) range is proposed. In a simple manner, this structure can be extended to provide isolated multioutputs with independent voltage regulation capability. Besides, the secondary side can be bidirectional to interface with energy storage elements such as batteries and ultracapacitors. The proposed topology is suitable for high voltage gain or high output dc-link voltage, low-power, and multioutput applications. The primary-side full-bridge switches operate complementarily to generate a two-level square wave. Therefore, the circulating energy is significantly smaller than conventional phase-modulated full-bridge converters. The output voltage is regulated by controlling the duty cycle of the auxiliarymosfeton the active/semiactive rectifier, which offers a simple control. Moreover, the ZVS operation range of full-bridgemosfets is enhanced due to the volt-second balance of magnetizing inductance and the secondary-side control. All the secondary-side rectifying diodes turnoffwith limiteddi/dt.A 1-kW prototype generating two 390 V, 330–400 V outputs from a 130–180 V dc link is designed to serve as a proof of concept. The full-load efficiency is 95.6% at 100 kHz switching frequency. [ABSTRACT FROM AUTHOR]

In this paper, a family of step-up resonant switched-capacitor converters (RSCs) with a continuously adjustable conversion ratio is proposed based on the Cuk dual-resonance core (DRC) featuring two resonant paths. By adopting the on-time fixed frequency modulation, the two resonant paths can conduct simultaneously and satisfy the charge balance of the flying capacitor during the overlapping period. As a result, the conversion ratio—upper bounded by two determined by the circuit configuration—can vary continuously and efficiently even at the light load condition (1/10 of the rated power). A comprehensive analysis—the operation principle, voltage-gain curve, and voltage/current stresses—about the DRC is given. In addition, the Cuk DRC was extended to the series–parallel switched-capacitor converter (SCC), Fibonacci SCC, and voltage doubler to form a family of step-up RSCs, whose voltage-gain curves of different topologies are derived. Moreover, a characteristic comparison between the Cuk DRC and the traditional “direct” and “indirect” resonant core is given. Then, a 48 to 62–94.6 V resonant core with peak efficiency 97.8% and maximum power 120 W under the open loop condition was built. The analyses were verified by both experimental and simulation results. [ABSTRACT FROM AUTHOR]

This paper presents a dynamic power flow management system for a solar photovoltaic (PV) system employing a single-stage single-inductor-based dual-input/output dc–dc converter feeding standalone dc loads backed up by a rechargeable battery. A time-sharing voltage-mode control scheme has been proposed for power flow management between solar PV, a battery, and a standalone dc load, and it also maintains a constant dc load voltage and performs maximum power point tracking operation of solar PV. The implementation of the control scheme has been described in detail. The steady-state performance of the single-stage converter has been explained with the relevant analytical expressions derived along with the characteristics. A state-space average model was developed for simulating the transient behavior and validating the working of the system for step changes in the input solar PV power and the dc loads. A hardware prototype of the proposed system has been fabricated, and the proposed controller has been implemented using the dSPACE DS1103 real-time interface board. The working of the proposed scheme for different levels of input solar insolation and dc load power demand has been satisfactorily demonstrated, and the corresponding results are also provided. [ABSTRACT FROM AUTHOR]

This paper presents a loss optimization method for phase-shifted LLC resonant converters operating with phase shift. The method features automatic detection of the preferred operational mode from the five possibilities for the topology. For a given power level and voltage transformation ratio it simultaneously determines the values of the resonant elements and transformer turns ratio. This work supplements and extends the available optimization tools for variable frequency operation and extends these to phase-shifted operation. The combined methods allow converters with much wider voltage regulation ranges to be designed. Numerical programming methods to determine the mode boundaries and to solve the steady-state solutions for the different modes of LLC converter are presented. A detailed loss model for a phase-shifted LLC is also discussed. A prototype of 30–403V, 295W is built to validate the optimization results. A comparison is made between an optimized and unoptimized converter. An extension to multipoint optimization, as required to optimize the European Weighted Efficiency is described. [ABSTRACT FROM AUTHOR]

This paper presents a new predictive power-factor-correction (PFC) controller for series-interleaved three-level boost (TLB) converters. Compared to the state-of-the-art TLB PFC controllers, where a two-cycle prediction and a detection of an operating region are necessary, the proposed controller achieves a low total harmonic distortion of the input current by using a single equation to predict the input current in all operating regions of the converter, in just one operating cycle. The average current control is achieved by sampling at the peak of the triangular carrier. The proposed PFC controller significantly reduces the distortion of the input ac current near the zero-crossing points, resulting in a low total harmonic distortion of the input current. The operation of the proposed controller was evaluated, and its stability and robustness to parameter changes was confirmed analytically. The controller operating principles were verified in simulations and validated by experiments on a medium-voltage 50-kW converter prototype. [ABSTRACT FROM AUTHOR]

High-frequency (HF) switching and ac-side unbalanced loads challenge smart transformer (ST)-fed hybrid grids (both ac and dc), causing common-mode (CM) voltage variations and dc-link oscillation. The HF switching introduces an HF CM voltage and the ac grid unbalanced loads introduce a fundamental frequency CM voltage in hybrid grids. The CM voltage in ST-fed distributed grids degrades the power quality, threatens the safety of the connected devices, and potentially constitutes a health risk for the operators of such devices. Therefore, this paper systematically analyzes the root causes of the ST CM voltage variations and the impacts on hybrid grids. Based on the two typical configurations (three- and four-leg converters), the performance and requirements of CM inductor filter and bypass CM filter on HF CM voltage suppression are studied in detail. By considering the CM voltage suppression and dc capacitor lifetime, a four-leg converter with improved modulation strategy and small dc bypass film capacitor is proposed. The simulation and experimental results clearly verify the feasibility and correctness of the proposed strategies. [ABSTRACT FROM AUTHOR]

This paper proposes a circuit structure that can improve the power-conversion efficiency \eta e of an AC–DC boost converter. The circuit uses a 1:1 transformer and a voltage boost circuit composed of an inductor, a capacitor, and a diode. The transformer forces the converter to operate in continuous conduction mode for input current, and thereby reduces input ripple. The voltage boost circuit enables the switch to operate in critical conduction mode, so the power loss due to turn-on of switch and reverse recovery of diode is minimized. The proposed converter has high \eta e over a wide range of AC input voltage $V_{{\rm{ac}}}:\,\eta _{e}= 92.9\% $ at $V_{{\rm{ac}}}= {\text{85}}\,{\text{V}} and \eta e= 97.4\% at V{\rm{ac}}= {\text{265}}\,{\text{V}}, when the converter was operated at DC output voltage of 400 V and output power of 500 W. The temperature of the switch stabilized at ∼84 °C for $V_{{\rm{ac}}}= {\text{85}}\,{\text{V}}$ , at ∼70 °C for $V_{{\rm{ac}}}= {\text{110}}\,{\text{V}}$, and at ∼42 °C for $V_{{\rm{ac}}}= {\text{220}}\,{\text{V}}$, whereas that in other AC–DC converters either failed or stabilized at much higher levels. The proposed converter is well suited to systems that require an input voltage range of 85–265 V in root-mean-square value. [ABSTRACT FROM AUTHOR]

This paper proposes a practical solution of high-frequency-link dc transformer based on switched capacitor (SCDCT) for medium-voltage dc (MVDC) power distribution application. Compared to the traditional dc transformer scheme, the proposed SCDCT can disconnect from MVDC distribution grid effectively as a dc breaker when a short fault occurs in the distribution, can enhance power transfer capacity, and always ensures high-frequency-link voltage match to improve current impact and efficiency performances, and the redundancy design can be achieved when some submodules fail to improve the reliability. In the paper, the topology, voltage and power characterization, control strategy, startup, and fault solution of SCDCT are presented and analyzed comprehensively. At last, an SCDCT prototype is built and the experimental results verify the correctness and effectively of the proposed solution. [ABSTRACT FROM AUTHOR]

This paper focuses on the zero-voltage ride-through (ZVRT) capability of a transformerless back-to-back (BTB) system intended for installation on a power distribution system with many distributed power generators. The BTB system consists of two modular multilevel cascade converters based on double-star chopper cells (MMCC-DSCC) and their ac sides are connected to a common ac mains via ac-link inductors. This paper provides an experimental discussion on the DSCC-based BTB system under voltage sags by using a three-phase 200-V 10-kW downscaled system. Experimental results show that the BTB system is equipped with ZVRT capability without causing any overvoltage and/or overcurrent even under the most severe voltage sags. [ABSTRACT FROM PUBLISHER]

This paper provides a theoretical discussion and comparison in energy balancing between a modular multilevel cascade inverter based on single-star bridge cells (SSBC) and that on single-delta bridge cells (SDBC). Attention is paid to applications involving asymmetric active-power generation in utility-scale grid-tied photovoltaic systems. Both qualitative and quantitative evaluation metrics to assess the energy-balancing capability are introduced and applied to both SSBC and SDBC inverters. As for the SSBC inverter, six zero-sequence voltage waveforms with different harmonic content enabling enhanced energy-balancing capability are analyzed and compared regarding their effectiveness. This paper also emphasizes on the SDBC as an alternative to the SSBC and highlights its superior operating characteristics under asymmetric active-power generation. [ABSTRACT FROM PUBLISHER]

New advanced power conversion systems play an essential role in the extension of range and life of batteries. This paper proposes a new modular multilevel converter with embedded electrochemical cells that achieves very low cell unbalancing without traditional balancing circuits and a negligible harmonic content of the output currents. In this new topology, the cells are connected in series by means of half-bridge converters, allowing high flexibility for the discharge and recharge of the battery. The converter features a cell balancing control that operates on each individual arm of the converter to equalize the state of charge of the cells. The paper shows that the proposed control does not affect the symmetry of the three-phase voltage output, even for significantly unbalanced cells. The viability of the proposed converter for battery electric vehicles and the effectiveness of the cell balancing control are confirmed by numerical simulations and experiments on a kilowatt-size prototype. [ABSTRACT FROM AUTHOR]

Full-bridge (FB)-boost converter is suitable for applications with wide input voltage range and galvanic isolation requirement, and a two-mode phase-shift (PS)-two-edge modulation (TEM) control scheme based on two modulation signals and one carrier can be used to achieve automatic mode shifting and high efficiency. In order to reduce the influence of the input voltage disturbance on the output voltage, the small-signal model of FB-boost converter is built and the input voltage feed-forward (IVFF) functions under different operating modes are derived in this paper. In view of the small-signal and large-signal control laws of the derived IVFF functions, the two-mode PS-TEM control schemes with small-signal and large-signal IVFF compensations are proposed, respectively. Both of them can realize automatic selection of operating modes and the corresponding IVFF compensations, and thus high efficiency and improved input transient response can be guaranteed. Besides, the IVFF function in boost mode is simplified for easy implementation, and the comparisons among the two-mode PS-TEM control schemes with small-signal, large-signal, and without IVFF compensations are presented in this paper. Finally, a 250-500-V input, 360-V output, and 6-kW-rated power prototype demonstrates the effectiveness of the proposed control schemes. [ABSTRACT FROM AUTHOR]

Two-stage single-phase inverters have been widely used as they can achieve voltage matching and galvanic isolation between the input and output. Due to the pulsating output power of the downstream inverter, an ac current at twice the output frequency, which is called second harmonic current (SHC), arises in the input side of the downstream inverter. This SHC will penetrate to the front-end dc-dc converter, leading to reduced conversion efficiency. This paper first analyzes the propagation mechanism of the SHC and load transient response of two-stage single-phase inverters from the viewpoint of output impedance. Then, based on the current mode control and load current feed forward, two control methods to achieve low SHC in the front-end dc-dc converter and fast dynamic performance during load transient are proposed in this paper. Finally, a 2-kW two-stage single-phase inverter prototype has been constructed and tested, and the experimental results are provided to verify the effectiveness of the proposed control methods. [ABSTRACT FROM AUTHOR]

A capacitor voltage balancing method for a modular multilevel dc (MMDC) converter is proposed. The MMDC inherits the desirable features of both a modular multilevel converter (MMC) and a dual active bridge, namely high voltage capability, fault tolerance, zero-voltage switching-on of all the power devices, and a wide operating range. These features are ideal for a dc transformer in any dc distribution system. One key MMDC control issue is balancing of submodule capacitor voltages. Conventional MMC balancing techniques cannot be used as arm currents of an MMDC change direction within each switching cycle. This paper establishes that the average capacitor voltage of each arm is self-balanced and proposes a method to balance voltages within an arm by assigning the gate signals with higher charge difference to the submodules with lower voltages and vice versa. Charge difference is obtained by the voltage difference, hence only the SM capacitor voltages are required for balancing control. Different from the existed methods, the proposed technique is not limited by voltage gain or load conditions. Simulation and experimental results show that capacitor voltages are balanced under different voltage gains, different output power, and also when the MMDC starts up, hence ensuring the stable MMDC operation. [ABSTRACT FROM PUBLISHER]

Axes cross-coupling in dq-frame will deteriorate the current control performance, especially for applications with low switching/sampling frequency. Decoupling methods based on inductor current state feedback and complex vector proportional-integrator controllers are usually performed to solve this problem. In this paper, an alternative decoupling method based on preprocessed reference current feed-forward is proposed, which can provide excellent decoupling performance even with considerable control delay. The current control of grid-connected voltage source converters is analyzed with complex-coefficient transfer functions. It is revealed that the decoupling performance of current control schemes is closely related to the symmetry of current closed-loop transfer function bode diagrams. The current closed-loop transfer function should show a standard symmetrical bode diagram with respect to 0 Hz, if axes cross-coupling is well decoupled. A quantitative index named cross-coupling function is then proposed to analyze and comprehensively compare the decoupling performance of the proposed method and existing ones in terms of the influence of the control delay, inductance estimation error, grid frequency deviation, and voltage disturbance. Experimental tests are implemented to validate the performance of the proposed method and comparison results. [ABSTRACT FROM AUTHOR]

Dynamic voltage restorer (DVR) technology has become a mature power quality product. In high-power applications, DVR using a multilevel converter is commonly used. However, DVR using a multilevel direct pulse width modulation (PWM) ac–ac converter has not been well studied. This paper presents a new DVR topology using a cascaded multilevel direct PWM ac–ac converter. In the proposed scheme, the unit cell of the multilevel converter consists of a single-phase PWM ac-ac converter using switching cell structure with coupled inductors. Therefore, the multilevel converter can be short- and open-circuited without damaging the switching devices. Neither lossy RC snubber nor a dedicated soft commutation strategy is required in the proposed DVR. This improves the reliability of the DVR system. The output voltage levels of the multilevel converter increase with the number of cascaded unit cells, and a high ac output voltage is obtained by using low-voltage-rating switching devices. Furthermore, a phase-shifted PWM technique is applied to significantly reduce the size of the output filter inductor. A 1-kW prototype of single-phase DVR is developed, and its performance is experimentally verified. Finally, the simulation results are shown for a three-phase DVR system. [ABSTRACT FROM AUTHOR]

This paper presents a primary-side regulation constant current constant voltage (CC-CV) Flyback ac–dc converter characterized by high efficiency and high precision of constant current control. In CC mode, a novel ring detection and demagnetization portion control technique is proposed, with which the ending time of demagnetization can be accurately determined and the portion of demagnetization in switching periods remain precisely the same, which enable the accuracy of constant current output that can reach a level as high as ±0.8%. In CV mode, a power stage model based on steady-state analyzing method is established, according to which the expression of static power loss is first obtained. After that total power loss expression including switching loss is derived as well. By MATLAB simulation, power loss mechanism is revealed and analysis is made in detail. According to which, optimized parameters of Flyback converter system and an effective CV regulation strategy are obtained eventually. Simulation as well as experimental results verifies the efficiency of the ac–dc converter system after optimization that can be more than 84.7% at 5 V/1 A application. [ABSTRACT FROM PUBLISHER]

This paper proposes a current-fed isolated bidirectional dc–dc converter (CF-IBDC) that has the advantages of wide input voltage range, low input current ripple, low conduction losses, and soft switching over the full operating range. Compared with conventional CF-IBDCs, the voltage spikes of the low-voltage (LV) side switches in the proposed converter can be eliminated without additional clamp circuits. The converter adopts the pulse width modulation plus hybrid phase-shift control scheme such that the bus voltage can match the output voltage by means of the transformer. Thus, the current stresses and conduction losses of the converter become lower. In addition, the practical zero voltage switching (ZVS) of the secondary-side switches can be realized by adjusting the phase-shift angle within the secondary side when in light load or no load condition. The operating principles and characteristics including the power transfer, root-mean-square (RMS) current, and soft switching are investigated in detail. Then the design guidelines of inductors are also given. Finally an experimental prototype with 30–60 V input and 400 V/2.5 An output is built to verify the correctness of theoretical analyses. [ABSTRACT FROM PUBLISHER]

The paper presents a new bridgeless power factor correction (PFC) topology, using a recently proposed controllable LCL filter, namely active virtual ground to achieve efficient power conversion, and high-frequency common mode voltage (CM) reduction. The proposed PFC circuit consists of high-frequency semiconductors for shaping inductor current and low-frequency semiconductors to form two different LCL structures for different conditions. This reduces grid differential mode current ripple or inductance. Besides, the PFC CM voltage, a main problem of bridgeless PFCs, is significantly reduced, since the capacitor in the LCL filter clamps the voltage between the grid and the converter ground. The performance of the proposed PFC is experimentally verified. The results show that the proposed PFC guarantees sinusoidal input current, low high-frequency common-mode voltage noise, and has a good agreement with the theoretical findings. [ABSTRACT FROM AUTHOR]

To solve some problems caused by the ac control method used for the existing modular multilevel dc–dc converter (MMDC), this paper has proposed a new MMDC topology with a wide range of output voltage by means of dc control. By reconstructing the submodule structure, the converter has a power branch and an auxiliary balance branch, which are used to transmit dc power and balance the capacitor voltage, respectively. Both dc analysis and control method are adopted to establish a mathematical model so as to deduce the mathematical relationships between the key electrical parameters. Moreover, what have been done include analyzing the self-balancing principle of the capacitor voltage and the mechanism of the auxiliary balance branch, calculating the related parameters of the capacitor voltage fluctuation, and presenting a dc closed-loop control strategy based on a small-signal model. The analyses of the steady and dynamic states in combination with simulation and experiment show that the proposed converter can stably operate in the mode of dc control, with a large adjustable range of output voltage, a small fluctuation in voltage of the capacitor, and the ability of the inductor to suppress the spike of the auxiliary balance current effectively so as to reduce the impact on the device. [ABSTRACT FROM AUTHOR]

Diode–capacitor-based dc–dc converters provide a simple and low cost solution for high step-up voltage regulation in solar and fuel cell generation. Transient modeling analysis reveals their worse influence of nonlinear and nonminimum-phase system characteristic due to right-half-plane (RHP) zero, especially in high voltage gain application. However, the process of energy transfer for diode–capacitor-based dc–dc converter is different from basic dc–dc converter. Based on the unique feature, this paper proposes an improved main circuit structure with parallel connection of resistive–capacitive damping network across the intermediate capacitor to achieve good dynamic performance. By optimal parameter design according to Routh–Hurtwitz criterion, all the RHP zeros in the transfer function of control-to-output voltage are eliminated completely. Then, by the case of diode–capacitor-based boost converter, the adaptive PI controller is designed to deal with nonlinear characteristic of voltage gain. It gets good dynamic performance under wide range output voltage. All the theoretical findings and design approaches are verified by simulation and experiment results. The existing diode–capacitor-based high step-up dc–dc converters with slight main circuit modification are more promising in renewable energy application. [ABSTRACT FROM AUTHOR]

Full-bridge single-stage (FBSS) ac-dc converters allow to regulate both the output voltage and the input current that achieves a near sinusoidal waveform using only the four bridge transistors. Independent of this feature, these converters still need to be optimized in order to become an interesting and attractive solution for modern switch mode power supplies with power factor corrector (PFC) function. One of the most important improvements needed is the downsizing of the dc bus capacitor C b with the inherent cost reduction. However, this action introduces complex issues in the regulation of the input current and it is also responsible for the generation of high output voltage ripple. The new contribution of this paper consists in the introduction of a set of power circuit optimizations and control techniques in a FBSS PFC converter that solves the referred issues in order to enable the reduction of the dc bus capacitor's size and cost. These procedures are based in the use of a free wheeling circuit that improves the light load operation and in the application of one shot nonlinear modulator, in order reduce the output voltage ripple even when the dc bus ripple is high. The possibility of using, in the proposed topology, a reduced ratio capacitance/watt lower than the typical values used in commercial applications (0.7-0.5 μF/W for 385-450 V, respectively), while maintaining the accurate input current regulation, is also theoretically proved. The developed concepts, solutions and design criteria are detailed described in the paper. The correspondent theoretical study is verified trough experimental results token in an optimized FBSS topology prototype. [ABSTRACT FROM AUTHOR]

This paper presents a method of mitigating the transient overshoots of dc-dc converters undergoing large load disturbances. The method involves a small power auxiliary circuit which exists as an energy buffer to provide a smooth absorption and release of excess energy from and to the main dc-dc converter during transients. In the method, the regulation of the current magnitude of the converter and energy storage of the auxiliary circuit are guaranteed by the relevant control schemes. The cost and size of the auxiliary circuit will be relatively small as it only operates intermittently. In this paper, the voltage deviation analysis and the auxiliary circuit design guideline are provided. Experimental results validating the approach are presented. [ABSTRACT FROM AUTHOR]

In this paper, a novel voltage-based charge balance control algorithm is presented, which is suitable for both digital and analog implementations for buck converter to achieve near-optimal dynamic performance. First, this paper presents a new derivation of practical charge balance equations based on simplified differential equations. This deviation is applicable to both fast input voltage and load step transients. The final algorithm does not require complex calculations and accurate knowledge of the output filter LC parameter. Second, the proposed voltage-based charge balance controller does not require accurate current sensor or fast analog-to-digital converter. Instead, to detect the critical time instant when the inductor current equals the new load current, a practical extreme voltage detector is introduced to capture the output voltage peak/valley information. Third, this algorithm is simple to be implemented by either low-cost digital signal processing devices (such as microcontroller unit) or analog circuits. Both digital and analog experimental prototypes are built to verify the feasibility and advantages of the new method. [ABSTRACT FROM AUTHOR]

A variable-on-time (VOT) control scheme for a critical conduction mode (CRM) integrated buck–flyback power factor correction (PFC) converter with high power factor (PF) and low total harmonic distortion (THD) is proposed in this paper. By utilizing the input and output voltage to modulate the on-time of both buck and flyback switches, the input current harmonics can be eliminated and high PF will be obtained. The operating principles of both the conventional constant-on-time (COT) control and VOT control for the CRM buck–flyback PFC converter are analyzed. A 100-W prototype has been built and tested in the lab. The experimental results demonstrate that the THD and PF of the converter with VOT control are greatly improved compared to those with COT control. [ABSTRACT FROM AUTHOR]

The inductor current ripple under hysteresis current control is sensitive to system as well as controller parameters, which often deviates from the desired band. Thus, a phase-locked-loop (PLL) is employed to regulate the switching frequency over the operating range. Digital platform drastically simplifies this using an all-digital PLL (ADPLL) and offers the controller-tuning flexibility for improved performance. However, the use of uniform voltage-sampling often leads to multilimit cycle instability. This paper proposes a mixed-signal hysteretic current controller (MSHCC) with the digital voltage-loop and the analog current-loop. A digital-to-analog converter is sufficient to generate time-multiplexed current references. This achieves robust stability and parameter-insensitive current ripple by sampling the error-voltage at the rising edge of the high-side gate signal. Stability analysis is carried out using discrete-time models. The proposed MSHCC can be configured to either of the peak, average, or valley current-mode techniques along with the flexibility to adjust the switching frequency using an ADPLL for real-time energy-optimization. The proposed scheme is implemented using an FPGA device and tested on a buck converter prototype. The MSHCC scheme can be extended to a multiphase buck converter. [ABSTRACT FROM AUTHOR]

This paper proposes an output current-differential (OCD) control scheme that has a master–slave structure. It can be applied in dc/dc converters to connect the input in series and the output in parallel. All of the control and sample circuits in the proposed control scheme are on the output side, which means that there is no further isolation in the control loops. The proposed control scheme, consisting of one output voltage regulator loop and individual load-current-sharing loops, will enable low-voltage converters to be used for high-voltage specifications. The master module regulates the output voltage through a common output voltage loop and provides current references to slave modules. The individual current-sharing loops residing in the slave modules regulate the current in each module equally. According to the power balance, input-voltage sharing is realized simultaneously. To prevent a situation in which system failure is caused by master module failure, a fault-tolerant automatic master–slave output current-differential (FOCD) control scheme is developed. The performance of the OCD and FOCD control schemes is validated on a 450-W prototype input-series–output-parallel system comprising three forward converters. [ABSTRACT FROM AUTHOR]

Primary-side controlled universal-line flyback converters have been widely used in low-power applications due to their simplicity, low standby power, and low cost. However, conventional OCP schemes normally cause too large a variation in output current when the input voltage varies. This paper proposes a self-compensating OCP circuit to achieve a high-precision constant current at different input voltages which can be used in flyback converters working in discontinues conduction mode. It consists of a CS S/H circuit and a dynamic OCP voltage reference circuit. Compared with conventional OCP circuits, it does not need an extra compensation circuit. To verify the feasibility and performance of the proposed circuit, an IC controller that uses the proposed OCP circuit has been designed and fabricated in 0.35 μm 5-V/40-V high-voltage CMOS technology. Experimental results show that the constant output current precision of the prototype is within ±1% when the input line voltage changes from 85 to 265 Vac. In the common line voltage range (\rm110-220\ Vac ), the output current regulation accuracy can reach a level as high as ± 0.3%. [ABSTRACT FROM AUTHOR]

In high-power applications, paralleling three-level T-type converters (3LT2Cs) with common ac and dc sides represents a modular solution for overcoming limitations of active switches. However, when independent space-vector pulse-width modulation (SVPWM) is employed, zero-sequence circulating currents (ZSCCs) will emerge. Especially, the use of three-phase three-limb inductors (3P3LIs) without additional passive components to reduce cost and improve density of overall parallel system, will worsen ZSCC problem. In this paper, detailed analysis of ZSCCs generated by SVPWM based on a derived average model taking into account magnetic circuit characteristics of 3P3LIs is proposed. Furthermore, four inter-related methods are sequentially proposed: 1) synchronizing sector numbers based on modified vector diagram partition for eliminating periodical ZSCC jumps caused by asynchronous change of starting small vector; 2) feeding forward grid voltages for eliminating starting current jumps due to computation delay; 3) connecting neural points to eliminating constant dc ZSCC caused by neural-point voltage deviation; and 4) adjusting relative duration of negative and positive small vectors in pairs to suppress remaining low-frequency ZSCCs. Consequently, 3LT2Cs operated in parallel can separately regulate output powers with a good circulating current suppression performance. The validity of theoretical analysis and proposed methods is verified by experiment with three-parallel 175-kw 3LT2Cs. [ABSTRACT FROM AUTHOR]

In this paper, a single-phase buck–boost matrix converter is proposed which can both buck and boost the input voltage with step-changed frequency. It consists of only six unidirectional current flowing bidirectional voltage blocking switches, two input and output filter capacitors, and one inductor. It has following advantages over the existing single-phase matrix converters: 1) it can both buck and boost input voltage solving the limited voltage transfer ratio (only boost or buck) problem; 2) it also has enhanced reliability as it is immune from shoot-through problem of voltage source when all switches are turned-on simultaneously, and, therefore, it has no need of PWM dead times and RC snubbers or dedicated soft-commutation strategies to solve the commutation problem; 3) it can also use high-speed power MOSFETs as their body diodes never conduct, which eliminate their poor reverse recovery problem. The operation principle of the proposed converter is given, and switching strategies are developed to obtain various multiples and submultiples of input frequency. To verify its performance, a laboratory prototype is fabricated and experiments are performed to produce step-down and step-up voltage with three different frequencies of 120, 60, and 30 Hz. [ABSTRACT FROM PUBLISHER]

This paper presents a three-phase back-to-back transformerless inverter for online uninterruptible power supply applications. The inverter consists of a three-phase three-wire input rectifier stage and a three-phase four-wire output inverter stage with their ground wires tied together. Both of the two stages are with sinusoidal pulse width modulation based division-summation (D-∑) digital control. The input stage controls the power flow between utility grid and dc link, regulates the dc-link voltage, and fulfills power factor correction. While the output stage tracks three-phase ac reference voltages. The proposed control laws derived based on the D-∑ digital approach take into account the effects of dc-link voltage fluctuation, grid-voltage distortion, and inductance variation due to different current levels. Thus, distortion of input current and output voltage, and filter inductor core size can be reduced significantly. Moreover, circulating current through their common ground can be suppressed effectively. Simulated and experimental results from a three-phase back-to-back transformerless inverter have confirmed the analysis and discussion of the proposed control approach. [ABSTRACT FROM PUBLISHER]