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A novel beta-quasi-Z-source (β-qZS) converter is proposed for an isolated dc–dc converter with the buck (step-down) and boost (step-up) functions. The β-qZS isolated dc–dc converter addresses the common challenges such as components stresses, reliability, suppression of switch voltage overshoot, size, and power derating in the existing impedance source-based isolated dc–dc converter applications. In addition, the proposed topology dealt with operating duty cycle constraint and loss of duty cycle phenomenon in the Z/qZS galvanically isolated dc–dc converters. This article describes the configuration and provides the principle of operation for the proposed β-qZS converter topology. A laboratory prototype is developed, and the experimental results satisfy the theoretical analysis. [ABSTRACT FROM AUTHOR]

This article proposes a three-phase six-switch switching-cell current source inverter (H6-SC-CSI) and a modified switching scheme of space vector pulsewidth modulation (SVPWM). Compared to the conventional three-phase six-switch current source inverter, the H6-SC-CSI has no open-circuit possibility, thus, the inverter reliability is greatly enhanced. In addition, the overlap-time can be minimized; hence, the output waveform quality can be considerably improved. Due to the presence of switching-cell capacitors, a conventional switching sequence of the SVPWM is modified for the inverter to operate properly. A 2.2 kW hardware prototype is built and tested to verify the effectiveness of the proposed PWM. [ABSTRACT FROM AUTHOR]

In this article, two configurations of the quadratic quasi-Z-source (Q-qZS) isolated dc–dc converters are presented, namely continuous input current and discontinuous input current configurations. The operating principles and steady state of the continuous input current topology analyzed in this article is characterized with the following features: 1) wider operating range due to buck and boost functions in a single stage; 2) draws continuous input current; 3) low input current ripple compared with other qZS-based isolated dc–dc converters; 4) better reliability and good immunity against EMI noise; 5) common ground between the input source and the inverting bridge; and 6) continuous control over the entire operating range. Compared with other qZS-based isolated dc–dc converters, lower current stresses on active and passive components used are realized. Hence, the total magnetic volume/energy required by the proposed converter is reduced with the potential of achieving high power density. A prototype of 40–140 V input voltage, 300 V output, and 550 W rated power is developed to validate the performance of the proposed converter presented. [ABSTRACT FROM AUTHOR]

The LLC resonant converter offers a promising solution for dc power conversion. The input-parallel output-parallel system offers low specification switches to use in high power applications. Moreover, the interleaving of the converters reduces the requirement of input and output passive filters, providing better dynamic performance and higher power density. However, the tolerances in the designed resonant components and the unavoidable mismatch in the circuit parameters lead to unbalanced estimated tank gain characteristics. Consequently, the zero-voltage-switching is lost, a serious imbalance in currents appears, and a thermal runaway is likely to occur. The proposed method utilizes the existing magnetics of resonant inductors and self-alleviates the imbalance in the output, input, and tank currents without increasing active or passive components, dedicated balancing controls, and expensive sensors. A 4 kW four-phase prototype is tested during steady state, transients, and phase-shedding modes to validate the performance of the proposed method. [ABSTRACT FROM AUTHOR]

This article proposes a single-phase five-level current source inverter with an inductor cell based on the switching-cell (SC) structure. Compared to the conventional inverter, the proposed structure has no open-circuit problem; thus, its reliability is greatly enhanced. The overlap-time inserted in the driving signals can also be minimized to improve the output waveform quality. The proposed inverter's operation is analyzed herein, and the design guideline of SC capacitors is discussed. In addition, the SC extension for the n-level topology is provided. A 1-kW prototype is built and tested to validate the effectiveness of the proposed structure. [ABSTRACT FROM AUTHOR]

Conventional dual-active-bridge dc–dc converters require a dedicated voltage balancer or feedback control when they are used for a bipolar LVdc distribution system. This article proposes a dual-active-half-bridge (DAHB) converter with voltage balancing. The proposed converter is composed of a conventional DAHB converter with an additional inductor and capacitor voltage balancer (LCVB). The LCVB is added between the transformer and the output of the DAHB converter. Although the LCVB increases the switch current, it can balance the two output voltages without additional active switching device and feedback control under unbalanced load conditions. In addition, compared with the conventional DAHB converter, the LCVB can increase the zero-voltage-switching range of the DAHB converter and has no detrimental effect on the operation modes and performances. A 2.8-kW prototype was built and tested to verify the performances of the proposed converter. [ABSTRACT FROM AUTHOR]

This letter proposes a modified double-dual-boost (DDB) high-conversion-ratio dc–dc converter. The proposed converter is derived from the existing DDB converter by a simple yet effective circuit modification and inherits all the advantages of the existing DDB converter. In addition, the input and output can share a common ground due to the circuit modification, enabling the use of low-side gate drivers for all switching devices. When compared with the existing DDB converter, the proposed converter requires only one more capacitor, and the seemingly two inductors can be integrated into a single inductor. Thus, the overall magnetic volume is the same as the conventional converter. To validate the performance of the proposed converter, a 2-kW prototype was fabricated and tested. [ABSTRACT FROM AUTHOR]

In this letter, an asymmetrical pulsewidth modulation (PWM) scheme is proposed to solve the output voltage unbalance and voltage gain issues of the input-parallel output-series (IPOS) converter when $D < 0.5$. With the proposed scheme, the natural output voltages balancing and high step-up conversion ratio can be extended when $D < 0.5$. As a result, combined with the conventional PWM scheme, the duty cycle limitation of the IPOS converter is completely eliminated without any extra components or dedicated control. A 2-kW prototype of the IPOS converter was built and tested to verify the performance of the proposed PWM scheme. [ABSTRACT FROM AUTHOR]

This article presents current-fed (CF) full-bridge (FB) isolated dc–dc converter using quasi-Z-source network. Owing to its inherent buck–boost and clamping function, the proposed converter can effectively eliminate start-up and switch voltage overshoot problems. In addition, when compared with the conventional CF-FB converter with active clamp, switch current stress is much reduced and less active switches are required. Operation principles of the proposed converter are analyzed in detail. The simulation and experimental results with 1-kW prototype are also provided to verify theoretical analysis. [ABSTRACT FROM AUTHOR]

This article presents a novel two-channel light emitting diode (LED) current balancing method using current-fed (CF) quasi-Z-source (qZS) dc–dc converter. The proposed LED driver is simple in structure and requires minimal number of component counts. Currents flowing through the two LED strings are automatically balanced owing to charge (amp–sec) balance condition on capacitors in the CF–qZS network. Since current balancing is achieved automatically by passive components, current control of the proposed converter is very simple. In addition, the proposed circuit uses only one active switch and one diode. The operating principle and characteristics of the proposed two-channel LED driver are analyzed in detail. To verify validity of the proposed converter, 80 W prototype is built and tested. [ABSTRACT FROM AUTHOR]

Conventional current source inverter (CSI) suffers from open-circuit problem during switch transition. Thus, overlap-time should be inserted in the gate signals for safe commutation. The overlap-time distorts output current waveforms and increases total harmonic distortion of output current. Moreover, it reduces the current source utilization of the inverter. To solve these problems, this article proposes a new three-phase four-leg switching-cell (SC) CSI, abbreviated as H8-SC-CSI. With the help of the SC structure, the reliability of the proposed inverter is greatly enhanced as open-circuit concerns are eliminated. In addition, with the intrinsic voltage clamping action of the SC structure, switch voltage overshoot at turn-off is mitigated. In the proposed CSI, the switch overlap-time can be minimized or eliminated because the inductor current can flow through the SC capacitors. A 1.2 kW prototype inverter is designed and tested to verify the feasibility of the circuit. [ABSTRACT FROM AUTHOR]

In this article, a current balancing coupled inductor (CBCL) based asymmetrically configured quad-active-bridge (QAB) converter is proposed. The QAB converter provides high-frequency isolation for a dc–dc stage of a solid state transformer (SST), as well as provides a modular solution to the SST medium-voltage (MV) dc link. The proposed CBCL ensures current balancing among/three active H-bridges without using any dedicated controller and without expensive sensors when transferring equal power from MV to low-voltage side. The proposed CBCL is the integration of three inductors of the active H-bridges required for power transfer and zero-voltage-switching and does not require an extra magnetic volume. Experimental results are presented to validate the performance. [ABSTRACT FROM AUTHOR]

In this article, a novel single-phase single-stage transformerless inverter is proposed for grid-tied and standalone applications. The proposed inverter has low leakage current and can perform both buck and boost functions. Furthermore, compared to the conventional state-of-the-art buck–boost inverters, it can achieve high power density because the magnetic volume is reduced significantly. In addition, high power conversion efficiency can be realized because only one switch operates at a high frequency. Moreover, there is no need for inserting high-frequency dead-time between the switches because only one switch receives high frequency and all other switches receive line frequency signals. In order to verify the performance of the proposed topology, a laboratory prototype is designed for 500 W, and the experimental results are provided as well. [ABSTRACT FROM AUTHOR]

In this letter, an asymmetrical pulsewidth modulated (PWM) scheme is proposed to solve the inductor current unbalance and nonlinear voltage gain issues of the series-capacitor (SC) high-conversion-ratio (HCR) converter when D > 0.5. With the proposed scheme, the converter can achieve both current balancing and high step-down conversion even for D > 0.5. As a result, combined with the conventional PWM scheme, the duty cycle limitation of the SC-HCR converter is completely eliminated. A 2-kW prototype SC-HCR converter was fabricated and tested to validate the performance of the proposed PWM scheme. [ABSTRACT FROM AUTHOR]

Unlike voltage-source inverters (VSI), the power decoupling element in current-source inverters (CSI) is inductor. Inductor has a long lifetime and makes the CSI more reliable. However, the traditional CSIs suffer from open-circuit problem and dangerous voltage spikes are generated if the input inductor current is intermitted. Therefore, to avoid the voltage spikes, overlap-time should be inserted in the gate signals. But these intervals distort the output currents and reduce the current source utilization. To address these issues, a novel three-phase CSI is proposed in this article. The proposed inverter is originated from the conventional seven-switch CSI named CSI7 by adding only two small film capacitors. The film capacitors along with the pre-existing diodes of the CSI7 ensure that there is always a path for the inductor current, and thus the open-circuit issue is resolved. The elimination of open-circuit issue enhances the reliability and allows to minimize the overlap-time. This will improve the quality of output currents as well. Moreover, simulation and experimental results are provided at the output power of 950 W to verify the effectiveness of the proposed inverter. [ABSTRACT FROM AUTHOR]

A self-current balancing structure of the dual-transformer-based triple-port active bridge (DT-TAB) converter with the reduced component count is proposed in this article for future sustainable energy system applications. The proposed topology inherits good characteristics of the conventional triple-port active bridge (TAB) with even power sharing in an open loop as a newly added function. It can achieve high power density over the existing TAB converters that possess current-balancing function because the need for an auxiliary magnetic circuit or sophisticated control scheme in equalizing the shared power is eliminated. Besides, the demagnetization and magnetic short-circuit problems associated with the conventional TAB converters that are been eradicated, decoupled power flow management is achieved at all conditions. Moreover, the phase shift between the input H bridges gives an additional control degree of freedom that permits the control of the output power or offers independent control of voltage magnitude during the reverse power flow, thereby extending the system-operating range. The proposed DT-TAB converter has been adapted for the two-port operation. All the power switches achieved zero-voltage-switching turn-on. Finally, the robust operation performances of the proposed DT-TAB converter have been verified by the laboratory prototype assembled and tested up to 1 kW. The experimental results presented at the end are the proof-of-concept. [ABSTRACT FROM AUTHOR]

The input-parallel output-parallel (IPOP) connected converter system allows the use of a low-power converter module for high-power applications. However, mismatches in the IPOP circuit parameters cause uneven current sharing between the constituent modules, which makes the IPOP system unreliable for practical use. Different techniques have been proposed so far; however, they cause an increase in magnetic volume, cost, component count, and complexity of the IPOP system. In this article, a reliable and efficient magnetic integration technique is proposed for the current sharing requirements of the IPOP system based on LLC resonant converter modules. The proposed integrated magnetic current balancing transformer (ICBT) based IPOP LLC resonant converter modules ensures both the input current sharing and output current sharing between constituent LLC modules, under the open-loop condition quite effectively. The proposed ICBT is the resonant inductors of LLC resonant converter modules, coupled to perform the function of current balancing as well as generates enough leakage inductances for resonance operation, simultaneously. Thus, it can reduce the overall magnetic volume and improves power density of the IPOP LLC modules. Moreover, the current sharing performances of the proposed ICBT in both the steady-state and dynamic-state are analyzed based on the magnetic and electrical model. To validate the performance of the proposed ICBT based IPOP two LLC modules, a 2.6-kW hardware prototype was designed, fabricated, and tested successfully. [ABSTRACT FROM AUTHOR]

In this article, a split-capacitor (SC) dual-active-bridge (DAB) converter is proposed. In the proposed converter, dc-link capacitors in both the input and output are split, and primary and secondary windings of the transformer are connected to midpoints of the input and output dc links, respectively. Although the switch current stress of the proposed converter is twice that of conventional DAB converter, its switch voltage stress is reduced to half. Therefore, the proposed converter has higher conduction loss than the conventional DAB converter, however, the switching loss can be reduced, which implies that the proposed converter is beneficial and can achieve higher efficiency than the conventional DAB converter as switching frequency increases. In addition, owing to the SC structure, no dc-blocking capacitors are required and the proposed converter can be readily connected to neutral-point-clamped or half-bridge-type converters without increasing the number of dc-link capacitors. To verify the performances of the proposed converter, 3-kW prototypes are built, tested, and compared. [ABSTRACT FROM AUTHOR]

This paper introduces a direct PWM split-phase boost ac-ac converter with a high voltage gain, natural output voltage balancing, no duty-ratio limitation, and no commutation problem. Although under unbalanced load conditions, the two output voltages still are equal without auxiliary circuits and/or any dedicated controller. Moreover, the input current ripple is significantly decreased due to the interleaving effect. With a proper asymmetrical PWM scheme, the converter can work in the entire range of the duty cycle. In addition, the commutation (dead-time) problem is eliminated. The operation of the proposed structure is validated with a 500 W prototype converter. Experiment results obtained under various load conditions such as inductive, nonlinear, extreme load, and load scenarios are also provided. [ABSTRACT FROM AUTHOR]

This article proposes a new single-phase three-level dual-buck inverter. The topology is derived from a simplified neutral-point clamped inverter that includes fewer active switches. The advantages of the proposed inverter are no shoot-through worries and no reverse recovery issue of the body diode of the power metal-oxide-semiconductor field-effect transistor compared with the conventional inverter; therefore, a high-reliability inverter can be achieved. The operation modes and pulsewidth modulation technique are analyzed in detail. Additionally, a switching scheme is proposed to balance and reduce the direct-current-link capacitance. A 1.4-kW prototype was built and tested to validate the theoretical analysis of the proposed converter. [ABSTRACT FROM AUTHOR]

In this paper, a novel six-switch dual-output inverter is proposed. The three-switch leg in the proposed inverter is implemented with the dual-buck structure. Hence, it retains all the benefits of the conventional dual-buck inverter such as improved reliability owing to the absence of short-circuit problems and higher efficiency with the use of power metal–oxide–semiconductor field-effect transistor (mosfet) without reverse recovery issues on its body diode. In addition, the passive components can be reduced with the high switching frequency operation. A 3.5-kW, 400-V dc input mosfet-based hardware prototype was built and tested to verify the performance of the proposed inverter and a maximum efficiency of 97% was achieved. [ABSTRACT FROM AUTHOR]

In this paper, dual-buck structured single-stage buck–boost inverters that use power MOSFETs to achieve high efficiency are presented. The proposed inverters require fewer switches, and half of the switches have reduced current stresses. They have no shoot-through problem; therefore, high system reliability can be obtained. The dead-time in PWM signals can be minimized or eliminated, which improving the quality of the output ac voltages and increasing the efficiency. In the proposed inverters, MOSFETs can be used without reverse-recovery issues of their body diodes to boost the efficiency and increase the switching frequency. To further improve the efficiency and stability of the proposed single-phase inverter, PWM strategies combining high- and low-frequency modulation are presented. A hardware prototype of the single-phase inverter was built and tested using resistive, inductive, and nonlinear loads. Experimental results for a 300-W prototype inverter show a 97.4% peak efficiency with a 25 kHz switching frequency. [ABSTRACT FROM AUTHOR]

A dual-buck inverter (DBI) possesses no shoot-through worries and can boost efficiency by using power mosfet s. The main drawback of a DBI is the use of more inductors than in a traditional full-bridge inverter. This greater number of inductors decreases the power density and increases system cost. In this paper, a modified cascaded full-bridge DBI is proposed. It retains all the advantages of the conventional cascaded full-bridge DBI, possesses no shoot-through issues, and can be operated with no pulse-width-modulation dead-times. It can be implemented with power mosfet without reverse recovery issues of a body diode to boost efficiency. The cascaded modules in the proposed inverter share current limiting inductors. Therefore, the number of inductors and inductor footprints, the complexity of the circuit layout, required inductance, and magnetic volume can all be decreased. To show the advantages of the proposed inverter, detailed theoretical analysis and experimental results are provided. [ABSTRACT FROM PUBLISHER]

In this paper, a new nonisolated high-voltage-gain boost converter is proposed. The proposed converter is constructed by adding an additional inductor to a conventional three-level-boost (TLB) converter. When compared with the conventional boost and TLB converters, the proposed converter can achieve higher voltage conversion ratio with reduced voltage and current stresses in the switches. In particular, the proposed converter can reduce the losses of active devices by reducing the current of the switch compared to conventional converters, thus, can achieve high efficiency. In addition, the proposed converter automatically balances the output voltages for an unbalanced load without the need for any additional control strategy or auxiliary circuit. A 2-kW prototype converter was built and tested to verify performances of the proposed converter. [ABSTRACT FROM PUBLISHER]

This paper proposes a new bidirectional direct pulse width modulation (PWM) three-phase buck-boost ac–ac converter. It requires six switches and three inductors and can provide wide range buck-boost voltage operation. It has high reliability due to no short-circuit and open-circuit risks in the converter. Therefore, it solves the commutation problem without using lossy snubber circuits, soft commutation strategies, and coupled inductors. It can be designed with metal–oxide–semiconductor field-effect transistors (MOSFETs) without their body diodes conducting, which eliminate reverse recovery issues and losses of body diodes. In addition, to reduce the ripples in input and output voltages and currents, the proposed converter can be operated with a PWM strategy-II. The PWM strategy-II is a type of interleaved PWM strategy with dead- and overlap-times operating modes. To validate the effectiveness of the proposed converter extensive analysis followed by detailed experimental results are provided. [ABSTRACT FROM PUBLISHER]

In this paper, a class of single-phase Z-source (ZS) ac–ac converters is proposed with high-frequency transformer (HFT) isolation. The proposed HFT isolated (HFTI) ZS ac–ac converters possess all the features of their nonisolated counterparts, such as providing wide range of buck-boost output voltage with reversing or maintaining the phase angle, suppressing the in-rush and harmonic currents, and improved reliability. In addition, the proposed converters incorporate HFT for electrical isolation and safety, and therefore can save an external bulky line frequency transformer, for applications such as dynamic voltage restorers, etc. The proposed HFTI ZS converters are obtained from conventional (nonisolated) ZS ac–ac converters by adding only one extra bidirectional switch, and replacing two inductors with an HFT, thus saving one magnetic core. The switching signals for buck and boost modes are presented with safe-commutation strategy to remove the switch voltage spikes. A quasi-ZS-based HFTI ac–ac is used to discuss the operation principle and circuit analysis of the proposed class of HFTI ZS ac–ac converters. Various ZS-based HFTI proposed ac–ac converters are also presented thereafter. Moreover, a laboratory prototype of the proposed converter is constructed and experiments are conducted to produce output voltage of 110 Vrms / 60 Hz, which verify the operation of the proposed converters. [ABSTRACT FROM AUTHOR]

In this letter, a modified series-capacitor (SC) high conversion ratio (HCR) dc–dc converter is proposed. The proposed converter inherits most of the advantages of the existing SC HCR dc–dc converter and can eliminate the problem of high switch-voltage stress of the existing converter encountered during start-up using a simple circuit modification. Although an extra capacitor is required in the proposed converter, total series capacitance of the proposed converter is the same as that of the conventional SC converter. In addition, input capacitance of the proposed converter can be reduced by less than half of that of the SC buck converter. A 1-kW prototype converter is built and tested to verify the performance of the proposed converter. [ABSTRACT FROM PUBLISHER]

In this paper, a new type of matrix converter (MC) also called a single-phase high-frequency transformer isolated (HFTI) buck MC is proposed. The proposed converter can provide step-down operation of the input voltage with various types of output voltages such as in-phase and out-of-phase output voltages, rectified (or positive) output voltage, and output voltage with step-changed frequency. By incorporating HFT isolation, the proposed MC saves an extra bulky line frequency transformer, which is required for the conventional MCs to provide electrical isolation and safety, when used in application such as dynamic voltage restorers, etc. Two different circuit variations of the proposed HFTI MC are presented with and without continuous output currents, with the latter having less passive components. The safe-commutation strategy is also employed for the proposed HFTI MC to provide current path for the inductor during dead-time, which avoids switch voltage spikes without adding any snubber circuits. The operation principle and circuit analysis of the proposed MC are presented, and switching strategies are also developed to obtain various output voltages. Moreover, a prototype of the proposed MC is fabricated, and experiments are performed to produce in-phase/out-of-phase and rectified output voltages, and output voltage with step-changed frequency. [ABSTRACT FROM PUBLISHER]

This paper presents a new single-phase switched-coupled-inductor dc–ac inverter featuring higher voltage gain than the existing single-phase qZ-source and semi-Z-source inverters. Similar to the single-phase qZ-source and semi-Z-source inverters, the proposed inverter also has common grounds between the dc input and ac output voltages, which is beneficial especially for photovoltaic inverter systems. The inverter volume and maximum current flowing can be reduced significantly through the coupling of all inductors. A theoretical analysis of the proposed inverter is described and a 280-W experimental prototype is built to verify the performance of the inverter. [ABSTRACT FROM AUTHOR]

To regulate an output ac voltage in inverter systems having wide input dc voltage variation, a buck–boost power conditioning system is preferred. This paper proposes a novel high-efficiency quasi single-stage single-phase buck–boost inverter. The proposed inverter can solve current shoot-through problem and eliminate PWM dead time, which leads to greatly enhanced system reliability. It allows bidirectional power flow and can use MOSFET as switching device without body diode conducting. The reverse recovery issues and related loss of the MOSFET body diode can be eliminated. The use of MOSFET contributes to the reduction of switching and conduction losses. Also, the proposed inverter can be operated with simple pulse width modulation (PWM) control and can be designed at higher switching frequency to reduce the volume of passive components. The detailed experimental results are provided to show the advantages of the proposed inverter. Efficiency measurement shows that using simple PWM control the proposed inverter can obtain peak efficiency of 97.8% for 1.1-kW output power at 30-kHz switching frequency. [ABSTRACT FROM AUTHOR]

In this paper, an isolated double step-down dc–dc converter with high efficiency and high step-down function is proposed. The proposed converter employs an additional capacitor in the primary side. Compared to the conventional full-bridge converters, the proposed converter has a double step-down feature with reduced voltage stress at the primary side of the transformer. Moreover, voltage stress of three primary-side switches reduces to half of the input voltage and zero voltage switching (ZVS) is naturally achieved for all switches with lower output capacitor energy of the switches. Therefore, the proposed converter requires smaller leakage inductance than the conventional full-bridge converter. Without adding complexity to the hardware and control, the proposed converter inherently prevents transformer saturation problem caused by the dc component of the transformer. A 3-kW experimental prototype is constructed to verify the performance of the proposed converter. [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]

In this paper, the switching cell concept is extended to isolated ac–ac converters, and a highly reliable double step-down ac–ac converter is proposed with high-frequency transformer (HFT) isolation. By using the switching cell structure and coupled inductors, the proposed converter has no commutation problem as it is immune from both short-circuit and open-circuit problems, even when all the switches are turned on or turned off, simultaneously. Therefore, it does not require pulse width modulation dead times along with bulky and lossy RC snubbers or voltage-sensing circuitry to implement soft-commutation strategies, resulting in high reliability and high quality output waveforms. The HFT in the proposed converter provides electrical isolation and safety which is required in applications such as dynamic voltage restorer (DVR), solid-state transformer (SST), etc., without the need for external bulky line frequency transformer. Moreover, all the passive components experience twice the switching frequency; therefore, their size can be reduced. The proposed converter is very suitable for application as DVR, to compensate both voltage sags and swells, owing to its ability to provide both inverting and noninverting outputs. A detailed theoretical analysis and operation of the proposed ac–ac converter are provided, and its applications as DVR and SST are also discussed. Experimental results with scaled-down prototype are also provided to verify its performance. [ABSTRACT FROM AUTHOR]

This paper extends the high-frequency transformer isolation concept to the Z-source (ZS) ac–ac converters and introduces a new family of high-frequency transformer isolated (HFTI) ZS ac–ac converters. The proposed HFTI-ZS converters retain all the benefits of their existing nonisolated counterparts, such as providing a larger range of output voltage with buck–boost functionality, reversing or maintaining the phase angle, reducing the in-rush and harmonic currents, and improving reliability. In addition to these benefits, the high-frequency (20 kHz) transformer (HFT) in the proposed ZS ac–ac converters provides the electrical isolation and safety with high power density as it eliminates the need for bulky and heavy line frequency (50 or 60 Hz) transformer for galvanic isolation, in applications such as dynamic voltage restorers, etc. The dc-blocking capacitor added in series with the HFT results in only ac voltage applied across its windings, which avoids its saturation. Various ZS-based HFTI ac–ac converters are proposed in this paper, and to verify their operation and advantages, example of quasi-ZS (qZS)-based isolated ac–ac converter is considered in detail. The commutation strategy is also developed to achieve the safe commutation, which avoids the current and voltage spikes without using any RC snubber. A 200-W laboratory prototype of HFTI-qZS ac–ac converter is fabricated and experiments are performed to validate the advantages of the proposed ac–ac converters. [ABSTRACT FROM PUBLISHER]

The Z-source inverters (ZSIs) have many advantages over the conventional voltage-fed and current-fed inverters, and extensive research works are conducing recently. However, relatively difficult physical layouts of the Z-source networks make them less attractive in the industry applications. A good hardware design is important to handle with the inherent long high-frequency loop in order to avoid generating severe voltage overshoot in the dc-rail voltage of the ZSIs. In this paper, the high-frequency loops in various ZSIs are investigated. A dc-rail voltage clamping technique for magnetically coupled impedance source converters is proposed by using an intuitive circuit modification and addition of a clamping diode. The proposed technique is verified through simulation and experiment. [ABSTRACT FROM PUBLISHER]

A single-phase inverting buck–boost ac–ac converter is proposed in this paper. The proposed converter has no shoot-through and dead-time problems and, like conventional dc–dc converters, it can be operated with simple pulse width modulation (PWM) control. It offers high frequency and high efficiency operation because high speed mosfet can be used as switching device without the reverse recovery issues and losses of its body diode. The proposed converter features quasi-continuous input and output currents, high input power factor, low total harmonic distortion of input and output currents, and high efficiency. A 460 W laboratory prototype was constructed and tested with interleaved and complementary PWM controls using a resistive and inductive load. Detailed experimental results are provided to show the novelty of the proposed converter. Experimental results confirmed that the proposed converter can obtain 93.8% efficiency at 60 kHz switching frequency. [ABSTRACT FROM PUBLISHER]

The switching cell (SC) direct pulse width modulation ac–ac converters have several advantages; in particular, they do not experience commutation issues, are not susceptible to reverse-recovery losses of body diode of switching devices, produce output waveforms of good quality, are more immune to EMI noise, and have smaller input/output current ripples. The magnetic components of the SC ac–ac converters consist of coupled inductors and a filter inductor. This paper proposes an integrated inductor which integrates all magnetic components of the SC ac–ac converters into one magnetic assembly. The leakage inductance of the proposed integrated inductor is utilized to eliminate the filter inductor in buck, boost, and buck—boost-type SC ac–ac converters. The proposed integrated inductor improves the power density and reduces cost and losses because it requires fewer magnetic cores, coils, and soldering connections. The proposed integrated inductor is especially suitable for high-current applications because it can avoid the magnetic core saturation caused by current, and can eliminate the air gap in the core. The proposed integrated inductor can realize high leakage inductance; therefore, the input and output current ripples can be reduced significantly. To verify the proposed integrated inductor, a 2.3-kW prototype inductor was built and tested. [ABSTRACT FROM AUTHOR]

This paper proposes single-phase direct pulsewidth modulation (PWM) buck-, boost-, and buck–boost-type ac–ac converters. The proposed converters are implemented with a series-connected freewheeling diode and MOSFET pair, which allows to minimize the switching and conduction losses of the semiconductor devices and resolves the reverse-recovery problem of body diode of MOSFET. The proposed converters are highly reliable because they can solve the shoot-through and dead-time problems of traditional ac–ac converters without voltage/current sensing module, lossy resistor–capacitor (RC) snubbers, or bulky coupled inductors. In addition, they can achieve high obtainable voltage gain and also produce output voltage waveforms of good quality because they do not use lossy snubbers. Unlike the recently developed switching cell (SC) ac–ac converters, the proposed ac–ac converters have no circulating current and do not require bulky coupled inductors; therefore, the total losses, current stresses, and magnetic volume are reduced and efficiency is improved. Detailed analysis and experimental results are provided to validate the novelty and merit of the proposed converters. [ABSTRACT FROM AUTHOR]

This paper proposes a new type of ac–ac converter which can operate as traditional noninverting buck and boost converters, and inverting buck–boost converter as well. The proposed converter uses six unidirectional current flowing and bidirectional voltage blocking switches, implemented by six reverse blocking insulated-gate bipolar transistors (IGBTs) or series mosfet–diode pairs, two input and output filter capacitors, and one inductor. It has no shoot-through problem of voltage source (or capacitor) even when all switches are turned-on, and therefore, pulsewidth modulation dead-times are not needed resulting in high-quality waveforms, and solves the commutation problem without using bulky and lossy RC snubbers or dedicated soft-commutation strategies. It has smaller switching losses because only two switches out of six are switched at high frequency during each half-cycle of input voltage, and it can use power MOSFETs as body diode never conducts, making it immune from mosfet failure risk which otherwise may occur due to poor reverse recovery problem of body diode. The noninverting buck–boost mode of the proposed converter can be used in applications with both step-up and step-down demand, while the inverting buck–boost mode can also be used in its application as dynamic voltage restorer. Detailed analysis of the proposed converter is given, and experimental results are also provided. [ABSTRACT FROM AUTHOR]

Recently, novel direct pulse width modulated (PWM) ac–ac converters using coupled inductors with switching cell structure have been introduced as an alternative to the conventional ac–ac converter topology. The converters can effectively solve the commutation problem associated with the conventional direct PWM ac–ac converters. However, the two coupled inductors used in the topology make the converters less attractive for use in practical applications. This paper proposes magnetic integration of the discrete-coupled inductors in single-phase ac–ac converters. The dc offset fluxes generated by the common-mode currents are cancelled out perfectly when the windings on the common core are appropriately arranged, resulting in ac flux only. The proposed inductor shows significant improvements over discrete-coupled inductors, in that the proposed inductor does not have an air gap, it exhibits symmetrical winding currents, and it has smaller volume with large current handling capability. The proposed inductor can handle an unlimited amount of current due to the absence of core saturation caused by the current and is well suitable for high power ac–ac conversion applications. The performance evaluation and comparison of the experimental results show that the net volume of the coupled magnetic components can be reduced by more than 50% with the proposed integration scheme. [ABSTRACT FROM PUBLISHER]

Z-source inverters have become a research hotspot because of their single-stage buck–boost inversion ability, and better immunity to EMI noises. However, their boost gains are limited, because of higher component-voltage stresses and poor output power quality, which results from the tradeoff between the shoot-through interval and the modulation index. To overcome these drawbacks, a new high-voltage boost impedance-source inverter called a switched-coupled-inductor quasi-Z-source inverter (SCL-qZSI) is proposed, which integrates a switched-capacitor and a three-winding switched-coupled inductor (SCL) into a conventional qZSI. The proposed SCL-qZSI adds only one capacitor and two diodes to a classical qZSI, and even with a turns ratio of 1, it has a stronger voltage boost-inversion ability than existing high-voltage boost (q)ZSI topologies. Therefore, compared with other (q)ZSIs for the same input and output voltages, the proposed SCL-qZSI utilizes higher modulation index with lower component-voltage stresses, has better spectral performance, and has a lower input inductor current ripple and flux density swing or, alternately, it can reduce the number of turns or size of the input inductor. The size of the coupled inductor and the total number of turns required for three windings are comparable to those of a single inductor in (q)ZSIs. To validate its advantages, analytical, simulation, and experimental results are also presented. [ABSTRACT FROM AUTHOR]

In this paper, a family of three-phase direct pulse-width modulated (PWM) ac–ac converters consisting of buck, boost, and buck–boost converters is proposed. The proposed converters have no commutation problem even if all of the switches are turned on or off simultaneously. They do not use lossy snubbers and do not sense the voltage polarity for commutation, and produce high-quality output voltage waveforms. The proposed converters allow the use of power MOSFETs and fast recovery freewheeling diodes independently. The use of power MOSFETs as active switches and freewheeling diodes with extremely fast recovery features lower the switching losses and enable us to reduce the volume of passive components by increasing switching frequency. The input (or output) filter inductor experiences twice of the converter switching frequency and thus can be designed with minimum size. To increase power density of the proposed converters, a three-phase three-limb coupled inductor is proposed. Three-phase coupled inductor integrates three separate coupled inductors of the proposed converters in one three-limb core. In comparison with separate coupled inductors, the three-phase coupled inductor has a smaller size with large current-handling capability. Experimental results obtained for the boost-type converter show the robustness of the proposed three-phase ac–ac converters. [ABSTRACT FROM AUTHOR]

High-power-density bidirectional dc–dc converters consisting of buck-and-boost converters are widely used in hybrid electric vehicles due to their smaller size and low cost. With a conventional constant-value inductor, the light-load efficiency of these converters is greatly reduced because of the large constant peak-to-peak current swing and relatively constant conduction losses that result in the entire load range. This paper proposes a new type of nonlinear inductor using magnetorheological (MR) fluid to improve the intermediate- and light-load efficiencies of these converters. The use of the proposed inductor does not require any additional circuit or complex control, and yet, it has benefits of decreasing conduction losses and improving efficiency due to its increasing inductance when the load is changed from heavy to light. A 3.7-kW prototype converter is built, and the efficiency from heavy to light load is measured separately using the proposed MR fluid-gap inductor and conventional air-gap inductor, which validates the advantages of the proposed MR fluid-gap inductor. [ABSTRACT FROM PUBLISHER]

This paper presents novel single-phase pulse width modulation (PWM) ac-ac converters that can solve the commutation problem in single-phase direct PWM ac-ac converters without sensing the input voltage polarity. By using a basic switching cell structure and coupled inductors, the proposed ac-ac converters can be short- and open-circuited without damaging the switching devices. Neither lossy RC snubber nor dedicated soft commutation strategy is required in the proposed converter. By replacing the conventional phase-leg of the PWM ac-ac converters with the switching cell structure and the coupled inductor, three novel buck, boost, and buck-boost type PWM ac-ac converters are developed. Although two coupled inductors are required for the proposed converter, the input inductor of the proposed converter can be much smaller than that of the conventional PWM ac-ac converters. The volume of the magnetic components can be further reduced by increasing switching frequency of the converter because very fast recovery diodes can be selected externally. In order to verify performance and robustness of the proposed converter, a 200-W boost type prototype converter was built and tested with both mismatched gate signals and highly distorted input voltage. [ABSTRACT FROM AUTHOR]

This study proposes a novel balanced two-phase interleaved LLC resonant converter. The LLC resonant converter offers a promising solution for dc power conversion. This converter can achieve high efficiency and high-power density by implementing zero-voltage-switching (ZVS) for primary switches and zero-current-switching for secondary rectifier diodes. The input-parallel output-parallel systems that increase their power ratings by connecting two converters in parallel allow the use of low-current rated switches in high-current applications. Moreover, the input and output passive filter size requirements in the two-phase interleaved converters are reduced, and they provide higher power density and improved dynamic performance. However, the estimated tank gain characteristics become imbalanced due to the tolerances and the inevitable mismatch of the resonant parameters. Accordingly, this system operates outside the ZVS region, and a severe current imbalance with thermal runaway will occur. The proposed method achieves the perfect balance of input, output, and tank currents utilizing the existing resonant inductor magnetics without additional active or passive components, expensive sensors, and dedicated control schemes. The proposed method retains the advantages of the conventional LLC resonant converter over the operating frequency range. Mathematical analysis and experimental results are presented to validate the performance of the proposed method. [ABSTRACT FROM AUTHOR]

This paper proposes a new variable inductor for efficient power conversion which consists of the ferrite core and the gap filled with a magnetorheological fluid. Its main feature is to show a large inductance variation versus inductor currents even with a simple inductor structure and so it leads to improving light and intermediate load efficiency of power conversion systems compared with the conventional inductor with an air gap. To predict the characteristics of the variable inductor, the nonlinear magnetic permeability of the magnetorheological fluid used is accurately measured because the inductance value strongly depends on the magnetic nonlinearity of the fluid. With the obtained B-H curve, a 1 kVA variable inductor is designed and analyzed with three-dimensional finite element method taking into account the losses of the core and the fluid. Finally, to prove the feasibility and usefulness of the proposed inductor, a prototype inductor is fabricated and its experimental results are compared with numerical ones. [ABSTRACT FROM PUBLISHER]

This paper presents a novel dc–dc converter incorporating a distributed impedance-source network ( Z-source network, or Z-network) to achieve the buck (step-down) and boost (step-up) function of a transformer-isolated dc–dc converter. In this paper, the distributed impedance source network composed of an array of inductors and capacitors is coupled between power source and main switching devices. The great and unique feature about the distributed impedance source network dc–dc converter is that unlike the traditional V-source or I-source converters, it can be open- and short-circuited without damaging switching devices. Therefore, the desired buck and boost function can be achieved. Moreover, converter reliability can be greatly improved. A dc–dc converter using this proposed concept is built and tested. Its performances are verified with experimental results. [ABSTRACT FROM PUBLISHER]

This paper presents the design and development of a transformer isolated high-power dc-dc converter. There are several design issues and challenges. In this paper, we focus on three major issues in high-power dc-dc converters. 1) An energy recovery passive snubber circuit and the selection of pulsewidth modulation control methods are first introduced to reduce the voltage spike and power loss across switching devices. 2) Interleaved winding method is then employed to reduce proximity effect in the transformer winding. 3) The voltage oscillation problem in the secondary rectifier diode is addressed, and solutions are provided. [ABSTRACT FROM AUTHOR]

This paper presents a dynamic voltage restorer to solve the voltage sag problem that causes the most serious economic loss among various grid accidents. The proposed method, which uses a direct AC–AC converter, consists of a simple controller unlike methods using a voltage source inverter that is controlled using complex operations. Unlike the energy storage type, there is no additional energy storage device, which reduces system cost. In addition, unlike the back-to-back type, energy conversion loss is reduced by one stage conversion. A switching cell-structured direct AC–AC converter that solves the rectification problem without a separate sensor is a reliable structure that reduces the volumes of the passive filters by increasing the switching frequency. In addition, using interphase voltages, it can take a wide range of voltage compensations to compensate for most of the voltage sag in various situations. In this paper, the background of the study is revealed and the operating principles and control schemes of the proposed system are analyzed. Then, the validity of this study is confirmed through simulation and experiment results. [ABSTRACT FROM AUTHOR]