Showing papers on "Converters published in 2005"

A comparison of three-level converters versus two-level converters for low-voltage drives, traction, and utility applications

Abstract: This paper evaluates three-level topologies as alternatives to two-level topologies in converters for low-voltage applications. Topologies, semiconductor losses, filter aspects, part count, initial cost, and life-cycle cost are compared for a grid interface, a conventional drive application, and a high-speed drive application.

Active-neutral-point-clamped (ANPC) multilevel converter technology

Abstract: This paper proposes a multilevel power conversion concept based on the combination of neutral-point-clamped (NPC) and floating capacitor converters In the proposed scheme, the voltage balancing across the floating capacitors is achieved by using a proper selection of redundant switching states, and the neutral-point voltage is controlled by the classical dc offset injection Experimental results are illustrated in the paper to demonstrate the system operation

Design and comparison of 4.16 kV neutral point clamped, flying capacitor and series connected H-bridge multi-level converters

Abstract: This paper describes the design of a (4-kV, 4.16-MVA) three-level neutral-point-clamped-, three-level flying-capacitor-, four-level flying-capacitor-, and nine-level seriesconnected H-bridge voltage-source converter on the basis of state-of-the-art 6.5-, 4.5-, 3.3- and 1.7-kV insulated gate bipolar transistors. The semiconductor loss distribution and the design of semiconductors and passive components are compared for a medium switching frequency assuming a constant converter efficiency of about 99%. To evaluate the converter characteristics in high switching frequency applications, a second comparison is realized for the maximum switching frequencies assuming a constant expense of semiconductors in all converters.

Active Neutral-Point-Clamped Multilevel Converters

Abstract: This paper proposes a multilevel power conversion concept based on the combination of neutral-point-clamped (NPC) and floating capacitor converters. In the proposed scheme, the voltage balancing across the floating capacitors is achieved by using a proper selection of redundant switching states, and the neutral-point voltage is controlled by the classical dc offset injection. Experimental results are illustrated in the paper to demonstrate the system operation

Boost DC-AC inverter: a new control strategy

Abstract: Boost dc-ac inverter naturally generates in a single stage an ac voltage whose peak value can be lower or greater than the dc input voltage. The main drawback of this structure deals with its control. Boost inverter consists of Boost dc-dc converters that have to be controlled in a variable-operation point condition. The sliding mode control has been proposed as an option. However, it does not directly control the inductance averaged-current. This paper proposes a control strategy for the Boost inverter in which each Boost is controlled by means of a double-loop regulation scheme that consists of a new inductor current control inner loop and an also new output voltage control outer loop. These loops include compensations in order to cope with the Boost variable operation point condition and to achieve a high robustness to both input voltage and output current disturbances. As shown by simulation and prototype experimental results, the proposed control strategy achieves a very high reliable performance, even in difficult transient situations such as nonlinear loads, abrupt load changes, short circuits, etc., which sliding mode control cannot cope with.

System identification of power converters with digital control through cross-correlation methods

Abstract: For digitally controlled switching power converters, on-line system identification can be used to assess the system dynamic responses and stability margins. This paper presents a modified correlation method for system identification of power converters with digital control. By injecting a multiperiod pseudo random binary signal (PRBS) to the control input of a power converter, the system frequency response can be derived by cross-correlation of the input signal and the sensed output signal. Compared to the conventional cross-correlation method, averaging the cross-correlation over multiple periods of the injected PRBS can significantly improve the identification results in the presence of PRBS-induced artifacts, switching and quantization noises. An experimental digitally controlled forward converter with an FPGA-based controller is used to demonstrate accurate and effective identification of the converter control-to-output response.

Voltage-balance limits in four-level diode-clamped converters with passive front ends

Abstract: Multilevel diode-clamped converters with more than three levels cannot maintain voltage balance in the dc-link capacitors for some operating conditions due to the existence of dc currents in the middle points. Since capacitors are either completely charged or discharged for those conditions, this circumstance severely limits practical application of these converters. The limit explored in this paper is that the four-level converter cannot achieve voltage balance. Proper redundant vectors are selected in the space-vector diagram so that a quadratic parameter related to the currents in the middle points is minimized.

Design and analysis of switched-capacitor-based step-up resonant converters

Abstract: A switched-capacitor-based step-up resonant converter is proposed. The voltage conversion of the converters is in step-up mode. By adding a different number of switched-capacitor cells, different output voltage conversion ratios can be obtained. The voltage conversion ratio from 2 to any whole number can therefore be generated by these switching-capacitor techniques. A resonant tank is used to assist in zero-current switching hence the current spike, which usually exists for classical switched-capacitor can be eliminated. Both high-frequency operations and high efficiency are possible. Generalized analysis and design method of the converters are also presented. Experimental results verified the theoretical analysis.

Power-Switching Converters

Abstract: From the Publisher: This hands-on reference/text provides in-depth practical coverage of switching converters - from the fundamental principles to the next generation of programmable devices - examining buck, boost, buck-boost, Cuk, quasi-resonant, loaded-resonant, and isolated configurations. Elucidating and comparing the advantages of switching converters to conventional converters, Power-Switching Converters describes the basic topologies of switching converters ... presents steady-state analyses for both continuous and discontinuous modes of operation ... discusses pulse-width modulation ... explains classic commercial integrated-circuit controllers for switching converters ... addresses stability considerations that arise in the design of switching converter controllers ... emphasizes the dynamic analysis of switching converters based on state-space averaging and linearization ... introduces PSPICE switching converter simulations ... illustrates the switching behavior of power semiconductor devices ... and more.

Optimal Modulation of Flying Capacitor and Stacked Multicell Converters using a State Machine Decoder

Abstract: Modulation of flying capacitor and stacked multicell converters (SMC) is complicated by the fact that these converters have redundant states that achieve the same phase leg voltage output. Hence a modulator must use some secondary criteria such as cell voltage balancing to fully define the converter switched state. Alternatively, the modulator can be adapted to directly specify the cell states, such as has been proposed for the harmonically optimal phase disposition (PD) strategy. However the techniques reported to date can lead to uneven distribution of switching transitions between cells, and the synthesis of narrow switched phase leg pulses. This paper presents an improved strategy that decouples the tasks of voltage level selection and switching event distribution. Conventional PD and CSVPWM strategies are used to define the target voltage level for the converter, and a finite state machine is then used to distribute the transitions to the converter cells in a cyclical fashion. Experimental results for a four level flying capacitor inverter are presented, verifying that the natural balancing properties of this converter has been preserved, the cell switching utilization is equal and the expected harmonic gains of PD and CSVPWM compared to phase shifted carrier PWM have been achieved

Power loss-oriented evaluation of high voltage IGBTs and multilevel converters in transformerless traction applications

Abstract: The low-frequency line transformer in todays ac rail vehicles suffers from poor efficiency and a substantial weight. Future traction drives may operate directly from the mains without this transformer. A feasible concept for a transformerless drive system consists of series connected medium voltage converters applying modern high-voltage insulated gate bipolar transistors (HV-IGBTs). In a first design step, the switching characteristics and losses of 6.5-kV IGBTs are compared to 3.3-kV and 4.5-kV IGBTs which are already commercially used in traction applications. Based on the considered HV-IGBTs, the properties of multilevel converters are analyzed and their applicability to the transformerless system is evaluated. The paper focusses on a loss analysis of the converters. Reliability aspects and harmonic spectra are briefly discussed. Taking these design aspects into account, the three-level neutral point clamped converter turns out to be a reasonable solution to realize line and motor converter modules in a transformerless traction system.

Digitally controlled boost power-factor-correction converters operating in both continuous and discontinuous conduction mode

Abstract: Whereas power-factor-correction (PFC) converters for low-power ranges (less than 250 W) are commonly designed for operation in the discontinuous conduction mode, converters for higher power levels are operated in the continuous conduction mode. Nevertheless, when these converters are operated at reduced power, discontinuous conduction mode will appear during parts of the line period, yielding input current distortion. This distortion can be eliminated by employing a dedicated control algorithm, consisting of sample correction and duty-ratio feedforward. The reduction of the harmonic distortion of the input current and the increase of the power factor are demonstrated by experiments on a 1-kW boost PFC converter.

New time-discrete modulation scheme for matrix converters

Abstract: While the known modulation strategies for matrix converters are based on pulsewidth modulation (PWM)-or vector modulation-this paper presents a novel time-discrete modulation method based on real-time prediction calculation to select the switching states. The decision about which switching state is to be set for the following sampling period is made by the use of a predictive quality function. Using this approach, unity displacement factor is seen at the supply side with minimum line current distortion while the load currents follow their reference values with good accuracy. The quality function is derived from a mathematical model of the matrix converter and the controlled system. Measurements taken on a model plant, consisting of a matrix converter and a standard induction machine with a rated output power of 11 kW, show that the matrix converter, equipped with the control method presented here, offers advantages over systems with conventional frequency converters, especially in terms of the input current distortion.

Single-phase Z-source PWM AC-AC converters

Abstract: The letter proposes a new family of simple topologies of single-phase PWM ac-ac converters with a minimal number of switches: voltage-fed Z-source converter and current-fed Z-source converter. By PWM duty-ratio control, they become "solid-state transformers" with a continuously variable turns ratio. All the proposed ac-ac converters in this paper employ only two switches. Compared to the existing PWM ac-ac converter circuits, they have unique features: providing a larger range of output ac voltage with buck-boost, reversing or maintaining phase angle, reducing in-rush and harmonic current, and improving reliability. The operating principle and control method of the proposed topologies are presented. Analysis, simulation, and experimental results are given using the voltage-fed Z-source ac-ac converter as an example. The analysis can be easily extended to other converters of the proposed family. The proposed converters could be used in voltage regulation, power regulation, and so on.

Modeling the closed-current loop of PWM boost DC-DC converters operating in CCM with peak current-mode control

Abstract: This paper derives the transfer function from error voltage to duty cycle, which captures the quasi-digital behavior of the closed-current loop for pulsewidth modulated (PWM) dc-dc converters operating in continuous-conduction mode (CCM) using peak current-mode (PCM) control, the current-loop gain, the transfer function from control voltage to duty cycle (closed-current loop transfer function), and presents experimental verification. The sample-and-hold effect, or quasi-digital (discrete) behavior in the current loop with constant-frequency PCM in PWM dc-dc converters is described in a manner consistent with the physical behavior of the circuit. Using control theory, a transfer function from the error voltage to the duty cycle that captures the quasi-digital behavior is derived. This transfer function has a pole that can be in either the left-half plane or right-half plane, and captures the sample-and-hold effect accurately, enabling the characterization of the current-loop gain and closed-current loop for PWM dc-dc converters with PCM. The theoretical and experimental response results were in excellent agreement, confirming the validity of the transfer functions derived. The closed-current loop characterization can be used for the design of a controller for the outer voltage loop.

A New Digital Control Algorithm to Achieve Optimal Dynamic Performance in DC-to-DC Converters

Abstract: In this paper, a new optimal control algorithm is proposed to achieve the best possible dynamic performance for DC-to-DC converters under load changes and input voltage changes. Using the concept of capacitor charge balance, the proposed algorithm predicts the optimal transient response for a DC-to-DC converter during the large signal load current change, or input voltage change. The equations used to calculate the optimized transient time and the optimized duty cycle series are presented. By using the proposed algorithm, the best possible transient performance, including the smallest output voltage overshoot/undershoot and the shortest recovery time, is achieved. In addition, since the large signal dynamic response of power converters is successfully predicted, the large signal stability is guaranteed. Experimental results show that the proposed method produces much better dynamic performance than a conventional current mode PID controller

Multi-input bidirectional DC-DC converter combining DC-link and magnetic-coupling for fuel cell systems

Abstract: This paper presents a new multi-input bidirectional dc-dc converter which connects a fuel cell, storage and load by a combination of a dc-link and magnetic-coupling. A boost-dual-half-bridge and a bidirectional direct-connected switching cell are used. The topology is simple and only needs six power switches. The load and the sources are galvanically isolated. Furthermore, the proposed converter draws/injects smooth current from the fuel cell and the supercapacitor. The system is suitable for medium-power applications where simple topology, autonomous operation, compact packaging, and low cost are required. Different control schemes to manage power flow are proposed and compared. Simulation and experimental results that verify the effectiveness of the topology and its control scheme are presented. Moreover, it is shown that the idea of combining the dc-link and magnetic-coupling can be further developed to a general topology for multi-port bidirectional dc-dc converters.

A DQ Synchronous Reference Frame Current Control for Single-Phase Converters

Abstract: This paper presents a current control using the dq synchronous reference frame for single-phase converters. This control method consists in transforming an orthogonal pair composed by the actual single-phase input current and a fictitious current, from a stationary to a rotating frame. The steady state current components in dq frame become DC instead of AC values so a zero error current control can be implemented. A single-phase PFC boost rectifier is used as an example application of this control. To validate the control method simulation and experimental results are presented

A family of high power density unregulated bus converters

Abstract: This paper begins by reviewing current bus converters and exploring their limitations. Next, a family of inductor-less bus converters is proposed to overcome the limitations. In the new bus converters, magnetizing current is used to achieve zero-voltage-switching (ZVS) turn-on for all switches. The resonant concept is used to achieve nearly zero-current-switching (ZCS) turn-off for the primary switches and no body diode loss for the synchronous rectifiers (SRs). Meanwhile, the self-driven method can be easily applied to save drive loss of SRs. Based on these concepts, a full-bridge bus converter is built in the quarter-brick size to verify the analysis. The experimental results indicate that it can achieve 95.5% efficiency at 500-W, 12-V/45-A output. Compared with industry products, this topology can dramatically increase the power density. These concepts are also applied to nonisolated dc/dc converters. As an example, a resonant Buck converter is proposed and experimentally demonstrated.

Application of a sinusoidal internal model to current control of three-phase utility-interface converters

Abstract: Three-phase voltage-source converters are used as utility interfaces. In such a case, the converter line currents are required to track sinusoidal references synchronized with the utility grid without a steady-state error. In this paper a current control method based on a sinusoidal internal model is employed. The method uses a sine transfer function with a specified resonant frequency, which is called an S regulator. The combination of a conventional proportional-integral (PI) regulator and an S regulator is called a PIS regulator. The PIS regulator ensures that the steady-state error in response to any step changes in a reference signal at the resonant frequency and 0 Hz reduces to zero. An experiment was carried out using a 1-kVA prototype of three utility-interface converters, a voltage-source rectifier, an active power filter, and static synchronous compensator. Almost perfect current-tracking performance could be observed.

Flyboost power factor correction cell and a new family of single-stage AC/DC converters

Abstract: A novel power factor correction (PFC) cell, called flyboost, is presented. The proposed PFC cell combines power conversion characteristics of conventional flyback and boost converters. Based on the flyboost PFC cell, a new family of single-stage (S/sup 2/) ac/dc converters can be derived. Prominent features of newly derived S/sup 2/ converters include: three power conversions, i.e., boost, flyback, and another isolated dc/dc power conversions are simultaneously realized that typically uses only one power switch and one simple controller; part of the power delivered to the load is processed only once; bulk capacitor voltage can be clamped to the desired level; and capable of operating under continuous current mode. Experimental results on example converters verify that while still achieving high power factor and tight output regulation, the flyboost PFC cell substantially improve the efficiency of the converter.

Design and experimentation of a dynamic voltage restorer capable of significantly reducing an energy-storage element

Abstract: This paper deals with a dynamic voltage restorer (DVR), or a voltage-sag compensator, which consists of a set of series and shunt converters connected back-to-back, three series transformers, and a dc capacitor installed on the common dc link. The DVR is characterized by installing the series converter on the source side and the shunt converter on the load side. This system configuration allows the use of an extremely small dc capacitor intended for smoothing the common dc-link voltage. This paper provides a design procedure of the dc capacitor under a voltage-sag condition, and proposes a control method for the series converter, which is capable of reducing the voltage ratings of both the series converter and the series transformers. Experimental results obtained from a 200-V, 5-kW laboratory system are shown to confirm the validity of the design procedure, and the effectiveness of the control method.

Modeling of capacitor impedance in switching converters

Abstract: Switched capacitor (SC) converters are gaining acceptance as alternatives to traditional, inductor-based switching power converters. Proper design of SC converters requires an understanding of all loss sources and their impacts on circuit operation. In the present work, an equivalent resistance method is developed for analysis, and equivalent resistance formulae are presented for various modes of operation. Quasiresonant converters are explored and compared to standard SC converters. Comparisons to inductor-based switching power converters are made. A number of capacitor technologies are evaluated and compared for applications to both SC converters and inductor-based converters. The resulting model can be used to accurately predict and optimize converter performance in the design phase.

Input impedance analysis of single-phase PFC converters

Abstract: The input impedance of single-phase boost power factor corrected (PFC) AC-DC converters is modeled and analyzed in this paper. A large-signal model is presented for the input impedance which overcomes the limitations of traditional piece-wise linearized models. The model is valid at frequencies ranging from the crossover frequency of the output voltage loop to half the switching frequency of the converter. Experimental results from a boost single-phase PFC converter are provided to validate the model. Input characteristics of typical boost PFC converters, such as input impedance dipping, leading phase of the input current, and responses to distorted input voltages are studied by using the model. A simple compensation technique to reduce the dipping in the input impedance, thereby improving converter performance and minimizing the potential for undesirable interactions with the input filter or the ac source, is also presented.

Digital current-mode controller for DC-DC converters

Abstract: This paper introduces a digital average current-mode controller architecture based on a low-resolution current A/D converter and a simple digital controller realization. The proposed approach combines advantages of practical digital realization and current-mode control for DC-DC converters operating at high switching frequencies. An experimental test circuit includes an experimental prototype current A/D converter, a controller implemented on an FPGA, and a 6 V-to-1.8 V, 10 A synchronous buck power stage operating at 200 KHz. Experimental verification results are described.

A Soft-Switched Three-Port Bidirectional Converter for Fuel Cell and Supercapacitor Applications

Abstract: This paper presents a zero-voltage-switching (ZVS) three-port bidirectional converter for fuel cell and supercapacitor applications. A simple and effective method to extend the soft-switching range is proposed, especially for the three-port active-bridge topology. By continuously adjusting the duty cycle on the supercapacitor side H-bridge according to the supercapacitor voltage level, soft-switching conditions are achieved over the full operating range due to the equivalent volt-second products applied to the transformer over half the switching period. Detailed analysis is provided for both the two-port and the three-port converters. Furthermore, a dual PI-loop based control strategy is proposed to achieve constant output voltage and precise power flow management, as well as soft-switching. The converter is implemented and tested

Evaluation of three-level rectifiers for low-voltage utility applications

Abstract: This paper evaluates the benefits of three-level topologies as alternatives to two-level topologies in low-voltage converters primarily operated in rectifier mode. The main evaluation aspects are input filter size, semiconductor losses, maximum switching frequency, part count, initial cost, and life cycle cost. Semiconductor loss characteristics of various three-level topologies are discussed. A detailed converter comparison is based on a 100-kW 400-V/sub rms/ rectifier using commercially available Si insulated gate bipolar transistor modules.

Implementation of pulse-width-modulation based sliding mode controller for boost converters

Abstract: This letter addresses the various issues concerning the implementation of a pulse-width modulation (PWM) based sliding mode (SM) controller for boost converters. The methods of modeling the system and translation of the SM control equations for the PWM implementation are illustrated. It is shown that the control technique is easily realized with simple analog circuitries. Various experiments are conducted to test the static and dynamic performances of the system.

Power management system for vehicles

Abstract: A power management system for a vehicle. The system may be flexibly configured using various arrangements of DC to AC inverters, DC to DC converters AC to DC converters, DC to AC converters and AC to AC converters, one or more batteries, and a control portion to provide an AC bus, a primary DC bus and a secondary DC bus. The control portion monitors the status of the buses, inverters and converters and controls the operation of the inverters and converters to supply and regulate the power supplied by the buses. The inverters and converters may be bidirectional, allowing power to be transferred between buses.

Hybrid switched-capacitor-Cuk/Zeta/Sepic converters in step-up mode

Abstract: The energy-transfer-capacitor in basic Cuk, Zeta and Sepic converters is split into two capacitors. The rectifier diode is replaced by two diodes that form with the two capacitors a switched-capacitor circuit, which appears connected between the input and output inductances of the original converter. As a result, hybrid circuits, presenting a higher DC voltage ratio than the classical Cuk, Zeta and Sepic converters, are obtained. Even if the new hybrid structures do not reach the DC gain of quadratic converters, they present a higher efficiency in processing the energy: unlike the cascaded converters whose efficiency is a product of the efficiencies of each block, the hybrid converters do not require an additional level of energy processing. A DC analysis, simulation and experimental results concerning the proposed circuits are presented.