High-frequency Isolation Transformer Research Articles

A New Class of Single-Phase High-Frequency Isolated Z-Source AC–AC Converters With Reduced Passive Components

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.

A High-Frequency Isolation (HFI) Charging DC Port Combining a Front-End Three-Level Converter with a Back-End LLC Resonant Converter

The high-frequency isolation (HFI) charging DC port can serve as the interface between unipolar/bipolar DC buses and electric vehicles (EVs) through the two-power-stage system structure that combines the front-end three-level converter with the back-end logical link control (LLC) resonant converter. The DC output voltage can be maintained within the desired voltage range by the front-end converter. The electrical isolation can be realized by the back-end LLC converter, which has the bus converter function. According to the three-level topology, the low-voltage rating power devices can be adapted for half-voltage stress of the total DC grid, and the PWM phase-shift control can double the equivalent switching frequency to greatly reduce the filter volume. LLC resonant converters have advance characteristics of inverter-side zero-voltage-switching (ZVS) and rectifier-side zero-current switching (ZCS). In particular, it can achieve better performance under quasi-resonant frequency mode. Additionally, the magnetizing current can be modified following different DC output voltages, which have the self-adaptation ZVS condition for decreasing the circulating current. Here, the principles of the proposed topology are analyzed in detail, and the design conditions of the three-level output filter and high-frequency isolation transformer are explored. Finally, a 20 kW prototype with the 760 V input and 200–500 V output are designed and tested. The experimental results are demonstrated to verify the validity and performance of this charging DC port system structure.

A Single-Phase Buck Matrix Converter With High-Frequency Transformer Isolation and Reduced Switch Count

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.

A Family of High-Frequency Isolated Single-Phase Z-Source AC–AC Converters With Safe-Commutation Strategy

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.

A Digital Predictive Current-Mode Controller for a Single-Phase High-Frequency Transformer-Isolated Dual-Active Bridge DC-to-DC Converter

This paper presents predictive current-mode control for a single-phase high-frequency transformer-isolated dual-active bridge dc-to-dc converter. The predictive control algorithm increases the bandwidth of the current loop of the converter which enables tracking of the current reference within one switching cycle. The paper further demonstrates that the application of the predictive control algorithm can remove transient dc offset from the current in high-frequency isolation transformer within one switching cycle. Direct control of the converter current protects the transformer from saturation even at transient conditions. The control algorithm has been implemented on an experimental setup and transient tests have been performed to validate controller performance. Since the predictive control algorithm is dependent on the measured value of the leakage inductance of the transformer, a compensator has been implemented to improve the parameter insensitivity of the proposed controller.

A Novel Zero-Voltage-Switching Push–Pull High-Frequency-Link Single-Phase Inverter

In this paper, a novel zero-voltage-switching (ZVS) push–pull high-frequency-link (PPHFL) single-phase inverter is proposed, which consists of a primary-side converter with three power switches ${\mathrm{ S}}_{1}{\sim } {\mathrm{ S}}_{3}$ , high-frequency isolation transformer, cycloconverter, and $LC$ filter circuit. First, the evolution process and the switching strategy of the proposed PPHFL inverter are discussed. Then, the steady operating principle and output characteristics of the inverter are analyzed in detail based on the switching sequence. In addition, the design guidelines and topologies in comparison with the ZVS phase-shift full-bridge topology and typical push–pull circuit are given. It shows that the switches (S1 and ${\mathrm{ S}}_{2})$ of the primary-side converter can realize ZVS easily and the switch S3 can also achieve ZVS when the leakage inductor energy is large enough. Furthermore, the cycloconverter can achieve ZVS commutation. Finally, a 20-kHz PPHFL inverter prototype is developed, whose efficacy is verified by experimental results.

A Single-Stage CCM Zeta Microinverter for Solar Photovoltaic AC Module

Low power inverter mounted on each photovoltaic (PV) panel, i.e., microinverter is gaining popularity due to the increased energy harvest. This paper proposes a novel single-stage zeta microinverter for PV applications. The proposed topology is based on zeta converter with high-frequency transformer isolation and a single low side switch on the primary side of transformer. The most distinctive feature of the proposed inverter is its ability to work in continuous conduction mode (CCM) over a wide load range, which results in high efficiency, reduced current stress, and components’ rating. All the secondary switches operate at line frequency, thus reducing switching losses significantly. A comprehensive study of the proposed microinverter, including steady-state analysis and design, has been reported. A 220-W microinverter was designed, developed, and tested in the laboratory. Experimental results are demonstrated to verify the CCM operation of the inverter and its performance over a wide load range.

Optimal ZVS Modulation of Single-Phase Single-Stage Bidirectional DAB AC–DC Converters

A comprehensive procedure for the derivation of optimal, full-operating-range zero voltage switching (ZVS) modulation schemes for single-phase, single-stage, bidirectional and isolated dual active bridge (DAB) ac-dc converters is presented. The converter topology consists of a DAB dc-dc converter, receiving a rectified ac line voltage via a synchronous rectifier. The DAB comprises primary and secondary side full bridges, linked by a high-frequency isolation transformer and a series inductor. ZVS modulation schemes previously proposed in the literature are either based on current-based or energy-based ZVS analyses. The procedure outlined in this paper for the calculation of optimal DAB modulation schemes (i.e., combined phase-shift, duty-cycle, and switching frequency modulation) relies on a novel, more accurate, current-dependent charge-based ZVS analysis, taking into account the amount of charge that is required to charge the nonlinear parasitic output capacitances of the switches during commutation. Thereby, the concept of “commutation inductance(s)” is shown to be an essential element in achieving full-operating-range ZVS. The proposed methods are applied to a 3.7 kW, bidirectional, and unity power factor electric vehicle battery charger which interfaces a 400 V dc-bus with the 230 Vac, 50-Hz utility grid. Experimental results obtained from a high-power-density, high-efficiency converter prototype are given to validate the theoretical analysis and practical feasibility of the proposed strategy.

Digitally Controlled Switch Mode Power Supply Based on Matrix Converter

In this paper, a switch mode power supply based on matrix converter is proposed for telecommunication applications. The conventional high power switch mode power supply consists of an AC/DC conversion with boost converter followed by a DC/DC converter to produce dc voltage. These rectifiers draw significant harmonic current from the utility, resulting in poor input power factor with high total harmonic distortion (THD). The proposed AC-AC system is an effective replacement of the conventional system which employs twostep power conversion. In the proposed approach, the matrix converter directly converts the low frequency (50 Hz, three phases) input to a high frequency (10 kHz, single phase) ac output without a dc-link. The output of the matrix converter is then processed via a high frequency isolation transformer to produce dc voltage. Control of the system ensures that output voltage is regulated and input currents are of high quality. The matrix converter forms an attractive topology of power converter for high power applications where factors such as absence of electrolytic capacitors, potentiality of increasing power density, reducing size and weight, good input power quality are fundamental. The proposed method is verified using Simulink toolbox of MATLAB 7.2 version.

Dual active bridge synchronous chopper control strategy in electronic power transformer

The electronic power transformer (EPT) is a novel transmission and transformation device, which consists of a series connection of two voltage-source H-bridge converters and a DC-DC converter with high-frequency isolation transformer. When applied to microgrids, EPT will not only deliver energy to loads from the utility grid but also inject some amount of excess power into the utility grid. Hence, the capability of bidirectional power flow is important for EPT, which depends on the DC-link stage. The traditional synchronous chopper control for a dual active bridge (DAB) converter has some limitations in the application of bidirectional power flow area. This study proposes a novel synchronous chopper control strategy for the DAB converter to implement bi-direction power flow, details the basic principle and steady-state operation and presents the mathematical derivations. A three-phase three-stage circuit configuration of 10 kV/400 V bi-direction EPT system based on the novel control DAB converter is designed, and corresponding control schemes for the system are discussed. The performance of this EPT system is validated by the MATLAB/Simulink-based simulations and the laboratory prototype experiments.

Analysis of the Internal Electrical Characteristics of Electronic Power Transformers

The modularized subunit of an electronic power transformer (EPT) is a series connection of two H-bridge voltage-source converters and a DC-DC converter with a high-frequency isolation transformer (HFIT). On the basis of cascading and paralleling the modularized subunits, EPT can be used in high-voltage and large-current applications in the power system. This paper discusses the steady state analysis of the modularized subunit of EPT. Theoretical analysis considers the influences of the two H-bridge voltage-source converters on the two sides of the DC-DC converter. We deduce the formulas of the theoretical calculation on the internal electrical characteristics of EPT (e.g., the voltages of the DC-bus capacitor and the primary side peak current of the HFIT). This paper provides guidance on the design and selection of EPT key elements (e.g., the DC-bus capacitors and HFIT). Experimental results are obtained from a single subunit of a laboratory model rated at 962 V, 15 kVA. All calculations, simulations, and experiments confirm the theoretical analysis of the subunit of EPT.

Proposal of Three-Phase High-Frequency Transformer Isolation UPS Topologies for Distributed Generation Applications

A family of double-conversion three-phase uninterruptible power systems with power factor correction and high-frequency transformer (HFT) isolation topologies are presented in this paper. This might be a suitable alternative compared to large conventional uninterruptible power systems (UPSs) that are based upon bulky transformers that operate at the grid frequency. This system is commonly used to supply critical loads, but nowadays, due to the increasing interest in renewable energy integration and distributed generation (DG), it can also be a suitable solution for energy storage in microgrids or renewable-power-conditioning interface applications. The three-phase HFT UPS used for further analysis is composed of single-phase chopper modules, one three-phase boost module, and one three-phase inverter. The relevant features are soft commutation of the controlled switches in the chopper stage, simple control strategy that can be implemented with well-known integrated circuits, and the use of a few batteries in series due to the same reference of the step-up stages. The principle of operation of the chosen system for experimental validation, the implemented control strategy, and the experimental results obtained for a 15-kVA prototype are presented.

Comprehensive Study of Single-Phase AC-DC Power Factor Corrected Converters With High-Frequency Isolation

Solid-state switch mode AC-DC converters having high-frequency transformer isolation are developed in buck, boost, and buck-boost configurations with improved power quality in terms of reduced total harmonic distortion (THD) of input current, power-factor correction (PFC) at AC mains and precisely regulated and isolated DC output voltage feeding to loads from few Watts to several kW. This paper presents a comprehensive study on state of art of power factor corrected single-phase AC-DC converters configurations, control strategies, selection of components and design considerations, performance evaluation, power quality considerations, selection criteria and potential applications, latest trends, and future developments. Simulation results as well as comparative performance are presented and discussed for most of the proposed topologies.

Proposal of Three-Phase High Frequency Transformer Isolation UPS Topologies

A family of double conversion three-phase uninterruptible power system with power factor correction and high-frequency transformer (HFT) isolation topologies is presented in this work. This might be an alternative compared to large conventional UPS systems that are based in bulky transformers that operates in the grid frequency. These systems are suitable for operation with line-to-line input voltages equal to 220-V or 380-V. For both input voltages, the proposed converter has almost the same efficiency processing the same output power. The three-phase HFT UPS used for further analysis is composed by single-phase chopper modules, one three-phase boost module, and one three-phase inverter. The relevant features are soft commutation of the controlled switches in the chopper stage, simple control strategy that can be implemented with well-known integrated circuits (IC), and the use of few batteries in series due to the same reference of the step-up stages. Principle of operation of the chosen system for experimental validation, implemented control strategy and experimental results obtained for a 15kVA prototype are presented.

Design and implementation of an energy efficient power conditioner for fuel cell generation system

The energy use of the world grows continuously and the development of a clean distributed power generation becomes environmentally important. Fuel cells are one such integral part of Renewable Energy Sources based clean energy supply; that they operate with hydrogen as fuel and water with heat as process waste. Due to the electrochemical reaction, fuel cell has the power quality of delivering low voltage with high current capability. Here an attempt is made to develop a power conditioner with a series of conversion to get a 220 V sinusoidal AC, 50 Hz single phase voltage of low distortion and fast dynamic regulation to cater load variations. A novel Polyphase Boost DC-to-DC switching converter based on parallel connection of 8 identical converters with current mode control is devised to have minimum reflected ripple current and voltage injected to fuel cell input. A full bridge converter with high frequency transformer isolation, step-up the DC voltage level from the low voltage fuel cell along with poly phase boost converter, deliver required DC to the PWM inverter, which generate AC utility power output. Recent trend of Ultra-capacitor based transient energy storage and retrieval system, to cater for the sluggish behavior of fuel cell, for load transients is incorporated. DSP and FPGA based digital real time controllers are used to realize the gating of MOSFETs and IGBTs used in the power conditioner. A 1 kW power conditioner is developed for a PAFC fuel cell system with 12 V DC nominal and their performance evaluations are satisfactory.

Dc-ac Power Supply For Battery Based Systems

This work presents the analysis, design, and experimental development of a single-phase inverter with high frequency transformer isolation and for a wide input voltage range. The high frequency input current ripple is mitigated by the use of a small input LC filter, and the low frequency current component injected by the inverter is attenuated by the voltage loop controller. The proposed topology can be used in dc voltage-sourced applications such as renewable energy and battery based systems.

DC link voltage optimized control for efficient residential fuel cell converter

The stack of a residential fuel cell power generation system has low voltage and high current output characteristics. Therefore, a high step up DC–DC converter with high frequency transformer isolation is necessary to elevate the voltage to the DC link as inverter input. This paper deeply investigates the relationship between the DC link voltage and the converter efficiency. The calculation and simulation results reveal that the lower the DC link voltage is, the higher the converter efficiency is. Based on this conclusion, a DC link voltage optimized control scheme is proposed to improve the converter efficiency, further the power conditioning system efficiency.

Some Design Considerations For High-power High-voltage Dc/dc Converter With Improved Power Density And Efficiency

This paper presents the findings of a R&D project connected to the development of 50 kW auxiliary power supply for the high-voltage DC-fed commuter trains. The aim was to design a new-generation power converter utilizing high-voltage IGBT modules, which can outpace the predecessor (6.5 kV IGBT based twolevel half-bridge DC/DC converter) in terms of power density, i.e. to provide more power for smaller volumetric space. The topology proposed is 3.3 kV IGBT based three-level neutral point clamped half-bridge with the high-frequency isolation transformer and the current doubler rectifier that fulfils all the targets imposed by the designers. Despite an increased component count the proposed converter is very simple in design and operation. The paper provides an overview of the design with several recommendations and guidelines as well as the feasibility and performance comparison of the proposed converter with the 6.5 kV IGBT based two-level half-bridge DC/DC converter.

Analysis and Design of 3.3 kV IGBT Based Three-Level DC/DC Converter with High-Frequency Isolation and Current Doubler Rectifier

Analysis and Design of 3.3 kV IGBT Based Three-Level DC/DC Converter with High-Frequency Isolation and Current Doubler RectifierThe paper presents the findings of a R&D project connected to the development of auxiliary power supply (APS) for the high-voltage DC-fed rolling stock applications. The aim was to design a new-generation power converter utilizing high-voltage IGBT modules, which can outpace the predecessors in terms of power density, i.e. to provide more power for smaller volumetric space. The topology proposed is 3.3 kV IGBT-based three-level neutral point clamped (NPC) half-bridge with high-frequency isolation transformer and current doubler rectifier that fulfils all the targets imposed by the designers. Despite an increased component count the proposed converter is very simple in design and operation. The paper provides an overview of the design with several recommendations and guidelines. Moreover, the simulation and experimental results are discussed and the performance evaluation of the proposed converter is presented.

A Zero-Voltage Switching Two-Inductor Boost Converter With an Auxiliary Transformer

The two-inductor boost converter with an auxiliary transformer has been previously proposed for applications where a large difference between the input and the output voltages is required. In the isolated version of the converter implementation, however, the leakage inductance of the high frequency isolation transformer presents an adverse effect and will cause significant voltage ringing on the primary switches. This paper presents a resonant version of this converter that is able to actively utilise the transformer leakage inductance as well as the metal-oxide-semiconductor field-effect transistor (MOSFET) drain source capacitance. The resonant arrangement produces softswitching conditions for both the primary MOSFETs and the output rectifier diodes. The detailed resonant state analysis is provided in the paper. The simulation waveforms are also produced for one specific operating condition. Finally, the experimental waveforms of a 1 MHz 100 W converter are provided to verify the theoretical analysis.