Large step down voltage converters for desalination

One percent of the world's drinking water is currently desalinated, and this will have to increase to 14% by 2025. Desalination is energy intensive, having significant commercial and ecological implications. One of the most promising methods of desalination is capacitive deionisation which only uses 1kWh/m3 but requires a voltage of less than 1.8V at currents of up to 1000A This thesis produced hardware capable of creating 550A at a voltage of 1.8V, giving over a 1kW power rating, with an input voltage of 340V dc. The converter designed was a bidirectional asymmetrical half-bridge flyback converter allowing for isolation at these high step down ratios. The converter was used to charge a bank of 17,000F supercapacitors from 0V to 1.8V, with an initial charging step down ratio in excess of 340:1 falling to 190:1 as the load charged. A novel Asymmetrical Half-Bridge Coupled-Inductor Buck converter is presented as the ideal solution for large step-down ratios with analysis comparing the ability to efficiently step down a voltage with other common converters, the buck and flyback converters. A comparison between a single-ended coupled-inductor buck converter employing a buck-boost voltage clamp and the novel asymmetrical half-bridge coupled-inductor buck converter circuit shows that the asymmetrical half-bridge converter is a more efficient circuit as leakage energy is recovered; the switch voltages are clamped to within the dc voltage rating of the bridge and the control strategy is simple. Passive and active snubbers are reviewed for efficiency, switch ratings and management of the effects of leakage inductance and compared against the novel designs presented. In the desalination application isolation is required so the flyback circuit is used. An isolated three switch bidirectional converter is constructed using silicon carbide MOSFETs and diodes switching at 40kHz. The converter uses novel current measuring techniques, an on-board microprocessor and closed loop control designed into the final DC-DC converter.One percent of the world's drinking water is currently desalinated, and this will have to increase to 14% by 2025. Desalination is energy intensive, having significant commercial and ecological implications. One of the most promising methods of desalination is capacitive deionisation which only uses 1kWh/m3 but requires a voltage of less than 1.8V at currents of up to 1000A This thesis produced hardware capable of creating 550A at a voltage of 1.8V, giving over a 1kW power rating, with an input voltage of 340V dc. The converter designed was a bidirectional asymmetrical half-bridge flyback converter allowing for isolation at these high step down ratios. The converter was used to charge a bank of 17,000F supercapacitors from 0V to 1.8V, with an initial charging step down ratio in excess of 340:1 falling to 190:1 as the load charged. A novel Asymmetrical Half-Bridge Coupled-Inductor Buck converter is presented as the ideal solution for large step-down ratios with analysis comparing the ability to efficiently step down a voltage with other common converters, the buck and flyback converters. A comparison between a single-ended coupled-inductor buck converter employing a buck-boost voltage clamp and the novel asymmetrical half-bridge coupled-inductor buck converter circuit shows that the asymmetrical half-bridge converter is a more efficient circuit as leakage energy is recovered; the switch voltages are clamped to within the dc voltage rating of the bridge and the control strategy is simple. Passive and active snubbers are reviewed for efficiency, switch ratings and management of the effects of leakage inductance and compared against the novel designs presented. In the desalination application isolation is required so the flyback circuit is used. An isolated three switch bidirectional converter is constructed using silicon carbide MOSFETs and diodes switching at 40kHz. The converter uses novel current measuring techniques, an on-board microprocessor and closed loop control designed into the final DC-DC converter

Single-Stage Power Electronic Converters with Combined Voltage Step-Up/Step-Down Capability

Power electronic converters are typically either step-down converters that take an input voltage and produce an output voltage of low amplitude or step-up converters that take an input voltage and produce an output voltage of higher amplitude. There are, however, applications where a converter that can step-up voltage or step-down voltage can be very useful, such as in applications where a converter needs to operate under a wide range of input and output voltage conditions such as a grid-connected solar inverter. Such converters, however, are not as common as converters that can only step down or step up voltage because most applications require converters that need to only step down voltage or only step up voltage and such converters have better performance within a limited voltage range than do converters that are designed for very wide voltage ranges. Nonetheless, there are applications where converters with step-down and step-up capability can be used advantageously. The main objectives of this thesis are to propose new power electronic converters that can step up voltage and step down voltage and to investigate their characteristics. This will be done for two specific converter types: AC/DC single-stage converters and DC-AC inverters. In this thesis, two new AC/DC single-stage converters and a new three-phase converter are proposed and their operation and steady-state characteristics are examined in detail. The feasibility of each new converter is confirmed with results obtained from an experimental prototype and the feasibility of a control method for the inverter is confirmed with simulation work using commercially available software such as MATLAB and PSIM

A Comprehensive Review of DC-DC Converters for EV Applications

DC-DC converters in Electric vehicles (EVs) have the role of interfacing power sources to the DC-link and the DC-link to the required voltage levels for usage of different systems in EVs like DC drive, electric traction, entertainment, safety and etc. Improvement of gain and performance in these converters has a huge impact on the overall performance and future of EVs. So, different configurations have been suggested by many researches. In this paper, bidirectional DC-DC converters (BDCs) are divided into four categories as isolated-soft, isolated-hard, non-isolated-soft and non-isolated-hard depending on the isolation and type of switching. Moreover, the control strategies, comparative factors, selection for a specific application and recent trends are reviewed completely. As a matter of fact, over than 200 papers have been categorized and considered to help the researchers who work on BDCs for EV application

IMPROVEMENT STUDY ON SOFT-SWITCHED QUASI-RESONANT DC/DC BOOST CONVERTER

This report describes a novel soft-switched quasi-resonant DC/DC boost converter. Recently, remarkable efforts have been made in soft-switched quasi-resonant DC/DC converters to reduce losses and improve power efficiency. This project presents a new technique and it had improved the performance of the most recent study on soft-switched quasi-resonant DC/DC boost converter, which is presented in Ba-Thunya and Prasad's study [1]. The proposed converter employs an active snubber circuit with an auxiliary switch in series with a clamp capacitor to reduce powerlosses in Ba-Thunya and Prasad's original an active snubber circuit with an auxiliary switch and a clamp diode to reduce power losses in Ba-Thunya and Prasad's original converter. The energy from the snubber inductor after the auxiliary switch turn-off is returned to the input or delivered to the output via the active snubber circuit, thus the voltage stress onthe main switch is reduced and switching losses are minimized. Furthermore, the proposed converter reduces the reverse-recovery-related losses of the boost rectifier by controlling the di/dt rate of the rectifier current with the snubber inductor. This report describes the principle of operation of the new soft-switched quasi-resonant DC/DC boost converter. The feasibility study of the proposed converter is investigated using PSPICE program

A New Single-Phase Single-Stage AC-DC Stacked Flyback Converter With Active Clamp ZVS

Single-stage AC-DC converters integrate an AC-DC front-end converter with a DC-DC back-end converter. Compared with conventional two-stage AC-DC converters, single-stage AC-DC converters use less components and only one controller, which is used to regulate the output voltage. As a result, the cost, size and complexity of AC-DC converters can be reduced, but single-stage converters do not perform as well as two-stage converters, and most have drawbacks that are related to the fact that the DC bus voltage is not controlled an can become excessive. A new single-phase single-stage AC-DC converter that uses stacked flyback converters is proposed in this thesis. The proposed converter consists of two low power flyback converters stacked on top of each other and an active clamp that helps the main switches operate with ZVS. The stacked structure helps reduce the voltage stresses typical fund in many single-stage converters. In the thesis, the operation of the converter is explained, the steady-state characteristics of the converter are determined and its design is discussed. The feasibility of the new converter is confirmed with experimental results obtained from a 100VAC~220VAC worldwide input, 48V output, 100kHz switching frequency and 200 W output power prototype converter

Study of a Symmetrical LLC Dual-Active Bridge Resonant Converter Topology for Battery Storage Systems

A symmetrical LLC resonant converter topology with a fixed-frequency quasi-triple phase-shift modulation method is proposed for battery-powered electric traction systems with extensions to other battery storage systems. Operation of the converter with these methods yields two unique transfer characteristics and is dependent on the switching frequency. The converter exhibits several desirable features: 1) load-independent buck-boost voltage conversion when operated at the low-impedance resonant frequency, allowing for dc-link voltage regulation, zero-voltage switching across a wide load range, and intrinsic load transient resilience; 2) power flow control when operated outside the low-impedance resonance for integrated battery charging; 3) and simple operational mode selection based on needed functionality with only a single control variable per mode. Derivation of the transfer characteristics for three operation cases using exponential Fourier series coefficients is presented. Pre-design evaluation of the S-LLC converter is presented using these analytical methods and corroborated through simulation. Furthermore, the construction of a rapid-prototyping magnetics design tool developed for high-frequency transformer designs inclusive of leakage inductance, which is leveraged to create the magnetic elements needed for this work. Two 2kW prototypes of the proposed topology are constructed to validate the analysis, with one prototype having a transformer incorporating the series resonant inductance and secondary clamp inductance into the transformer leakage and magnetizing inductance, respectively. A test bench is presented to validate the analysis methods and proposed multi-operational control scheme. Theoretical and experimental results are compared, thus demonstrating the feasibility of the new multi-mode operation scheme of the S-LLC converter topology

Soft-Switching DC-DC Converters

Power electronics converters are implemented with switching devices that turn on and off while power is being converted from one form to another. They operate with high switching frequencies to reduce the size of the converters\u27 inductors, transformers and capacitors. Such high switching frequency operation, however, increases the amount of power that is lost due to switching losses and thus reduces power converter efficiency. Switching losses are caused by the overlap of switch voltage and switch current during a switching transition. If, however, either the voltage across or the current flowing through a switch is zero during a switching transition, then there is no overlap of switch voltage and switch current so in theory, there are no switching losses. Techniques that ensure that this happens are referred to as soft-switching techniques in the power electronics literature and there are two types: zero-voltage switching (ZVS) and zero-current switching (ZCS). For pulse-width modulated (PWM) Dc-Dc converters, both ZVS and ZCS are typically implemented with auxiliary circuits that help the main power switches operate with soft-switching. Although these auxiliary circuits do help improve the efficiency of the converters, they increase their cost. There is, therefore, motivation to try to make these auxiliary circuits as simple and as inexpensive as possible. Three new soft-switching Dc-Dc PWM converters are proposed in this thesis. For each converter, a very simple auxiliary circuit that consists of only a single active switching device and a few passive components is used to reduce the switching losses in the main power switches. The outstanding feature of each converter is the simplicity of its auxiliary circuit, which unlike most other previously proposed converters of similar type, avoids the use of multiple active auxiliary switches. In this thesis, the operation of each proposed converter is explained, analyzed, and the results of the analysis are used to develop a design procedure to select key component values. This design procedure is demonstrated with an example that was used in the implementation of an experimental prototype. The feasibility of each proposed converter is confirmed with experimental result obtained from a prototype converter

Analysis and design of a high gain non-isolated zero current switching bidirectional DC–DC converter for electric vehicles

This paper presents a dual inductor based current-fed bidirectional non-isolated DC–DC converter for energy storage applications. The main idea of this converter is to achieve a higher voltage conversion ratio by obtaining the operation of zero current switching. The proposed soft-switching bidirectional DC–DC converter reduces the turn-off switching losses with the aid of auxiliary network, where, the auxiliary network comprised with the resonant inductor and the resonant capacitor. This converter operates under two different operating modes such as a boost (discharge) and buck (charge) modes. In both the modes of converter operations, the IGBTs are operating under zero current turn-off in order to minimize the switch turn-off losses and to improve the efficiency of the converter. The principle of the operations and its theoretical analysis are validated by the experimental results using a 300W (50 V/250 V) converter system