The work in this dissertation presents the first attempt in the literature to propose the concept of "soft switch". The goal of "soft switch" is to develop a standard PWM switch cell with built-in adaptive soft switching capabilities. Just like a regular switch, only one PWM signal is needed to drive the soft switch under soft switching condition. The core technique in soft switch development is a built-in adaptive soft switching circuit with minimized circulation energy. The necessity of minimizing circulation energy is first analyzed. The design and implementation of a universal controller for implementation of variable timing control to minimize circulation energy is presented. The controller has been tested successfully with three different soft switching inverters for electric vehicles application in the Partnership for a New Generation Vehicles (PNGV) project. To simplify the control, several methods to achieve soft switching with fixed timing control are proposed by analyzing a family of zero-voltage switching converters. The driver based soft switch concept was originated from development of a base driver circuit for current driven bipolar junction transistor (BJT). A new insulated-gate-bipolar-transistor (IGBT) and power metal-oxide-semiconductor field-effect-transistor (MOSFET) gated transistor (IMGT) base drive structure was initially proposed for a high power SiC BJT. The proposed base drive method drives SiC BJTs in a way similar to a Darlington transistor. With some modification, a new base driver structure can adaptively achieve zero voltage turn-on for BJT at all load current range with one single gate. The proposed gate driver based soft switching method is verified by experimental test with both Si and SiC BJT. The idea is then broadened for "soft switch" implementation. The whole soft switched BJT (SSBJT) structure behaves like a voltage-driven soft switch. The new structure has potentially inherent soft transition property with reduced stress and switching loss. The basic concept of the current driven soft switch is then extended to a voltage-driven device such as IGBT and MOSFET. The key feature and requirement of the soft switch is outlined. A new coupled inductor based soft switching cell is proposed. The proposed zero-voltage-transition (ZVT) cell serves as a good candidate for the development of soft switch. The "Equivalent Inductor" and state plane based analysis method are used to simply the analysis of coupled inductor based zero-voltage switching scheme. With the proposed analysis method, the operational property of the ZVT cell can be identified without solving complicated differential equations. Detailed analysis and design is proposed for a 3kW boost converter example. With the proposed soft switch design, the boost converter can achieve up to 98.9% efficiency over a wide operation range with a single gate drive. A high power inverter with coupled inductor scheme is also designed with simple control compared to the earlier implementation. A family of soft-switching converters using the proposed "soft switch" cell can be developed by replacing the conventional PWM switch with the proposed soft switch. / Ph. D.

Important considerations of a photovoltaic (PV) source are achieving a high voltage and drawing currents with very little ripple component from it. Furthermore, the output from such a source is variable depending on irradiation and temperature. In this research, literature review of prior methods employed to boost the output voltage of a PV source is examined and their limitations identified. This research then proposes a multi-phase tapped-coupled inductor boost DC-DC converter that can achieve high voltage boost ratios, without adversely compromising performance, to be used as an interface to a PV source. The proposed converter achieves minimal current and voltage ripple both at the input and output. The suitability of the proposed converter topology for variable input voltage and variable power operation is demonstrated in this dissertation. The proposed converter is also shown to have good performance at high power levels, making it very suitable for high power applications. Detailed analysis of the proposed converter is done. Advantages of the proposed converter are explained analytically and confirmed through simulations and experimentally. Regulation of the converter output voltage is also explained and implemented using a digital controller. The simulation and experimental results confirm that the proposed converter is suitable for high power as well as variable power, variable voltage applications where high voltage boost ratios are required. / Dissertation (MEng)--University of Pretoria, 2013. / gm2014 / Electrical, Electronic and Computer Engineering / Unrestricted

General-purpose step-down converter is essential in electronic system for processing energy from high-voltage rail to low-voltage circuits. The applications can be found at the auxiliary supplies in automobile, industrial and communication systems. Buck converter is a common circuit topology to fulfill step-down conversion, especially in low-power application since it is well-studied and straightforward. However, it suffers from low duty cycle under high step-down condition, and typically operates in continuous conduction mode (CCM) that generates large switching loss. On the other hand, as an extension of the buck converter, tapped-inductor (TI) buck converter has larger duty cycle while maintaining the structural simplicity. Therefore, the main objective of this thesis is to explore the potential of TI buck converter as a wide conversion range, high power density and high efficiency topology for low power application. To achieve high efficiency at switching frequency of MHz-level, synchronous conduction mode (SCM) is applied for turn-on losses elimination. The operation principle and power stage design of SCM TI buck is first introduced. The design of high switching frequency coupled inductor is emphasized since its size plays a critical role in power density. Loss breakdown is also provided to perform a comprehensive topological study. Secondly, detailed zero-voltage-switching (ZVS) condition of SCM TI buck is derived so that the converter does not experience redundant circulating energy. The experimental results of 15-W SCM TI buck converter prototypes are provided with 90.7% of peak power stage efficiency. The size of coupled inductor is down to 116 mm3. To enhance light-load efficiency, a variable frequency control scheme based on derived ZVS conditions is implemented with the switching frequency ranging from 2 MHz to 2.9 MHz. / Master of Science / General-purpose step-down converter is essential in electronic system for processing energy from high-voltage rail to low-voltage circuits. The applications can be found at the auxiliary supplies in automobile, industrial and communication systems. Typically, the ultimate goals of general-purpose step-down converter are versatility, high efficiency and compact size. Recently, tapped-inductor (TI) buck converter is studied since it could overcome the drawback of commonly used buck converter under high step-down conversion. Therefore, the potential of TI buck converter as a general-purpose step-down converter candidate is explored in this thesis, including control method, hardware design, etc. The thesis verifies that TI buck converter could have compact size while remaining efficient and adaptable.

The telecommunications market is one of the large rapidly growing fields in today’s power supply industry due to the increasing demand for telecom distributed power supply (DPS) systems. The half-bridge LLC (Inductor-Inductor-Capacitor) resonant converter is currently the most attractive topology for the design and implementation of 24V/48V DC telecom power converters. The current doubler rectifier (CDR) converter topology was invented and described in the early 1950s which can offer the unique characteristic of halving the output voltage while doubling the output current compared to a standard rectifier. In this thesis, the current doubler converter topology with its unique characteristic is evaluated as a complementary solution to improve the LLC resonant converter performance, especially for the low output voltage and high output current telecommunication applications. A novel half-bridge LLC resonant current doubler converter (LLC-CDR) is proposed in this thesis which can offer several performance benefits compared to conventional LLC-standard rectifier design . The unique characteristics of the LLC-CDR topology can offer significant improvements by transformation of a 48V converter into a 24V converter with the same power density. This thesis introduces a new SPICE-based simulation model to analyse the operation of this novel LLC-CDR converter circuit design. This model can be used to define the critical component parameters for the LLC -CDR circuit output inductor values. It can also be used to predict the circuit overall performance under different load conditions. Both time-domain based transient simulation analysis and frequency-domain based AC analysis provided by this simulation model showed favourable results in comparison to bench measurement results on a prototype. The model provides a valuable insight to reveal some of the unique characteristics of this LLC -CDR topology. It demonstrates a proof of concept that the conventional LLC resonant converter can be easily redesigned for low voltage, high current applications by using the LLC-CDR topology without requiring a new design for the LLC resonant stage components and the power transformer. A new magnetic integration solution was proposed to significantly improve the overall performance in the LLC-CDR topology that had not been published before. The LLC-CDR converter hardware prototypes with two output inductors coupled and uncoupled configurations were extensively modelled, constructed and bench tested.Test results demonstrated the suitability of an integrated coupled inductors design for the novel LLC-CDR converter application. The integrated coupled inductors design can significantly improve the LLC-CDR converter frequency-domain based AC simulation analysis results. In addition, these results also illustrate the potential benefit of how the magnetic integration design in general could reduce the magnetic component size, cost, and weight compared to the uncoupled inductors design. Finally, a hardware prototype circuit was constructed based on a commercial 1800 W single phase telecom power converter to verify the operation of this novel half bridge LLC-CDR topology. The converter prototype successfully operated at both no load and full load conditions with the nominal output voltage halved from 48VDC to 24VDC, and doubled the output current to match the same output power density. It also demonstrates that the efficiency of this novel half bridge LLC –CDR is 92% compares to 90% of EATON’s commercial 24VDC LLC resonant converter, which can fulfill the research goals.