Thesis (MScEng)--Stellenbosch University, 2003. / ENGLISH ABSTRACT: This report presents the design and development of a multilevel inverter for DC reticulation. Two main multilevel inverter topologies are introduced and discussed. The research focusses on the flying capacitor multilevel topology, since it became evident that it is more suitable for DC reticulation than the diode clamped multilevel topology. A bootstrap power supply for the gate drive circuits of a multilevel inverter is developed and its feasibility verified experimentally. A self-starting auxiliary power supply, that aims at addressing the power supply problem of DC to AC and DC to DC converters, is designed and its functionality demonstrated on a flying capacitor multilevel inverter. An FPGA based digital controller for implementing the inverter's control algorithms is also discussed. This controller incorporates a feed-forward output voltage regulation technique. Experimental results obtained with the four-level flying capacitor multilevel inverter, using the FPGA based digital controller and the self-starting auxiliary power supply, are presented in this report. / AFRIKAANSE OPSOMMING: In hierdie verslag word die ontwerp en ontwikkelling van 'n multivlak omsetter vir GS retikulasie bespreek. Twee hoof multivlak omsetter topologië word voorgestel en bespreek. Die navorsing fokus op die "vlieënde-kapasitor" multivlak topologië omdat dit duidelik geword het dat dit 'n beter opsie is vir die GS retikulasie as die diode-klamp multivlak topologië. 'n Kragbron vir die hekaandryfbane van die multivlak omsetter is ontwikkel en die werking daarvan is met experimentele toetse bevestig. 'n Self-begin kragbron, wat die probleem van die kragtoevoer aan die GS na WS en die GS na GS omsetters aanspreek, is ontwerp en die funksionaliteit is gedemonstreer met die "vlieënde-kapasitor" multivlak . omsetter. 'n Digitale beheerder, gebaseer op 'n FPGA, wat gebruik word om die omsetter se beheer algoritmes te implementeer, word ook bespreek. Hierdie beheerder inkorporeer 'n vorentoe-voer uittree spannings regulasie tegniek. Eksperimentele resultate wat gekry is met 'n vier-vlak "vlieënde-kapasitor" multivlak omsetter, wat van die FPGA gebaseerde digitale beheerder en die self-begin kragbron gebruik maak, word ook in die verslag bespreek.

M.Ing. / This study focuses its attention on conducted common mode EMI. Common mode current is the current that flows from an electrical circuit to a zero reference plane and back to the circuit again. It is known that the manner in which the common mode current flows is through stray capacitances that form between the electrical circuit and the zero reference. This study was aimed at developing a method to measure the value of the stray capacitance of a switching circuit. Determining the value of the stray capacitance by taking physical measurements on a circuit board is a challenge for a number of reasons, one of which is that great care should be taken not to add to the stray capacitance by means of the measuring equipment. By measuring the value of the stray capacitance, it will be possible to model the occurrence of Common Mode EMI better and more accurately. This could, in turn, lead to a reduction in EMI. Any body of an arbitrary shape, size and material exhibits a self-capacitance with respect to a zero reference frame. This principle, together with the principle of conservation of charge, also applies to electrical components, or circuits as a whole. The experimental work was performed on a buck DC-DC converter. The circuit was simplified to aid in analysis. By varying the value of an external capacitance and taking basic measurements, it is then possible to uniquely determine the absolute values of the self-capacitances. For every pair of external capacitance values placed in the circuit (of which one can be =0pF), a value for stray capacitance is calculated. Many data points were recorded with many different external capacitors in the circuit, resulting in a variety of stray capacitance values. In order to obtain a single value, a weighted mean of all the values was calculated. The values obtained in this proposed method of measuring the stray capacitance compares well with that obtained using the Finite Element Method. The advantage of the method presented here is that the self-capacitances are determined under the actual operational conditions, no specialised equipment is required and no unique handling of parasitics is needed. The method relies on very simple measurements and no complex data manipulations are required.

Indiana University-Purdue University Indianapolis (IUPUI) / The inner structural characteristics of non-minimum phase DC-DC converters pose a severe limitation in direct regulation of voltage when addressed from a control perspective. This constraint is reflected by the presence of right half plane zeros or the unstable zero dynamics of the output voltage of these converters. The existing controllers make use of one-to-one correspondence between the voltage and current equilibriums of the non-minimum phase converters and exploit the property that when the average output of these converters is the inductor current, the system dynamics are stable and hence they indirectly regulate the voltage. As a result, the system performance is susceptible to circuit parameter and load variation and require additional controllers, which in turn increase the system complexity. In this thesis, a novel approach to this problem is proposed for second order non-minimum phase converters such as Boost and Buck-Boost Converter. Different solutions have been suggested to the problem based on whether the converter is modeled as a linear system or as a nonlinear system. For the converter modeled as a linear system, the non-minimum phase part of the system is decoupled and its transfer function is converted to minimum phase using a parallel compensator. Then the control action is achieved by using a simple proportional gain controller. This method accelerates the transient response of the converter, reduces the initial undershoot in the response, and considerably reduces the oscillations in the transient response. Simulation results demonstrate the effectiveness of the proposed approach. When the converter is modeled as a bilinear system, it preserves the stabilizing nonlinearities of the system. Hence, a more effective control approach is adopted by using Passivity properties. In this approach, the non-minimum phase converter system is viewed from an energy-based perspective and the property of passivity is used to achieve stable zero dynamics of the output voltage. A system is passive if its rate of energy storage is less than the supply rate i.e. the system dissipates more energy than stores. As a result, the energy storage function of the system is less than the supply rate function. Non-minimum phase systems are not passive, and passivation of non-minimum phase power converters is an attractive solution to the posed problem. Stability of non-minimum phase systems can also be investigated by defining the passivity indices. This research approaches the problem by characterizing the degree of passivity i.e. the amount of damping in the system, from passivity indices. Thus, the problem is viewed from a system level rather than from a circuit level description. This method uses feed-forward passivation to compensate for the shortage of passivity in the non-minimum phase converter and makes use of a parallel interconnection to the open-loop system to attain exponentially stable zero dynamics of the output voltage. Detailed analytical analysis regarding the control structure and passivation process is performed on a buck-boost converter. Simulation and experimental results carried out on the test bed validate the effectiveness of the proposed method.

In this dissertation, a new satellite platform power architecture based on paralleled three-port DC/DC converters is proposed to reduce the total satellite power system mass. Moreover, a fourport DC/DC converter is proposed for renewable energy applications where several renewable sources are employed. Compared to the traditional two-port converter, three-port or four-port converters are classified as multi-port converters. Multi-port converters have less component count and less conversion stage than the traditional power processing solution which adopts several independent two-port converters. Due to their advantages multi-port converters recently have attracted much attention in academia, resulting in many topologies for various applications. But all proposed topologies have at least one of the following disadvantages: 1) no bidirectional port; 2) lack of proper isolation; 3) too many active and passive components; 4) no softswitching. In addition, most existing research focuses on the topology investigation, but lacks study on the multi-port converter’s control aspects, which are actually very challenging since it is a multi-input multi-output control system and has so many cross-coupled control loops. A three-port converter is proposed and used for space applications. The topology features bidirectional capability, low component count and soft-switching for all active switches, and has one output port to meet certain isolating requirements. For the system level control strategy, the multi-functional central controller has to achieve maximal power harvesting for the solar panel, the battery charge control for the battery, and output voltage regulation for the dc bus. In order to design these various controllers, a good dynamic model of the control object should be obtained first. Therefore, a modeling procedure based on a traditional state-space averaging method is v proposed to characterize the dynamic behavior of such a multi-port converter. The proposed modeling method is clear and easy to follow, and can be extended for other multi-port converters. In order to boost the power level of the multi-port converter system and allow redundancy, the three-port converters are paralleled together. The current sharing control for the multi-port converters has rarely been reported. A so called “dual loop” current sharing control structure is identified to be suitable for the paralleled multi-port converters, since its current loop and the voltage loop can be considered and designed independently, which simplifies the multi-port converter’s loop analysis. The design criteria for that dual loop structure are also studied to achieve good current sharing dynamics while guaranteeing the system stability. The renewable energy applications are continuously demanding the low cost solution, so that the renewable energy might have a more competitive dollar per kilowatt figure than the traditional fossil fuel power generation. For this reason, the multi-port converter is a good candidate for such applications due to the low component count and low cost. Especially when several renewable sources are combined to increase the power delivering certainty, the multi-port solution is more beneficial since it can replace more separate converters. A four-port converter is proposed to interface two different renewable sources, such as the wind turbine and the solar panel, one bidirectional battery device, and the galvanically isolated load. The four-port converter is based on the traditional half-bridge topology making it easy for the practicing power electronics engineer to follow the circuit design. Moreover, this topology can be extended into n input ports which allow more input renewable sources. vi Finally, the work is summarized and concluded, and references are listed.

In the field of power electronics there is always a push to create smaller and more efficient power conversion systems. This push is driven by the industry that uses the power systems, and can be realized by new semiconductor devices or new techniques. This examination describes a novel technique for a small and highly efficient method of converting relatively high DC voltage to a very low voltage for use in the telecommunications industry. A modification to the standard Forward Resonant converter results in improvements in component stress, system efficiency, response time, and control circuitry. This examination describes background information needed to understand the concepts in DC to DC power systems, "soft-switching" topologies, and control methods for these systems. The examination introduces several topologies that are currently being used, and several types that have been previously analyzed, as a starting point for the detailed analysis of the proposed converter topology. A detailed analytical analysis is given of the proposed topology, including secondary effects, and component stresses. This analysis is compared to the results found from both Pspice simulation, and a working DC to DC converter. Finally, the topology is examined for potential improvements, and possible refinements to the model described.

High conversion efficiency and fast transient response at high switching frequency are the two main challenges for low-voltage high-current DC-DC converters, which are the motivations of the dissertation work. To reduce the switching power loss, soft switching is a desirable technique to keep power loss under control at high switching frequencies. A Duty-Cycle-Shift (DCS) concept is proposed for half-bridge DC-DC converters to reduce switching loss. The concept of this new control scheme is shifting one of the two symmetric PWM driving signals close to the other, such that ZVS can be achieved for the lagging switch due to the shortened resonant interval. By applying a basic DCS concept to a conventional half-bridge DC-DC converter, Zero-Voltage-Switching is achieved for one of the two primary switches. To achieve ZVS for the other switch, a ZVS half-bridge topology is proposed. Basically, by adding an active branch to the conventional half-bridge topology, the leakage inductance energy is trapped during the freewheeling time, and the energy is released to achieve ZVS for the other switch. In addition, a modified ZVS half-bridge topology is proposed to ground the auxiliary switch, and thus, a simple drive circuitry can be applied to the auxiliary switch. Leakage inductance leads to ringing issue in a half-bridge DC-DC converter. An active-clamp snubber topology is presented in the half-bridge DC-DC converters to recycle the leakage inductance energy and attenuate the ringing. Since dissipative snubbers are removed, a converter can operate more efficiently. Body-diode reverse-recovery-related loss in SRs increases with the switching frequency. To reduce this reverse-recovery loss, two passive snubber circuits are proposed for SR rectifiers in a current dubler rectifier. The proposed snubbers attenuate reverse recovery ringing and higher efficiencies are achieved. A unified DC model is derived based on the state-space average equation, which is suited for both symmetric and asymmetric half-bridge DC-DC converters. Furthermore, the DC analysis is conducted based on the unified DC model for symmetric and asymmetric half-bridge DC-DC converters with current-doubler rectifier. The AC model of isolated DC-DC converters is also established, and output impedance is analyzed for the purpose of transient response investigation. A two-stage approach is a trade-off between conversion efficiency and fast transient response. Full-Duty-Cycle (FDC) two-stage architecture is proposed to achieve desirable open-loop output impedance and fast transient response. Class-D resonant converters are investigated and recognized as potential topologies to reduce switching loss and SR conduction loss. Considering the limited regulation capability of class-D resonant converters, low-Q SRC and LLC resonant converters are proposed as candidate topologies in two-stage approaches.

A computer-aided design procedure has been developed to determine the minimum weight design solutions, for singly wound EE- and EI-cored inductors that perform the functions of energy storage and transfer in switching DC–DC power converters. Three converter topologies, namely the Buck, the Boost, and the Buck/Boost have been considered in implementing the closed form Lagrange Multiplier-based solution to the design problem. A notable feature is the interactive use of design graphs to facilitate a trade-off study between the weight of the inductor, the total losses in the inductor and the peak current stress in the switching transistor and diode. Thus useful insight is obtained by bringing aspects of converter design into view. Practical core and magnetic material data from manufacturer's catalogs can be specified and the design optimized for the minimal weight. / Master of Science

The large-signal transient response of duty ratio controlled dc-to-dc converters is investigated using the phase-plane technique. The transition pattern of large-signal trajectories is provided in terms of the circuit parameter and operating conditions. Several transient trajectories of practical interest including start-up, step input voltage change and step-load change are analyzed. The effect of large-signal characteristics of the feedback controller on the transient trajectory is presented. / Master of Science

The methods of modeling and measurement of switching converters are discussed in this thesis. The average model is very simple but is inaccurate when modulation frequencies approach to one-half of the switching frequency. The discrete model can accurately predict the modulation envelope of the output voltage, but it is very complex. The discrete-average model combines both average and discrete modeling techniques so that it possesses the advantages of both techniques. The experimental results show the deficiency of analog modulation technique for loop gain measurement under certain circumstances. The digital modulator, on the other hand, always provides accurate measurement data. The accuracy of the discrete-average model is verified by measurement data using the digital modulator. / Master of Science

Open-loop stability analysis of multi-loop current-injected switching regulator is performed using a small signal model containing a power stage, an error processor and a duty cycle pulse modulator. Two design constraints and the effects of various critical control circuit parameters are pinpointed and the analysis-based design guidelines are established in order to optimize the switching regulator performances. In addition, an external ramp slope is proposed to obtain the optimal performance and eliminate the 50% duty cycle instability when operated in constant frequency mode. The effects of the second stage output filter are also examined. / Master of Science