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.

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

A generalized, discrete-time modeling and simulation program, applicable to any PWM, resonant or quasi-resonant converter, has been developed. From a circuit description, this program automatically generates state-space equations corresponding to each switching interval and performs time-domain simulations by using state-transition equations with a fast-convergence algorithm for topological changes. / M.S.

Two novel clamped-mode resonant converters are analyzed. These clamped-mode converters operate at a constant frequency while retaining many desired features of conventional resonant converters such as fast responses, zero-voltage turn-on or zero-current turn-off, and low EMI levels, etc. The converters are able to regulate the output from no load to full load and are particularly suitable for off-line, high-power applications. To provide insights to the operations and derive design guidelines for the clamped-mode resonant converters, a complete dc characterization of both the clamped-mode series-resonant converter and the clamped-mode parallel-resonant converter, operating above and below resonant frequency, is performed. State-plane analysis techniques are employed. By portraying the converters' operation on a state-plane diagram, various circuit operating modes are identified. The boundaries between different operating modes are determined. The regions for natural and force commutation of the active switches are defined. Important dc characteristics, such as control-to-output transfer ratio, rms inductor current, peak capacitor voltage, rms switch currents, average diode currents, switch turn-on currents, and switch turn-off currents are derived to facilitate the converter designs. To illustrate the converter designs in different operating regions, several design examples are given. Finally, three prototype circuits are built to verify the analytical results. / Ph. D.

The power requirements for future spacecraft power systems will be on the order of a few hundred kilowatts to a few megawatts. Because of these power levels, a high-voltage, high-power distribution subsystem may be utilized to transmit power from the source to the different loads. Using current state-of-the-art power conditioning electronics, complex series and parallel configurations will be required at the interface between the source and the distribution subsystem and between the distribution subsystem and the loads. The dynamic response of such a spacecraft power system may be obtained using a general purpose program such as SPICE2. However, for large and complex spacecraft power systems, the input file will be large and complex with correspondingly large computation times. As an alternative, the spacecraft power system can be considered as an interconnection of modular components. Each component is treated as a two-port network, and a state model is written with the port voltages as the inputs. The state model of each component is solved using the state transition matrix and assuming that the port voltages are . clamped for each time step. This calculation proceeds as if all two-port networks are decoupled. After the state variables have been updated, the inputs to all components are calculated using network analysis principles. The solution procedure alternates between solving the dynamic model of all components and the network equations for the component inputs. The modular state variable approach and SPICE2 are compared using two example systems. This comparison shows the advantages of the modular state variable approach. First, for the modular state variable approach the system is considered as an interconnection of modular components. In SPICE2, the system is treated as an interconnection of circuit elements. As a result, the system description for large and complex spacecraft power systems is much _ larger and more complex than a modular state variable description. Secondly, the modular state variable approach requires less CPU time than SPICE2. For one of the example systems presented here, the modular state variable approach uses one-twentieth of the CPU time used by SPICE2. / Ph. D.

Basic PWM dc-to-dc converter structure is examined wherein a basic substructure of converters, known as a converter cell, is identified. Converter cells can be used in generation and classification of basic PWM dc-to-dc converters. A large number of new converters are generated. Converter analysis, whereby the nonlinear response of the system to perturbations in the control or the input, is determined by two different methods. A classical approach to nonlinear systems analysis is first used wherein the system is represented by a Volterra functional series. The alternative approach presented concentrates on deriving circuit models for the PWM switch. The PWM switch represents the static nonlinear substructure of the vast majority of converter cells. Analysis of converters then proceeds in an analogous fashion to ordinary transistor circuit analysis whereby the nonlinear device is replaced by its circuit model. Topological considerations of single-phase dc-to-ac inverters are discussed. A number of zero-current switching quasi-resonant inverter topologies are derived. Schemes that permit these topologies to handle reactive loads are identified. / Ph. D.