This diploma thesis deals with design and comparison of selected DC/DC converters, where the better of them is practically realized. The first part of the diploma thesis is focused on the general analysis of DC/DC power converters. The following part is theoretical analysis focused on the first selected topology – step-up converter. The second analysed topology is forward converter with full bridge on the primary side. The theoretical analysis also includes a description of synchronous rectifier, the differences between hard and soft switching, and the types of secondary rectifiers. Another part specializes in the detailed calculation of main components of selected converters and their subsequent power dimensioning. Both designed topologies are compared according to the required aspects. The selected better topology is supplemented by the design of control circuits and an auxiliary power supply. Practical realization of converter and commissioning follows. The diploma thesis ends with verification measurements on the realized converter and their subsequent analysis.

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With the fast development of communication systems, computers and consumer electronics, the power supplies for telecoms, servers, desktops, laptops, flat-panel TVs, LED lighting, etc. are required for more efficient power delivery with smaller spaces. The LLC resonant converter has been widely adopted for these applications due to the advantages in high efficiency, high power density and holdup time operation capability. However, LLC resonant converter meets some issues, especially in high output current applications. Those issues include magnetic design, start-up, short-circuit protection, synchronous rectifier drive, EMI noise and integration, etc. To solve those issues, like start-up and short-circuit protection, SR driving and EMI, etc., a synthesis method is proposed to find the similar resonant topologies like LLC. Based on this method, lots of multi-element resonant converters are found to solve the issues that LLC resonant converter cannot handle. To evaluate the performance of found numerous valuable topologies. Thus, a general evaluation system is required. State-plane analysis with new normalization factors is utilized. Based on it, the voltage stress, current stresses and apparent power of resonant converters are easy to compare. This method can help select suitable circuit topology for certain applications. Meanwhile, it also can help resonant converters' design. The important performance factors, like start-up, short-circuit protection, SR driving, integration and EMI performance, are also taken into account for the whole evaluation system. The high switching frequency is needed recently for high power density requirement. However, LLC resonant converter suffers high transformer loss. Matrix transformer is introduced to reduce winding loss and total volume. Flux cancellation method is utilized to reduce core size and loss. Synchronous Rectifier (SR) devices and output capacitors are integrated into secondary windings to eliminate termination related winding losses, via loss and reduce leakage inductance. The passive integration is necessary for high power density resonant converter, especially for high order system. Based on stress, suitable passive components are chosen for integration. Then, the magnetic integration method is shown based on multi-winding transformer structure. The passive integration principles are discussed. A novel passive integration method is proposed for multi-elements resonant converters. In conclusion, this work is focus on the topology analysis and integration of resonant converters. Searching the suitable topologies for certain application, and evaluate the performance of them. Then, improve the system power density by integration techniques. / Ph. D.

Evolutions in microprocessor technology require the use of a high-frequency synchronous buck converter (SBC) in order to achieve low cost, low profile, fast transient response and high power density. However, high frequency also causes more power loss on MOSFETs. Optimization of the MOSFETs plays an important role in the system performance. Circuit and device modeling is important in understanding the relationship between the device parameters and the power loss. The gate-to-drain charge (Qgd) is studied by a novel nonlinear model and compared with the simulation results. A new switching model is developed, which takes into account the effect of parasitic inductance on the switching process. Another model for dv/dt-induced false triggering-on relates the false-trigger-on voltage with the parasitic elements of the device and the circuits. Some techniques are proposed to reduce the simulation time of FEA in the circuit simulation. Based on this approach, extensive simulations are performed to study the switching performance of the MOSFET with the effect of the parasitic elements. Directed by the analytical models and the experience acquired in the circuit simulation, the MOSFET optimization is realized using FEA. Different optimization algorithms are compared. The experimental results show that the optimized MOSFETs surpass the mainstream commercialized products in both cost and performance. / Ph. D.

Dielectrics are materials that can be polarized by an applied electric field. As the essential property for dielectrics, the relationship between electric field and dielectric polarization, has been widely studied and used in the area of electrical engineering. Representative applications are insulators and electrical energy storage capacitors. For some types of dielectrics, the dielectric polarization is not only decided by the applied electric field, but also is affected by mechanical and thermal properties. This work studies the electro-mechanical and electro-thermal energy inter-conversions and proposes the design of power converters for these materials. Piezoelectric effect is a cross-coupling between mechanical property and electrical property of dielectrics. It is a reversible process where external electric potential can generate internal mechanical force while external mechanical force can also generate internal electric potential. This effect is utilized to build a piezoelectric transformer (PT) by combining two sets of piezoelectric material together. One set is used as the input, to cause a geometric strain by applied electric field, while the other set is used as the output, to generate an electric charge by the coupled mechanical stress. Compared to traditional magnetic transformers, PTs store energy in mechanical inertia and compliance and therefore they do not generate electromagnetic noise. They are suitable for batch mass manufacturing since there is no winding requirement. Among many types of PTs, radial PT and Rosen-type PT are most widely used. To provide a guide for the design of PT-based converters, the electrical characteristics of PTs are first analyzed. The accuracy and applicability of different levels of models of PTs are compared and discussed. The detailed universal attributes of PTs, which include the gain characteristic, the input impedance characteristic and the efficiency characteristic, are also derived. In addition, with the assistance of additional compensation component(s), PTs can provide better performance. The impacts of the input and output inductors and capacitors on gain and efficiency characteristics of a PT are analyzed. Tunable PT is a recently developed raidal PT with three ports: input, output, and control ports. When connected with different impedance at the control port, tunable PT has different voltage gain characteristics. It is proposed to use this property for output voltage regulation while keeping constant switching frequency to ensure high efficiency operation of the PT in PT-based power converters. A closed-loop control scheme is proposed, where the regulation is done by a duty cycle controlled switched capacitor at the tunable PT control port. Two types of output filter are also analyzed and compared. Dc-dc converters with power rating ranging from 30 W to 100 W are built to verify the proposed design. Rosen-type PT features natural mechanisms for high transform ratio in a compact planar form, which provide an alternative solution for dc bus-fed high step-down voltage-ratio auxiliary power supplies in medium-/high-voltage systems without using bulky magnetic transformer with high turns numbers. The design procedure of the Rosen-type PT-based high step-down voltage-ratio dc-dc converter is presented. The proposed design is validated by a prototype with height of 1 cm, whose nominal output power is 5 W, input voltage ranges from 200 V to 1.5 kV, regulated output voltage is 5 V. Pyroelectric effect is a cross-coupling between thermal property and electrical property in some dielectrics. It is also reversible. The pyroelectric effect refers to the polarization change caused by temperature change, while the reversed pyroelectric effect refers to a temperature change generated by a electric field change. The reversed pyroelectric effect can be used for building a environmentally friendly thermodynamic system. Electrical characterization of the pyroelectric material is executed to facilitate the design of the power converter needed in the corresponding thermodynamic system. Specifically, this work proposes an energy recovery circuit to increase the coefficient of performance of the system since during the thermodynamic cycle, part of the electrical driving work does not pump heat and may therefore be recovered. / Doctor of Philosophy / When a dielectric material is placed in an electric field, electric charges slightly shift from their average equilibrium positions, causing dielectric polarization. In the area of electromagnetism, dielectric material is widely used as an electrical insulator and to build capacitors. For some types of dielectrics, dielectric polarization is not only affected by electric field. Strong couplings between electrical and mechanical characteristics, and between electrical and thermal characteristic also exist and can be utilized in practical applications. Piezoelectric effect is a coupling between electrical and mechanical characteristics. It is a reversible process where external electric potential can generate internal mechanical force and vice versa. It can be utilized to build transformers, which do not require coil winding nor generate electromagnetic interference compared to their magnetic counterparts. This work analyzed the electrical characteristics of piezoelectric transformers and proposed the design of dc-dc converters based on different types of piezoelectric transformers for different applications, which include tunable radial piezoelectric transformer-based power converters and Rosen-type piezoelectric transformer-based step-down converter with high voltage conversion ratio. (Reversed) pyroelectric effect is a coupling between electrical and thermal characteristics in some dielectrics. An adiabatically applied or removal electric field results in an increase or decrease in the temperature of the corresponding material. This effect can be used to build a environmentally friendly thermodynamic system instead of the most prevalent vapor compression method which involves the use of hydro-fluorocarbon gases leading to global warming and ozone depletion. Electrical characterization is executed first to facilitate the design of the power converter needed by the thermodynamic system. In addition, during the thermal cycle, part of the work done to drive representative cycles does not pump heat and may therefore be recovered. This work proposed circuit featuring energy recovery to provide the desired electric field for driving the thermodynamic system and charge recycling to improve the system efficiency.

Buck converter is a power converter which drops high input voltage into a low output voltage in high efficiency. With this characteristic, it has been used in a great number of applications. Optimized the maximum load to increase the buck converter's efficiency at the cost of light load efficiency is a general way used in a traditional buck converter because it has a higher impact on power consumption. We propose a novel way of designing the two-phase buck converter with light load efficiency improvement in this thesis. The purposed two-phase buck converter uses RC delay to control switch frequency. Different frequency will affect the buck converter in output value and efficiency. RC delay includes two parts; part one connect with phase one, part two connect with phase two. After the test, when resister's value of part one is 100kΩ, and the capacitor's value is 50 pF, the resister's value of path two is 40kΩ, and the capacitors' value is 50 pF, the buck converter can reach maximum efficiency. The inspiration of the neural network is derived from the biological brain, neural is similar with the human neural, and the synaptic weights can treat as the connection between two nodes. Reservoir computing can be seen as an extension of the neural network since it is a framework for computation. Echo State Network(ESN) is one of the major types of reservoir computing, and it is a recurrent neural network. Compared with a neural network, it only trains output weights, which can save a lot of time but keep the accuracy of the training at the same time. The efficiency of the two-phase buck converter and power loss for each phase in the control scheme were measured. The input voltage set to be 30V, with the switch frequency change from 40's to 100's, the output voltages change from 9.2V to 6V, the output current range is 18 mA to 30 mA. The efficiency ranges are 94% to 98%. The teaching target set for the ESN is the output voltage of the two-phase buck converter. The ESN will read data from two-phase buck converter's simulation, including input voltage, the frequency of the switches and based on that to compute the output voltage. / Master of Science / Buck converter is a power converter which drops high input voltage into a low output voltage in high efficiency. With this characteristic, it has been used in a great number of applications. Most of the buck converter optimized the maximum load to increase the efficiency, however, it will also increase the power consumption of the buck converter. For this reason, we propose a novel way of designing the two-phase buck converter optimize with Echo State Network(ESN). The inspiration of neural network is derived from the biological brain, similar with a human brain, the neural network also have self-learning ability. Reservoir computing is one kind of neural network, it can save more time on computing data and increase the efficiency at the same time. Compare with normal two-phase buck converter, the purposed two-phase buck converter optimize with ESN can increase the efficiency and also decrease the running time.

The battery charger plays an important role in the development of electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs).This thesis focuses on the DC-DC converter for high voltage battery charger and is divided into four chapters. The background related to EV battery charger is introduced, and the topologies of isolated DC-DC converter possibly applied in battery charge are sketched in Chapter 1. Since the EV battery charger is high voltage high power, the phase-shifted full bridge and LLC converters, which are popularly used in high power applications, are discussed in detail in Chapter 2. They are generally considered as high efficiency, high power density and high reliability, but their prominent features are also limited in certain range of operation. To make full use of the advantages and to avoid the limitation of the phase-shifted full bridge and LLC converters, a novel hybrid resonant and PWM converter combining resonant LLC half-bridge and phase shifted full-bridge topology is proposed and is described in Chapter 3. The converter achieves high efficiency and true soft switching for the entire operation range, which is very important for high voltage EV battery charger application. A 3.4 kW hardware prototype has been designed, implemented and tested to verify that the proposed hybrid converter truly avoids the disadvantages of LLC and phase-shifted full bridge converters while maintaining their advantages. In this proposed hybrid converter, the utilization efficiency of the auxiliary transformer is not that ideal. When the duty cycle is large, LLC converter charges one of the capacitors but the energy stored in the capacitor has no chance to be transferred to the output, resulting in the low utilization efficiency of the auxiliary transformer. To utilize the auxiliary transformer fully while keeping all the prominent features of the previous hybrid converter in Chapter 3, an improved hybrid resonant and PWM converter is proposed in Chapter 4. The idea has been verified with simulations. The last chapter is the conclusion which summaries the key features and findings of the two proposed hybrid converters. / Master of Science

With the ever-growing developments of sustainable energy sources such as fuel cells, photovoltaics, and other distributed generation, the need for a reliable power conversion system that interfaces these sources is in great demand. In order to provide the highest degree of flexibility in a truly distributed network, it is desired to not only interface multiple sources, but to also interface multiple loads. Modern multi-port converters use high frequency transformers to deliver the different power levels, which add to the size and complexity of the system. The different topological variations of the proposed multi-port dc-dc converter have the potential to solve these problems. This thesis proposes a unique dc-dc current source converter for multi-port power conversion. The presented work will explain the proposed multi-port dc-dc converter's operating characteristics, control algorithms, design and a proof of application. The converter will be evaluated to determine its functionality and applicability. Also, it will be shown that our converter has advantages over modern multi-port converters in its ease of scalability from kW to MW, low cost, high power density and adaption to countless combinations of multiple sources. Finally we will present modeling and simulation of the proposed converter using the PSIM software. This research will show that this new converter topology is unstable without feedback control. If the operating point is moved, one of the source ports of the multiport converter becomes unstable and dies off supplying very little or no power to the load while the remaining source port supplies all of the power the load demands. In order to prevent this and add stability to the converter a simple yet unique control method was implemented. This control method allowed for the load power demanded to be shared between the two sources as well as regulate the load voltage about its desired value.

A single-phase, buck-boost based, dual-output AC-DC converter is studied in this thesis. The converter has two DC outputs with opposite polarities, which share the same ground with the input power line. The power stage performance, including the input filter, is studied and procedure to select power components is given. The circuit model is analyzed to develop appropriate control. Zerocrossing distortion of the source input current is addressed and a solution is proposed. Experimental results are satisfactory in that a high power factor line current results for steady-state operation.

A worldwide awareness of the environment is accelerating fuel cell vehicle development. With respect to power electronics, special requirement is on the development of high efficiency, high voltage ratio bi-directional DC/DC converter for fuel cell energy system management. In this paper, two bi-directional DC/DC converters, which are developed for Ford Motor Company’s fuel cell vehicle, are compared from different aspects. Comparison is concentrated on the circuit topology and EMI performance. Emphasis is placed on soft-switch, hard-switch, synchronized rectification, auxiliary start-up winding and their effect on EMI performance in this kind of isolated bi-directional converter. Comparison includes circuits analysis and test result. The EMI test setup is described. EMI measurements are given and explained. EMI solutions for bi-directional DC/DC converter are discussed. An EMI filter is designed and final test result is given. / Master of Science