Photovoltaic (PV) systems are renewable, DC sources which provide non-linear output power with respect to PV panel operating voltage or current. The majority of PV sources yield poor conversion efficiencies between available solar radiation and electrical output. Additionally, they are expensive compared to other conventional power sources. Power electronic converters are capable of harvesting the most energy from these resources due to their configurability and high-efficiency. These converters form a power conditioning stage which allows for numerous control methods and energy management options. Traditional systems group PV sources into arrays in order to increase operating voltage and power to levels where it is practical to connect them to the utility grid. Grid-tied PV has the potential to increase the acceptance of PV energy by reducing end-user complexity — there are no batteries to manage and additional wiring can be kept to a minimum. However, these arrays of PV panels have significant drawbacks when they are subjected to non-ideal conditions. If a single panel is shaded, or covered in some way, then it will have greatly reduced output current. As a result, any other panel which is connected in series with the affected panel is also subject to the same output current reduction. This series grouping of panels may then indirectly affect other series-sets of panels which are connected in parallel to it by tricking the power electronics unit into operating at a point which is not the true maximum-power-point (MPP). By connecting a single PV panel to a single DC-DC converter, these array-effects can be avoided. Reliability and power output of the whole system should increase at the expense of additional hardware. The outputs of several PV-connected DC-DC converters can be connected either in series or in parallel. If they are connected in parallel, the converters must be able to boost the PV panel voltage up to a level greater than the desired utility-grid voltage. This thesis focuses on the design and control of a high-boost-ratio DC-DC converter suitable for use in a parallel-connected, grid-tied PV system. It demonstrates the feasibility of boost-ratios of up to 10 times while still achieving high efficiency. The design avoids the use of electrolytic capacitors in favor of smaller ceramic capacitors and a few large film-capacitors. A simplified model is proposed which is still suitable for use in the design of high-bandwidth control loops. Testing is done with a PV source showing preliminary results with a maximum-power-point-tracker (MPPT) which achieves very good steady-state performance. / Master of Science

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Tese de mestrado integrado. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 2010

Piezoelectric ceramics are characterized as smart materials and have been widely used in the area of actuators and sensors. The principle operation of a piezoelectric transformer (PT) is a combined function of actuators and sensors so that energy can be transformed from electrical form to electrical form via mechanical vibration. Since PTs behave as band-pass filters, it is particularly important to control their gains as transformers and to operate them efficiently as power-transferring components. In order to incorporate a PT into amplifier design and to match it to the linear or nonlinear loads, suitable electrical equivalent circuits are required for the frequency range of interest. The study of the accuracy of PT models is carried out and verified from several points of view, including input impedance, voltage gain, and efficiency. From the characteristics of the PTs, it follows that the efficiency of the PTs is a strong function of load and frequency. Because of the big intrinsic capacitors, adding inductive loads to the PTs is essential to obtain a satisfactory efficiency for the PTs and amplifiers. Power-flow method is studied and modified to obtain the maximum efficiency of the converter. The algorithm for designing a PT converter or inverter is to calculate the optimal load termination, YOPT, of the PT first so that the efficiency (power gain) of the PT is maximized. And then the efficiency of the dc/ac inverter is optimized according to the input impedance, ZIN, of the PT with an optimal load termination. Because the PTs are low-power devices, the general requirements for the applications of the PTs include low-power, low cost, and high efficiency. It is important to reduce the number of inductive components and switches in amplifier or dc/ac inverter designs for PT applications. High-voltage piezoelectric transformers have been adopted by power electronic engineers and researchers worldwide. A complete inverter with HVPT for CCFL or neon lamps was built, and the experimental results are presented. However, design issues such as packaging, thermal effects, amplifier circuits, control methods, and matching between amplifiers and loads need to be explored further. / 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.

This thesis introduces the ‘tunable’ piezoelectric transformers (TPT) which provide an extra control terminal, used in this case, to regulate the output voltage. A detailed mathematical analysis is done on the electrical equivalent circuit of the TPT to understand the effect of control terminal loading on the circuit performance. Based on this analysis, a variable capacitor connected across the control terminal is proposed to regulate the output voltage for line and load variations is suggested. The concept of ‘tunability’ in a TPT is introduced and mathematical conditions are derived to achieve the required ‘tunability’. This analysis can help a TPT designer to design the TPT for a specific application and predict the load and line regulations limits for a given design. A circuit implementation of the variable capacitor, intended for control, is presented. With the proposed control circuit design, the effective value of a fixe capacitor can be controlled by controlling the duty cycle of a switch. Hence, this enables pulse width modulated (PWM) control for the TPT based converter operating at a constant frequency. Fixed frequency operation enables a high efficiency operation of TPT near its resonant frequency and the complete secondary control requires no isolation in the voltage feedback and control circuit. This prevents any ‘cross-talk’ between primary and secondary terminals and reduces the component count. The design of series input inductor for achieving zero voltage switching (ZVS) in the inverter switches for the new control is also discussed. Experimental results for two different TPT designs are presented. Their differences in structure and its effect on the circuit performance has been discussed to support the mathematical analysis. / Master of Science / Piezoelectric transformers (PTs) are electromechanical devices which can transfer electrical energy by using acoustic coupling. Piezoelectricity is a phenomenon where certain crystalline materials develop electric potential across their surface when subjected to a mechanical stress (transducer). This can also occur in inverse i.e. an electric field inside such a material can produce a mechanical strain inside them (actuator). These direct and indirect piezoelectric effects are used to make a PT which has a transducer and an actuator coupled together to transfer electrical energy. Power electronics is a rapidly growing field which relies heavily on conventional electromagnetics to store energy (inductors), step-down and step-up voltage (magnetic transformers) and to act as band pass circuits (resonant converter topologies) etc. to enable power conversion. Piezoelectric transformers behave as band pass circuits as such they resonate at a certain frequency and hence allow only a narrow range of frequencies to pass through them. Owing to their light weight, high power density and automated manufacturing capability, they are seen as a potential replacement for electromagnetic transformers in power converters. This thesis introduces a new structure of PTs, namely the tunable piezoelectric transformers, which allow for better control techniques as compared to standard PTs. Using the extra ‘control’ terminal provided in such a structure the design of a DC-DC converter using TPT is discussed in detail. Mathematical analysis to support the design is presented and the two hardware prototypes, with distinctive designs, are developed to verify the results.

Since 2002, the department of electricity at Uppsala university has pushed the Lysekil project. The project has a number of wave energy converters installed in the sea southwest of Lysekil. The purpose of this work is to design, build and test a DC-DC converter, which will later be used as a necessary part of the grid connection of a wave energy converter. Since a wave energy converter does not generate electricity at a constant frequency, it is not possible to use a gearbox. Instead, power is rectified and, if there are several wave power energy converters, are put together with the others before it is inverted and transformed to the correct voltage level, and finally connected to the grid [1]. The designed DC-DC converter is a converter of the type "inverting buck-boost", i.e. a converter that can both lower and raise the voltage, and inverts the polarity of the output. Although the voltage in normal circumstances will only be increased, the simulations showed that the efficiency and cost of components did not differ much between a "boost" and "buck-boost" converter, thus considered flexibility to be able to lower the voltage if needed. The project also includes a small part to the construction of a bridge rectifier, but as the most difficult moment in the project is the DC-DC converter, the greatest focus will be there.

In this thesis, coupled inductors have been applied to multiphase DC-DC converters. Detailed analysis has been done to investigate the benefits of directly coupled inductors and inversely coupled inductors, compared to conventional uncoupled inductors. In general, coupled inductors for multiphase DC-DC converters have inherent benefits such as excellent current sharing characteristics, immunity to component tolerance and reduction in current control complexity. Specifically, by employing directly coupled inductors for multiphase DC-DC converters, overall current ripple can be effectively reduced, compared to that of uncoupled inductors. For inversely coupled inductors, phase current ripple can be reduced if operating points and coupling coefficients are carefully chosen. As for small-signal characteristics, inversely coupled inductors have the advantages of broadening the bandwidth of multiphase DC-DC converters and being more immune to load variation at low frequencies. On the other hand, directly coupled inductors have the benefit of low sensitivity to input variation at high frequencies. In addition, the proposed new structure for multiphase DC-DC converters has excellent current sharing performance and reduced current ripple. Computer simulations have been done and hardware prototypes have been built to validate the concepts.

Power consumption of CMOS digital logic designs has increased rapidly for the last several years. It has become an important issue, not only in battery-powered applications, but also in high-performance digital designs because of packaging and cooling requirements. At multi-GHz clock rates in use today, charging and discharging CMOS gates and wires, especially in clocks with their relatively large capacitances, leads to significant power consumption. Recovering and recycling the stored charge or energy about to be lost when these nodes are discharged to ground is a potentially good strategy that must be explored for use in future energy-efficient design methodologies. This dissertation investigates a number of novel clock energy recycling techniques to improve the overall power dissipation of high-performance logic circuits. If efficient recycling energy of the clock network can be demonstrated, it might be used in many high-performance chip designs, to lower power and save energy. A number of chip prototypes were designed and constructed to demonstrate that this energy can be successfully recycled or recovered in different ways: • Recycling clock network energy by supplying a secondary DC-DC power converter: the output of this power converter can be used to supply another region of the chip, thereby avoiding the need to draw additional energy from the primary supply. One test chip demonstrates energy in the final clock load can be recycled, while another demonstrates that clock distribution energy can be recycled. • Recovering clock network energy and returning it back to the power grid: each clock cycle, a portion of the energy just drawn from the supply is transferred back at the end of the cycle, effectively reducing the power consumption of the clock network. The recycling methods described in this thesis are able to preserve the more ideal square clock shape which has been a limitation of previous work in this area. Overall, the results provided in this thesis demonstrate that energy recycling is very promising and must be pursued in a number of other areas of the chip in order to obtain an energy-efficient design.

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