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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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.

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.

Photovoltaic (PV) systems are a rapidly growing segment in the renewable energy industry.  Though they have humble origins and an uncertain future, the commercial viability of PV has significantly increased, especially in the past decade.  In order to make PV useful, however, significant effort has to go into the power conditioning systems that take the low-voltage dc from the panel and create utility compatible ac output.  Popular architectures for this process include the centralized inverter and the distributed micro-inverter, each with its own advantages and disadvantages.  One attempt to retain the advantages of both architectures is to centralize the inverter function but construct PV panel-level micro-converters which optimize the panel output and condition the power for the inverter.  The main focus of this work is to explore the technical challenges that face the evolution of the dc-dc micro-converter and to use them as a template for a vertically integrated design procedure. The individual chapters focus on different levels of the process:  topology, modulation and control, transient mitigation, and steady-state optimization.  Chapter 2 introduces a new dc-dc topology, the Integrated Boost Resonant (IBR) converter, born out of the natural design requirements for the micro-converter, such as high CEC efficiency, simple structure, and inherent Galvanic isolation.  The circuit is a combination of a traditional PWM boost converter and a discontinuous conduction mode (DCM), series resonant circuit.  The DCM operation of the high-frequency transformer possesses much lower circulating energy when compared to the traditional CCM behavior.  When combined with  zero-current-switching (ZCS) for the output diode, it results in a circuit with a high weighted efficiency of 96.8%.  Chapter 3 improves upon that topology by adding an optimized modulation scheme to the control strategy.  This improves the power stage efficiency at nominal input and enhances the available operating range.  The new, hybrid-frequency method utilizes areas where the modulator operates in constant-on, constant-off, and fixed-frequency conditions depending on duty cycle, the resonant period length, and the desired input range.  The method extends the operating range as wide as 12-48V and improves the CEC efficiency to 97.2% in the 250-W prototype.  Chapter 4 considers the soft-start of the proposed system, which can have a very large capacitive load from the inverter.  A new capacitor-transient limited (CTL) soft-start method senses the ac transient across the resonant capacitor, prematurely ending the lower switch on-time in order to prevent an excessive current spike.  A prototype design is then applied to the IBR system, allowing safe system startup with a range of capacitive loads from 2μF to 500μF and a consistent peak current without the need for current sensing.  Chapter 5 further investigates the impact of voltage ripple on the PV output power.  A new method for analyzing the maximum power point tracking (MPPT) efficiency is proposed based on panel-derived models.  From the panel model, an expression demonstrating the MPPT efficiency is derived, along with a ripple "budget" for the harmonic sources.  These ripple sources are then analyzed and suggestions for controlling their contributions are proposed that enable circuit designers to make informed and cost-effective design decisions.  Chapter 6 illustrates how results from a previous iteration can provide a basis for the next generation's design.  A zero-voltage-switching (ZVS) version of the circuit in Chapter 2 is proposed, requiring only two additional MOSFETs and one inductor on the low-voltage side.  The maximum switching frequency is then increased from 70kHz to 170kHz, allowing for a 46% reduction in converter volume (from 430cm³ to 230cm³) while retaining greater than 97% weighted efficiency. / Ph. D.

The duty cycle control (DCC) modulation scheme for the three-phase dual-active-bridge (3p-DAB) DC-DC converter is a promising three degree-of-freedom modulation scheme which can extend the converter’s soft-switching range and reduce conduction losses under partial loading and wide voltage variations. However, the prior suggested methods to implement DCC in 3p-DABs have drawbacks such as requiring a multi-frequency approximation and offline optimization process or achieving less than optimal efficiency. To overcome these challenges, this research first proposes an optimal DCC modulation strategy (OMS) for the 3p-DAB based on a novel piece-wise time-domain analysis (TDA) and optimization process that obtains the optimal control parameters for minimum RMS phase current. Secondly, this research proposes a novel closed-form minimum current stress optimization (MCSO) DCC scheme based on the theoretical findings of the TDA optimization. The MCSO reduces the transformer phase currents and extends soft-switching operation under partial loading and wide voltage variations. Experimental results via open-loop testing show that the proposed closed-form MCSO DCC scheme has virtually identical efficiency as the OMS, making this the first research to provide a closed-form DCC modulation scheme for a 3p-DAB that achieves efficiency results equivalent to a fully-optimized offline scheme, but without the drawbacks of the offline optimization process. / Thesis / Master of Applied Science (MASc)

Electric vehicle (EV) adoption continues to rise, yet EV sales still represent a small portion of vehicle sales in most countries. An expansion of the DC fast charging (DCFC) network is likely to accelerate this revolution towards sustainable transportation, giving drivers more flexible options for charging on longer trips. However, DCFC presents a large load on the grid which can lead to costly grid reinforcements and high monthly operating costs – adding energy storage to the DCFC station can help mitigate these challenges. This thesis first performs a comprehensive review of DCFC stations with energy storage, including motivation, architectures, power electronic converters, and detailed simulation analysis for various charging scenarios. The review is closely tied to current state-of-the-art technologies and covers both academic research contributions and real energy storage projects in operation around the world. It is identified that the battery energy storage systems (BESSs) with active front end converter provides high efficiency with reasonable power density in a DCFC station. It is also realized that the isolated DC/DC converter interfacing BESS and EV determines the overall efficiency of a DCFC station with a low grid connection. Secondly, this thesis analyzes the impact of active front end based DCFC stations connected to a grid distorted with background voltage harmonics. In active front end based DCFC stations, background voltage harmonics produce current not only at the frequencies of the distorted voltage, but also at other coupled frequencies. Various mitigation techniques, such as increasing inner control loop gain, grid voltage feedforward, and selective harmonic compensation, have been adopted in industry to reduce the emissions originating from distorted background voltage. However, although these techniques are effective in suppressing the current at the harmonic orders present in the background voltage, they deteriorate the emission at coupled frequencies. This thesis provides the theoretical explanation of this phenomenon, which is verified by simulation of a two-level active front end in PSCAD/EMTDC. This thesis also discusses the proper treatment of current emission due to background voltage harmonics. Thirdly, the thesis identifies the semi dual active bridge (semi-DAB) converter as an ideal candidate as the interfacing isolated DC/DC converter between the BESS and the BEV. A novel control strategy is proposed for the semi-DAB converter to achieve wide voltage gain while increasing the efficiency at operational points with high input voltage and low output voltage, which is a commonly occurring scenario when the BESS is fully charged, and the EV battery is at low charge. Furthermore, this thesis also provides an algorithm to determine the required phase-shift in real time for any operating point, eliminating the need to devise the control trajectory offline. A 550 V, 10 kW experimental prototype is built and tested to validate the proposed control strategy. With a 25 A constant charging current, the prototype shows the proposed control strategy can improve efficiency by up to 3.5% compared to the well-known dual phase shift control at operating points with high input voltage (450 – 550 V) and low output voltage (150 – 275 V), with a peak efficiency of 97.6%. Finally, this thesis proposes a novel variable turns-ratio semi-DAB converter to improve its overall efficiency even further when the input voltage is high and the output voltage is low. Furthermore, a control law is also proposed to determine the turns-ratio, i.e., the operational structure of the converter, which reduces the converter peak and rms current. The 550 V, 10 kW prototype is modified to accommodate the variable turns-ratio high frequency transformer to test the proposed converter and control. The proposed converter with control can further improve the efficiency at many operating points compared to single turns-ratio semi-DAB with DPS control. The peak efficiency achieved is 98.5%. / Thesis / Doctor of Philosophy (PhD)

Tunability and adaptability for radio frequency (RF) front-ends are highly desirable because they not only enhance functionality and performance but also reduce the circuit size and cost. This thesis presents a number of novel design strategies in DC-DC converters, impedance networks and adaptive algorithms for tunable and adaptable RF based mobile telecommunication systems. Specifically, the studies are divided into three major directions: (a) high voltage switch controller based DC-DC converters for RF switch actuation; (b) impedance network designs for impedance transformation of RF switches; and (c) adaptive algorithms for determining the required impedance states at the RF switches. In the first stage, two-phase step-up switched-capacitor (SC) DC-DC converters are explored. The SC converter has a simple control method and a reduced physical volume. The research investigations started with the linear and the non-linear voltage gain topologies. The non-linear voltage gain topology provides a higher voltage gain in a smaller number of stages compared to the linear voltage gain topology. Amongst the non-linear voltage gain topologies, a Fibonacci SC converter has been identified as having lower losses and a higher conversion ratio compared to other topologies. However, the implementation of a high voltage (HV) gain Fibonacci SC converter is complex due to the requirement of widely different gate voltages for the transistors in the Fibonacci converter. Gate driving strategies have been proposed that only require a few auxiliary transistors in order to provide the required boosted voltages for switching the transistors on and off. This technique reduces the design complexity and increases the reliability of the HV Fibonacci SC converter. For the linear voltage gain topology, a high performance complementary-metaloxide- semiconductor (CMOS) based SC DC-DC converter has been proposed in this work. The HV SC DC-DC converter has been designed in low voltage (LV) transistors technology in order to achieve higher voltage gain. Adaptive biasing circuits have been proposed to eliminate the leakage current, hence avoiding latch-up which normally occurs with low voltage transistors when they are used in a high voltage design. Thus, the SC DC-DC converter achieves more than 25% higher boosted voltage compared to converters that use HV transistors. The proposed design provides a 40% power reduction through the charge recycling circuit that reduces the effect of non-ideality in integrated HV capacitors. Moreover, the SC DC-DC converter achieves a 45% smaller area than the conventional converter through optimising the design parameters. In the second stage, the impedance network designs for transforming the impedance of RF switches to the maximum achievable impedance tuning region are investigated. The maximum achievable tuning region is bounded by the fundamental properties of the selected impedance network topology and by the tunable values of the RF switches that are variable over a limited range. A novel design technique has been proposed in order to achieve the maximum impedance tuning region, through identifying the optimum electrical distance between the RF switches at the impedance network. By varying the electrical distance between the RF switches, high impedance tuning regions are achieved across multi frequency standards. This technique reduces the cost and the insertion loss of an impedance network as the required number of RF switches is reduced. The prototype demonstrates high impedance coverages at LTE (700MHz), GSM (900MHz) and GPS (1575MHz). Integration of a tunable impedance network with an antenna for frequency-agility at the RF front-end has also been discussed in this work. The integrated system enlarges the bandwidth of a patch antenna by four times the original bandwidth and also improves the antenna return loss. The prototype achieves frequency-agility from 700MHz to 3GHz. This work demonstrates that a single transceiver with multi frequency standards can be realised by using a tunable impedance network. In the final stage, improvement to an adaptive algorithm for determining the impedance states at the RF switches has been proposed. The work has resulted in one more novel design techniques which reduce the search time in the algorithm, thus minimising the risk of data loss during the impedance tuning process. The approach reduces the search time by more than an order of magnitude by exploiting the relationships among the mass spring’s coefficient values derived from the impedance network parameters, thereby significantly reducing the convergence time of the algorithm. The algorithm with the proposed technique converges in less than half of the computational time compared to the conventional approach, hence significantly improving the search time of the algorithm. The design strategies proposed in this work contribute towards the realisation of tunable and adaptable RF based mobile telecommunication systems.

A novel, variable-gain switched-capacitor DC-DC converter is designed, constructed and tested. The proposed converter minimizes many of the problems which have traditionally hindered switched-capacitor DC-DC converters. The converter has high efficiency, strong regulation and low output voltage ripple across a wide variation in the line and load. The converter utilizes an optimized switching configuration that contains the maximum number of ideal conversion ratios for the given number of capacitors driven by a two-phase clock. The switched-capacitor converter is controlled by a gain-hopping feedforward control scheme in conjunction with duty-cycle, pulse-width modulation feedback control. The proposed control technique enhances the efficiency and regulation capability of switched-capacitor DC-DC converters, which are typically limited when there is a large variation in the line. Because the converter is optimized, programmable and capable of providing buck and/or boost operation (stepping-up and/or stepping-down the input voltage), the new switched-capacitor DC-DC converter is well-suited for a variety of applications and operating conditions. In addition, a novel algorithm based on graph theory and network analysis is developed which enumerates all possible ideal conversion ratios for a given switched-capacitor DC-DC converter structure. In particular, this algorithm can be used as a design tool to greatly improve the operation of multi-gain switched-capacitor converters, where the aim is to maximize the number of ideal conversion ratios while minimizing the number of switches and capacitors. Furthermore, the structure of all attainable positive, ideal conversion ratios of a two-phase switched-capacitor DC-DC converter, utilizing up to five capacitors, is enumerated. As a result, the design process for switched-capacitor converters is greatly simplified and a suitable converter structure can be more easily selected for a given application. / Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2013-04-03 23:27:24.183

With the increasing interest in offshore wind power, the related technologies, including HVDC networks, are gaining similar levels of attention. For large scale wind farms far from shore, high voltage DC transmission can provide several advantages over traditional high voltage AC transmission. This thesis focuses on DC/DC converters, a core part of the HVDC network, especially for use in the high voltage, high power and offshore wind environment. The thesis examines a wide range of possible DC/DC converter topologies for the application. Different topologies are compared and evaluated in detail for use in a high power situation. Based on these results, three DC/DC converter topologies are selected for more detailed modelling. The simulation processes and results are presented in the thesis, which reveals the limitations and behaviour of the topologies when they are used at the MW level. In addition, the high power semiconductor switching devices are discussed and evaluated for each topology. To assess the suitability of the DC/DC converter topologies in the offshore wind application, the selected converter topologies are also analysed and modelled combined with a PMSG wind turbine. Finally, a down-scaled DC/DC converter prototype is built to verify the analysis and simulation results.