Three-phase rectifiers/inverters are widely used in grid-tied applications. Take the electric vehicle (EV) charging systems as an example. Within a certain space designated for the chargers, quick charging yet high efficiency are demanded. According to the current industry practice, with a power rating between 10 and 30 kW, the power density are limited by silicon (Si) power semiconductor devices, which make the systems operate at only up to around 30 kHz. The emerging wide bandgap (WBG) power semiconductor devices are considered as game changing devices to exceed the limits brought by their Si counterparts. Much higher switching frequency, higher power density and higher system efficiency are expected to be achieved with WBG power semiconductor devices. Among different types of WBG power semiconductor devices, Silicon Carbide Metal-Oxide-Semiconductor Field-Effect Transistors (SiC MOSFETs) are more popular in current research conducted for tens of kW power converter applications. However, the commonly adopted hard switching operation in this application still leads to significant switching loss at high frequency operation even for SiC-based systems. With the unique feature that the turn-off energy is almost negligible compared with the turn-on energy, critical conduction mode (CRM) based zero voltage soft switching turn-on operation is preferred for the SiC MOSFETs to eliminate the turn-on loss with small penalty on the conduction loss and on the turn-off loss. With this soft switching operation, switching frequency of SiC-based systems is able to be pushed to more than ten times higher than Si-based systems, and therefore higher power density yet even higher system efficiency can be achieved. The CRM-based soft switching is applied to three-phase rectifiers/inverters under the unity power factor operating condition first. Decoupled CRM-based control is enabled, and the inherent drawback of wide switching frequency variation range at CRM-based operation is overcome by the proposed novel modulation technique. It is the first time that CRM-based soft switching modulation is demonstrated in the most conventional three-phase H-bridge ac–dc converter, and more than three-time size reduction compared with current industry practice yet 99.0% peak efficiency are achieved at above 300 kHz switching frequency operation. Then this proposed soft switching modulation technique is extended to non-unity power factor operating conditions especially for grid-tied inverter system applications. With several improvements on the modulation, a generalized CRM-based soft switching modulation technique is proposed, which is applicable to both the unity and non-unity power factor conditions. With the power factor down to 0.8 lagging or leading according to commercial products, above 98.0% peak efficiency is achieved with the generalized soft switching modulation technique at above 300 kHz switching frequency operation. Furthermore from the aspect of electromagnetic interference (EMI), compared with the traditional Si-based design, CRM operation brings higher differential-mode (DM) EMI noise, and higher dv/dt with SiC MOSFETs brings higher common-mode (CM) EMI noise. What's more, hundreds of kHz switching frequency operation makes the main components of the system EMI spectrum located within the frequency range related to the EMI standard (150 kHz – 30 MHz). Therefore, several methods are adopted for the reduction of EMI noise. The total inductor current ripple is reduced with multi-channel interleaving control in order to reduce DM EMI noise. The balance technique is applied in order to reduce CM EMI noise. With PCB winding coupled inductors, the well-controlled parasitic parameters make the balance technique able to be effective for a uniform reduction of CM EMI noise from 150 kHz to above 20 MHz. In addition, PCB winding based magnetic designs are beneficial to achieving manufacture automation and reducing the labor cost. / Doctor of Philosophy / Power electronics and power conversion are crucial to many applications related to electricity, such as consumer electronics, domestic and commercial appliances, automobiles, data centers, utilities and infrastructure. In today's market, quality and reliability are usually considered as a given; high efficiency (low loss), high power density (small size and weight) and low cost are the main focuses in the design of power electronics products. In the past several decades, significant achievements in power electronics have been witnessed thanks to the silicon (Si) semiconductor technology, especially the Si power semiconductor devices. Nowadays, the development of Si power semiconductor devices is already close to the theoretical limits of the material itself. Therefore, in order to meet the increasing demands from customers in different applications, wide bandgap (WBG) based power semiconductor devices, namely Gallium Nitride (GaN) and Silicon Carbide (SiC), are becoming attractive because of its great potential compared with their Si counterparts. In literature, great contributions have already been made to understanding the WBG based power semiconductor devices. It is exciting and encouraging that some of the GaN-based power electronics products featuring high efficiency, high power density and low cost have been commercialized in consumer electronics applications. However, when pursuing these objectives, previous literature has not shown any applications of high frequency soft switching technology into the high power ac–dc conversion (usually three-phase ac–dc) in a simple way as the low power ac–dc conversion (usually single-phase ac–dc) in consumer electronics products. The key to achieving high efficiency, high power density and low cost is the high frequency soft switching operation. For single-phase ac–dc systems, the research on the realization of soft switching by control strategies instead of additional physical complexity has been intensively conducted, and this technology has also been adopted in the current industry practice. Therefore, the major achievement of this work is the development of a generalized soft switching control strategy for three-phase ac–dc systems, without adding any physical complexity, which is applicable to the simplest and most conventional three-phase ac-dc circuit topology. The proposed soft switching control strategy features bidirectional (rectifiers/inverters) power conversion, active/reactive power transfer, grid-tied/stand-alone modes, and scalability to multi-channel interleaved operation. Furthermore, with high frequency, the integration of magnetic components with embedded windings in the printed circuit board (PCB) becomes feasible, which is also beneficial to achieving electromagnetic compatibility (EMC) and manufacture automation. Based on the proposed control strategy and design methodology, a SiC-based 25-kW three-phase high frequency soft switching rectifier/inverter is developed for various applications such as electric vehicle (EV) charging stations, uninterruptible power supplies (UPS) and renewable energy based utilities.

One of the greatest technological challenges of the world today is reducing the size and weight of the existing products to make them portable. Specifically, in electric vehicles such as electric cars, UAVs and aero planes, the size of battery chargers and inverters needs to be reduced so as to make space for more parts in these vehicles. Electromagnetic Interference (EMI) filters take up a more than 80 % of these power converters, the size of these filters can be reduced by pushing the switching frequency higher. High frequency operation (> 300 kHz) leads to a size in reduction of EMI filters though it also leads to an increase in switching losses thus compromising on efficiency. Thus, soft switching becomes necessary to reduce the losses, adding more electrical components to the converter to achieve soft switching is a common method. However, it increases the physical complexity of the system. Hence, advanced control methods are adopted for today's power converters that enable soft switching for devices specifically ZVS turn-on as the turn-off losses of next generation WBG devices are negligible. Thus, the goal of this research is to discover novel switching algorithms for soft turn-on. The state-of the-art control methods namely CRM and TCM achieve soft turn-on by enabling bi-directional current such that the anti-parallel body diode starts conducting before the device is turned on. CRM and TCM result in variable switching frequency which leads to asynchronous operation in multi-phase and multi-converter systems. Hence, TCM is modified in this dissertation to achieve constant switching frequency, as the goal of this research is to be able to achieve ZVS turn-on for a three-phase converter. Further, Triangular Current Mode (TCM) to achieve soft switching and phase synchronization for three-phase two-level converters is proposed. It is shown how soft switching and sinusoidal currents can be achieved by operating the phases in a combination of discontinuous conduction mode (DCM), TCM and clamped mode. The proposed scheme can achieve soft switching ZVS turn-on for all the three phases. The algorithm is tested and validated on a GaN converter, 99% efficiency is achieved at 0.7 kW with a density of 110 W/in3. The discussion of TCM in current literature is limited to unity power factor assumption, however this limits the algorithm's adoption in real world applications. It is shown how proposed TCM algorithm can be extended to accommodate phase shift with all the three phases operating in a combination of DCM+TCM+Clamped modes of operation. The algorithm is tested and validated on a GaN converter, 99% efficiency is achieved at 0.7 kVA with a density of 110 W/in3. TCM operation results in 33 % higher rms current which leads to higher conduction losses, as WBG devices have lower on-resistance, these devices are the ideal candidates for TCM operation, hence to accurately obtain the device parameters, a detailed device characterization is performed. Further, proposed TCM+DCM+Clamped control algorithm is extended to three-level topologies, the control is modified to extract the advantage of reduced Common Mode Voltage (CMV) switching states of the three-level topology, the switching frequency can thus be pushed to 3 times higher as compared to state-of-the-art SVPWM control while maintaining close to 99 % efficiency. Two switching schemes are presented and both of them have a very small switching frequency variation (6%) as compared to state-of-the-art methods with >200% switching frequency variation. / Doctor of Philosophy / Power supplies are at the heart of today's advanced technological systems like aero planes, UAVs, electrical cars, uninterruptible power supplies (UPS), smart grids etc. These performance driven systems have high requirements for the power conversion stage in terms of efficiency, density and reliability. With the growing demand of reduction in size for electromechanical and electronic systems, it is highly desirable to reduce the size of the power supplies and power converters while maintaining high efficiency. High density is achieved by pushing the switching frequency higher to reduce the size of the magnetics. High switching frequency leads to higher losses if conventional hard switching methods are used, this drives the need for soft switching methods without adding to the physical complexity of the system. This dissertation proposes novel soft switching techniques to improve the performance and density of AC/DC and DC/AC converters at high switching frequency without increasing the component count. The concept and the features of this new proposed control scheme, along with the comparison of its benefits as compared to conventional control methodologies, have been presented in detail in different chapters of this dissertation.

The critical conduction mode (CRM) power factor correction converters (PFC) are widely used in industry for low power offline switching mode power supplies. For the CRM PFC, the main advantage is to reduce turn-on loss of the main switch. However, the large inductor current ripple in CRM PFC creates huge DM EMI noise, which requires a big EMI filter. The switching frequency of the CRM PFC is variable in half line cycle which makes the EMI characteristics of the CRM PFC are not clear and have not been carefully investigated. The worst case of the EMI noise, which is the baseline to design the EMI filter, is difficult to be identified. In this paper, an approximate mathematical EMI noise model based on the investigation of the principle of the quasi-peak detection is proposed to predict the DM EMI noise of the CRM PFC. The developed prediction method is verified by measurement results and the predicted DM EMI noise is good to evaluate the EMI performance. Based on the noise prediction, the worst case analysis of the DM EMI noise in the CRM PFC is applied and the worst case can be found at some line and load condition, which will be a great help to the EMI filter design and meanwhile leave an opportunity for the optimization of the whole converter design. What is more, the worst case analysis can be extended to 2-channel interleaved CRM PFC and some interesting characteristics can be observed. For example, the great EMI performance improvement through ripple current cancellation in traditional constant frequency PFC by using interleaving techniques will not directly apply to the CRM PFC due to its variable switching frequency. More research needs to be done to abstract some design criteria for the boost inductor and EMI filter in the interleaved CRM PFC. / Master of Science

This dissertation presents control, analysis, and design of silicon carbide (SiC)-based critical conduction mode (CRM) high-frequency soft-switching three-phase ac-dc converters (inverter and rectifier). The soft-switching technique with SiC devices grounded in CRM makes the operation of the ac-dc converter at hundreds of kHz possible while maintaining high efficiency with high power density. This is beneficial for rapidly growing fields such as electric vehicle charging, photovoltaic (PV) systems, and uninterruptable power supplies, etc. However, for the soft-switching technique to be practically adopted to real products in the markets, there are a lot of challenges to overcome. In this dissertation, four types of the challenges are carefully studied and discussed to address them. First, the grid-tied inverters used for distributed energy resources, such as PV systems, must continue operating to deliver power to the grid, when it faces flawed grid conditions such as voltage drop and voltage rise. During abnormal grid conditions, delivering constant active power from the inverter to the grid is essential to avoid large voltage ripples on the dc side because it could trigger over-voltage protection or harm the circuitries, eventually shutting down the inverter. Hence, in such cases, unbalanced ac currents need to be injected into the grid. When the grid voltages and the ac currents are not balanced, there is a chance for the CRM soft-switching inverter to lose its soft-switching capability. Continuous conduction mode operation emerges, causing hard-switching where discontinuous conduction mode (DCM) operation is expected. This leads to huge turn-on loss and high dv/dt noise at the active switch's turn-on moment. To eradicate the hard-switching problem, two improved modulation schemes are developed; one with off-time extension in the CRM phase, the other by skipping switching pulses in the DCM phase. The DCM pulse skipping is applied for a variety of grid imbalance cases, and it is proven that it can be a generalized solution for any kinds of unbalanced grid conditions. Second, the CRM soft-switching scheme with 2-channel interleaving achieves high efficiency at heavy load. Nevertheless, the efficiency plunges as the output load is reduced. This is not suitable for PV inverters, which take account of light load efficiency in terms of "weighted efficiency". Small inductor currents at light load cause the switching frequency to soar because of its CRM-based operation characteristic, causing large switching loss. To increase the inductor current dealt with by the first channel, a phase shedding control is proposed. Gate signals for the second channel are not excited, increasing the first channel's inductor current, thus cutting down the first channel's switching frequency. To prevent the unwanted circulating current formed by shared zero-sequence voltage in the paralleled structure, only two phases in the second channel working in high frequency are shed. The proposed phase shedding control achieves a 0.5 to 3.9 % efficiency improvement with light loads. Third, due to the usage of SiC devices, high dv/dt generated at switching nodes over the system parasitic capacitance causes substantial common mode (CM) noise compared to that with Si devices. In this case, a balance technique with PCB winding inductors can effectively reduce the CM noise. First, winding interleaving structure is selected to minimize the eddy current loss in the windings. But the interwinding capacitance caused by the winding interleaving structure aggravates the CM noise. Impact of the interwinding capacitance on the CM noise is analyzed with a new inductor model containing the interwinding capacitance. Then, finally, a novel inductor structure is proposed to remove the interwinding capacitance and to improve the CM noise reduction performance. The soft-switching ac-dc converter built with the final PCB magnetics features almost similar efficiency compared to that with litz-wire inductor and 14 to 18 dB CM noise reduction up to 15 MHz. Lastly, the soft-switching technique is extended to inverters in standalone mode. To meet tight ac voltage total harmonic distortion requirements, a current control in dq-frame is introduced. As for the ac voltage regulation at no-load, on top of the improved phase shedding control, a frequency limiting with fixed frequency DCM method is applied to prevent excessive increase in the switching frequency. Then, how to deal with short-circuit at the output load is investigated. Since the soft-switching modulation violates inductor voltage-second balance during the short-circuit, the modulation method is switched to a conventional sinusoidal PWM at fixed frequency. It is concluded that all the additional requirements for the standalone inverters can be satisfied by the introduced control strategies. / Doctor of Philosophy / The world is facing an unprecedented weather crisis. Global warming is getting more severe because of excessive amount of carbon emission. In an effort to overcome this crisis, paradigm of energy and lifestyle of people have changed. Penetration of distributed energy resources (DERs) such as wind turbines, and photovoltaic systems has been dramatically increased. Instead of internal combustion engine vehicles (EVs), electric vehicles hit the mainstream. In these changes, power electronics plays a critical role as the key element of the systems. Especially, three-phase inverter/rectifiers are essential parts in such applications. Most important aspects of the three-phase inverter/rectifier are efficiency and power density. In the past decades, Silicon (Si) power devices were mostly used for the systems and the technology based on Si has almost reached to its physical limits. The switching frequency of Si-based inverter/rectifier is limited below 20 – 30 kHz to reduce switching loss. This impedes high power density due to bulky passive components such as inductors and capacitors. Nowadays, the advent of wideband gap such as Silicon Carbide (SiC) and Gallium Nitride (GaN) power devices gives us a great opportunity to improve the efficiency and the power density with its high switching speed capability, low switching energy and low on-resistance. The SiC power devices are more suitable for DERs and EVs due to higher voltage rating. Using SiC power devices allows to increase inverter/rectifier' switching frequency about five times to have similar efficiency with those based on Si power devices, making the power density high. However, there is still room to push the switching frequency even higher to hundreds of kHz with soft-switching. In this sense, studies on soft-switching techniques for three-phase inverter/rectifier have been intensively conducted. Particularly, soft-switching techniques based on critical conduction mode (CRM) are regarded as the most promising solutions because it does not have any additional circuits to achieve the soft-switching, keeping the system as straightforward as possible. However, most of the studies for the CRM-based soft-switching three-phase inverter/rectifier mainly focus on limited occasions such as ideal operation conditions. For this technique to be widely used and adopted in industry, more practical cases for the systems need to be studied. In this dissertation, the soft-switching three-phase inverter/rectifier under diverse situations are investigated in depth. First, behavior of the soft-switching inverter/rectifier under unbalanced grid conditions are analyzed and control methods are developed to maintain its soft-switching capability. Second, how to improve light load efficiency is explored. Circulating current issue for the light load efficiency improvement is analyzed and a control method is proposed to eliminate the circulating current. Third, a design methodology and considerations of inductors based on PCB magnetics are discussed to reduce electromagnetic noise and improve system efficiency. Lastly, the soft-switching technique is extended to standalone mode applications dealing with strict voltage regulation, no-load operation, and output short-circuit.

With the increasing demands on electronic loads (e.g. desktop, laptop, monitor, LED lighting and server) in modern technology-driven lives, performance of switched-mode power supply (SMPS) for electronics have been growing to prominence. As front-end converters in typical SMPS structure, ac-dc power-factor correction (PFC) circuits play a key role in regulations of input power factor, harmonics and dc output voltage, which has a decisive effect on entire power-supply performances. Universal ac-line and low-power system (90–264 Vrms, up to 300–400 W) is one of the most common power-supply specifications and boost-derived PFC topologies have been widely used for the purpose. In order to concurrently achieve high efficiency and low-cost system in the PFC stage, zero-current switching (ZCS) control schemes are highly employed in control principles. Representative schemes are discontinuous conduction mode (DCM) and critical conduction mode (CRM). Both modes can realize ZCS turn-on without diode reverse recovery so that low switching losses and low-cost diode utilizations are obtainable. Among various boost-family PFC topologies, three-level boost (TLB) converter has generated considerable research interest in high-voltage high-power applications. It is mainly due to the fact that the topology can have halved component voltage stresses, improved waveform qualities and electromagnetic interference (EMI) from phase interleaved continuous conduction mode (CCM) operations, compared to other two-level boost PFC converters. On the other hand, in the field of universal-line low-power applications, TLB PFC has been thoroughly out of focus since doubled component counts and increased control complexity than two-level topologies are practical burden for the low-cost systems. However, recent researches on TLB PFC with ZCS control schemes have found that cost-competitiveness of the topology is actually comparable to two-level boost PFC converters because the halved component voltage stresses enable usage of low voltage-rating components of which unit prices are cheaper than higher-rating ones. Based on the justification, researches on ZCS control schemes for TLB PFC have been conducted to get enhanced waveform qualities and performance factors. Following the research stream, a three-level current modulation scheme that can be adopted in both DCM and CRM is proposed in Chapter 2 of this dissertation. Main concept of the proposed current modulation is additional degree-of-freedom in current-slope shaping by differentiating on-times of two active switches, which cannot be found from any other single-phase boost-derived PFC topologies. Using the multilevel feature, proposed operations in one switching period consist of three steps: common-switch on-time, single-switch on-time and common-switch off-time. The single-switch on-time step is key design factor of the proposed modulation that can be utilized either in fixed or adjustable form depending on control purpose. Based on the basic modulation concept, three-level CRM control scheme, adjustable three-level DCM control scheme, and spread-spectrum frequency modulation (SSFM) with adjustable three-level DCM scheme are proposed in Chapter 3–5, respectively. In each chapter, implemented control scheme aims to improve different performance factors. In Chapter 3, the proposed three-level CRM scheme uses increased single-switch on-time period to reduce peak inductor current and magnitude of variable switching frequency. It is generally accepted fact that CRM operations suffer from high switching losses and poor efficiency at light load due to considerable increment of switching frequency. Thus, efficiency improvement effect by the proposed CRM scheme becomes remarkable as load condition goes lighter. In experimental verifications, maximum improvement is measured by 1.2% at light load (20%) and overall efficiency is increased by at least 0.4% all over the load range. In Chapter 4, three-level DCM control scheme adopts adjustable single-switch on-time period in fixed switching-frequency framework. The purpose of adjustable control scheme is to widen the length of non-zero inductor current period as much as possible so that discontinued current period and high peak current of DCM operations can be minimized. Experiment results show that, compared to conventional two-level DCM control, full-load peak inductor currents are reduced by 20.2% and 17.1% at 110 and 220 Vrms input voltage conditions, respectively. Moreover, due to turn-off switching energy decrements by the turn-off current reductions, efficiency is also improved by at least 0.4% regardless of input voltage and load conditions. In Chapter 5, a downward SSFM technique is developed first for DCM operations of boosting PFC converters including two-level topologies. This chapter aims to achieve significant reduction of high differential-mode (DM) EMI amplitudes from DCM operations, which is major drawback of DCM control. By using the simple linearized frequency modulation, peak DM EMI noise at full load condition is reduced by 12.7 dBμV than conventional fixed-frequency DCM control. On top of the proposed SSFM, the adjustable three-level DCM control scheme in Chapter 4 is adopted to get further reductions of EMI noises. Experimental results prove that the collaborations of SSFM and adjustable DCM scheme reduce the EMI amplitudes further by 2.5 dBμV than the result of SSFM itself. The reduced EMI amplitudes are helpful to design input EMI filter with higher cut-off frequency and smaller size. Different from two-level boosting PFC converters, TLB PFC topology has two output capacitors in series and inherently suffers from voltage unbalancing issue, which can be noted as topological trade-off. In Chapter 6, two simple but effective voltage balancing schemes are introduced. The balancing schemes can be easily built into the proposed ZCS control schemes in Chapter 3–5 and experimental results validate the effectiveness of the proposed balancing principles. For all the proposed control schemes in this dissertation, detailed operation principles, derivation process of key equations, comparative analyses, implementation method with digital controller and experimental verifications with TLB PFC prototype are provided. / Doctor of Philosophy / Electronic-based devices and loads have been essential parts of modern society founded on rapid advancements of information technologies. Along with the progress, power supplying and charging of electronic products become routinized in daily lives, but still remain critical requisites for reliable operations. In many power-electronics-based supplying systems, ac-dc power-factor correction (PFC) circuits are generally located at front-end to feed back-end loads from universal ac-line sources. Since PFC stages have a key role in regulating ac-side current quality and dc-side voltage control, the importance of PFC performances cannot be emphasized enough from entire system point of view. Thus, advanced control schemes for PFC converters have been developed in quantity to achieve efficient operations and competent power qualities such as high power factor, low harmonic distortions and low electromagnetic interferences (EMI) noises. In this dissertation, a sort of PFC topologies named three-level boost (TLB) converter is chosen for target topology. Based on inherent three-level waveform capability of the topology, multiple zero-current switching (ZCS) control schemes are proposed. Compared to many conventional two-level PFC topologies, TLB PFC can provide additional degree-of-freedom to current modulation. The increased control flexibility can realize improvements of various waveform qualities including peak current stress, switching frequency range, harmonics and EMI amplitude. From the experimental results in this dissertation, improvements of waveform qualities in TLB PFC with the proposed schemes are verified with comparison to two-level current control schemes; in terms of efficiency, the results show that TLB PFC with the proposed schemes can have similar converter efficiency with conventional two-level boost converter in spite of increased component counts in the topology. Further, the proposed three-level control schemes can be utilized in adjustable forms to accomplish different control objectives depending on system characteristics and applications. In each chapter of this dissertation, a novel control scheme is proposed and explained with details of operation principle, key equations and digital implementation method. All the effectiveness of proposals and analyses are validated by a proper set of experimental results with a TLB PFC prototype.