As a scalable, high-efficiency, and simple converter topology, an interleaved, multiphase buck converter has been widely used to power microprocessors in information industry. As modern microprocessors continuously advance, the required current for high-performance microprocessors used in data center applications could be several hundreds of amperes with a current slew rate larger than 1000 A/μs. This poses great challenges for a high-efficiency, high-power-density voltage regulator design with a fast transient response. On the other hand, the design challenges of voltage regulators in mobile applications are also increasing due to the stringent requirement on the device thickness and the battery life. In a multiphase buck converter, discrete inductors are widely used as energy storage elements. However, this solution has a limited transient response with a large size of magnetic components. To overcome these issues, coupled inductor is proposed to realize a small steady-state current ripple, a fast transient response, and a small inductor size at the same time. Although lots of studies have been conducted in the topic of the coupled inductor, there are still several challenges unsolved in this area. These challenges are addressed through a comprehensive study in this dissertation. First, a comprehensive analysis of different coupled inductor structures is crucial to identify the benefits and limitations of each inductor structure and provide design guidance under different application requirements. Based on the coupling mechanism, different coupled inductor structures are categorized as a direct-coupled inductor (DCL), an indirect-coupled inductor (ICL) or a hybrid-coupled inductor (HCL) in this work. The performance of these three types of coupled inductors is analyzed in detail through the equivalent inductance analysis and the magnetic flux analysis. For the applications that require a small phase number, a DCL can achieve the smallest inductor size with a given inductance requirement. As the phase number increases, it is beneficial to use an ICL and an HCL due to their symmetrical, simple, and scalable inductor structures. As compared to an ICL, an HCL can achieve a smaller inductor size due to the flux-cancellation effect. The difference between a DCL, an ICL and an HCL are revealed quantitively with several design examples through this study. Second, the steady-state inductance (Lss) and the transient inductance (Ltr) are two key design parameters for coupled inductors. A large Lss and a small Ltr are preferred from the circuit performance point of view. However, there is a design conflict in an ICL and an HCL under the inductor size constraint, where reducing Ltr also results in a smaller Lss. A variable coupling coefficient concept is proposed to overcome this issue. With the same Lss, the proposed method can achieve a smaller Ltr during load transients as compared with the conventional method. This concept is realized by applying a nonlinear inductor in the additional winding loop with the current in this loop as the control source. Compared with the conventional structure, the proposed structure can achieve a great output voltage spike reduction and output capacitance reduction. Third, although an ICL and an HCL are promising candidates for multiphase coupled inductors, an extra inductor is required in the additional winding loop to adjust the coupling coefficient. This additional inductor occupies extra space. To shrink the total inductor size, several improved magnetic core structures are proposed to achieve the controllable coupling through the magnetic integration for an ICL and an HCL. Furthermore, the thickness of the core plate can be significantly reduced by the improved core structure for an HCL. Overall, it is demonstrated that the inductor footprint is greatly reduced by the proposed core structure, as compared with the conventional solution. Lastly, a novel PCB-embedded coupled inductor structure is proposed for a 20MHz integrated voltage regulator (IVR) for mobile applications. To achieve a small inductor footprint and a low profile, the inductor structure with a lateral flux pattern and direct coupling is adopted. Compared with the state-of-the-art solution, the proposed structure can adjust the coupling in a simple core structure by changing the inductor winding pattern. The proposed structure integrates multiple inductors into one magnetic core and is embedded into PCB with a total thickness of 0.54 mm. In contrast to prior arts, the proposed inductor structure features a large inductance density and quality factor with a much smaller DC resistance (DCR), thus is seen as a promising candidate for IVR applications. / Doctor of Philosophy / As modern microprocessors continuously advance in the information industry, the required current for high-performance microprocessors used in data center applications could be several hundreds of amperes with a current slew rate larger than 1000 A/μs. This poses great challenges for the power converter design. On the other hand, the design challenges of power converters in mobile applications are also increasing due to the stringent requirement on the device thickness and the battery life. As a scalable, high-efficiency, and simple converter topology, an interleaved, multiphase buck converter has been widely used to power these processors. In a multiphase buck converter, discrete inductors are widely used as energy storage elements. However, this solution has a limited transient response with a large size of magnetic components. To overcome these issues, coupled inductor is proposed to realize a small steady-state current ripple, a fast transient response, and a small inductor size at the same time. Although lots of studies have been conducted in the topic of the coupled inductor, there are still several challenges unsolved in this area. These challenges are addressed through a comprehensive study in this dissertation. First, a comprehensive analysis and comparison of different coupled inductor structures is crucial to identify the benefits and limitations of each inductor structure and provide design guidance under different application requirements. Based on the coupling mechanism, different coupled inductor structures are categorized as a direct-coupled inductor (DCL), an indirect-coupled inductor (ICL) or a hybrid-coupled inductor (HCL) in this work. The performance of these three types of coupled inductors is analyzed in detail through the equivalent inductance analysis and the magnetic flux analysis. The difference between a DCL, an ICL and an HCL are revealed quantitively with several design examples through this study. Second, the steady-state inductance (Lss) and the transient inductance (Ltr) are two key design parameters for coupled inductors. A large Lss and a small Ltr are preferred from the circuit performance point of view. However, there is a design conflict in an ICL and an HCL under the inductor size constraint, where reducing Ltr also results in a smaller Lss. A variable coupling coefficient concept is proposed to overcome this issue. This concept is realized by applying a nonlinear inductor in the conventional structure. Compared with the conventional structure, the proposed structure can achieve a great output voltage spike reduction and output capacitance reduction. Third, although an ICL and an HCL are promising candidates for multiphase coupled inductors, an extra inductor is required in the additional winding loop to adjust the coupling coefficient. This additional inductor occupies extra space. To shrink the total inductor size, several improved magnetic core structures are proposed to achieve the controllable coupling through the magnetic integration for an ICL and an HCL. Lastly, a novel PCB-embedded coupled inductor structure is proposed for a 20MHz integrated voltage regulator (IVR) for mobile applications. Compared with the state-of-the-art solution, the proposed structure can adjust the coupling in a simple core structure by changing the inductor winding pattern. In contrast to prior arts, the proposed inductor structure features a large inductance density and quality factor with a much smaller DC resistance (DCR), thus is seen as a promising candidate for IVR applications.

No description available.

Point-of-load (POL) converters have been used extensively in IT products. Today, almost every microprocessor is powered by a multi-phase POL converter with high output current, which is also known as voltage regulator (VR). In the state-of-the-art VRs, the circuits are mostly constructed with discrete components and situated on the motherboard, where it can occupy more than 1/3 of the footprint of the motherboard. A compact POL is desirable to save precious space on motherboards to be used for some other critical functionalities. Recently, industry has released many modularized POL converters, in which the bulky inductor is integrated with the active components to increase the power density. This concept has been demonstrated at current levels less than 5A and power density around 600-1000W/in³. This might address the needs of small hand-held equipment such as smart phones, but it is far from meeting the needs for the applications such as laptops, desktops and servers, where tens and hundreds of amperes are needed. A 3D integrated POL module with an output current of tens of ampere has been successfully demonstrated at the Center for Power Electronic Systems (CPES), Virginia Tech. In this structure, the inductor is elaborated with low temperature co-fire ceramic (LTCC) ferrite, as a substrate where the active components are placed. The lateral flux inductor is proposed to achieve both a low profile and high power density. Generally, the size of the inductor can be continuously shrunk by raising the switching frequency. The emerging gallium-nitride (GaN) power devices enable the creation and use of a multi-MHz, high efficiency POL converter. This dissertation firstly explores the LTCC inductor substrate design in the multi-MHz range for a high-current POL module with GaN devices. The impacts of different frequencies and different LTCC ferrite materials on the inductor are also discussed. Thanks to the DC flux cancellation effect, the inverse coupled inductor further improves the power density of a 20A, 5MHz two-phase POL module to more than 1kW/in³. An FEA simulation model is developed to study the core loss of the lateral flux coupled inductor, which shows the inverse coupling is also beneficial for core loss reduction. The ceramic-based 3D integrated POL module, however, is not widely adopted in industrial products because of the relatively high cost of the LTCC ferrite material and complicated manufacturing process. To solve that problem, a printed circuit board (PCB) inductor substrate with embedded alloy flake composite core is proposed. The layerwise magnetic core is laminated into a multi-layer PCB, and the winding of the inductor then is formed by the copper layers and conventional PCB vias. As a demonstration of system integration, a 20A, 1.5MHz integrated POL module is designed and fabricated based on a 4-layer PCB with embedded flake core, which realizes more than 85% efficiency and 600W/in³ power density. The application of standardized PCB processes reduces the cost for manufacturing the integrated modules due to the easy automation and the low temperature manufacturing process. Combining the PCB-embedded coupled inductor substrate and advanced control strategy, the two-phase 40A POL modules are elaborated as a complete integrated laptop VR solution. The coupled inductor structure is slightly modified to improve its transient performance. The nonlinearity of the inductance is controlled by adding either air slots or low permeability magnetic slots into the leakage flux path of the coupled inductor. Then the leakage flux, which determines the transient response of the coupled inductor, can be well controlled. If we directly replace the discrete VR solution with the proposed integrated modules, more than 50% of the footprint on the motherboard can be saved. Although the benefits of the lateral flux inductor have been validated in terms of its high power density and low profile, the planar core is excited under very non-uniform flux. Some parts of the core are even pushed into the saturation region, which totally goes against the conventional sense of magnetic design. The final part of this dissertation focuses on evaluating the performance of the planar core with variable flux. The counterbalance between DC flux and AC flux is revealed, with which the AC flux and the core loss density are automatically limited in the saturated core. The saturation is essentially no longer detrimental in this special structure. Compared with the conventional uniform flux design, the variable flux structure extends the operating point into the saturation region, which gives better utilization of the core. In addition, the planar core with variable flux also provides better thermal management and more core loss reduction under light load. As conclusions, this research first challenges the conventional magnetic design rules, which always assumes uniform flux. The unique characteristics and benefits of the variable flux core are proved. As an example of taking advantages of the lateral flux inductor, the PCB integrated POL modules are proposed and demonstrated as a high-density VR solution. The integrated modules are cost-effective and ready to be commercialized, which could enable the next technological innovation for the whole computing and telecom industry. / Ph. D.

This document explores the isolated multiple-input single ended primary inductor converter (IMISEPIC) and discusses its application. This thesis proposes the following control methods such as current feed-forward control, voltage feedback control and maximum power point control to analyze the IMISEPIC. Zero-ripple technique is also applied to IMISEPIC in order to increase the converter’s life-time. Design strategy and concerns about the IMISEPIC are also presented, and simulations and circuit experiments are conducted to verify the analysis. Finally, the discussion about control limitation is used for future design consideration. / text

With the rapid growth in the wireless industry there has been increasing demand to make devices with better performance. This means lower power, lower voltage, smaller, and in general more efficient. This has lead to the interest in and necessity for good quality passive components. Good quality passive components make better filters, baluns, voltage controlled oscillators, and matching networks. There has been a lot of work over the last ten years focused on improving the quality of inductors. Typical inductors fabricated on silicon have Q factors of approximately 10. This is because silicon is conductive and therefore acts like a lossy ground plane and develops interfering currents. Improvements that have been attempted include thicker metal layers, thicker dielectric layers, patterned ground shields, as well as using multiple metal layers. These methods, however, still do not improve inductors to the quality of those built on insulating substrates such as glass. The main successful attempt on silicon has been where the inductor coil is released so that it is in the air supported by posts. In some work the inductor coil is raised 50 to 100??m above the underpass by methods like etching or photoresist molding. The suspended inductor approach was applied to an insulating substrate to fabricate and characterize unique suspended inductors and transformers. Inductors were released to have 1??m of air underneath the coil by the use of a release etch. Transformers were made in a similar way except two released layers where used. The top coil, done in plated gold, was released as well as an interconnection layer. Such a small air gap and the transformers with two released metal layers are a couple of the unique features of this thesis work. The devices were characterized up to 20GHz before and after release. An improvement in the peak Q factor (up to 70), as well as in the self-resonance frequency (up to 4GHz higher) was noticed after release. This is expected due to the reduction in parasitics. The results were then compared with simulations and a couple closed form expressions, both of which were able to give a reasonable accuracy. There was also success in getting good high frequency transformers. Even though some good high-Q inductors were fabricated as part of this thesis, there is still further work that can be done. This includes packaging, integration with capacitors, and further optimization.

Indutores Ativos são circuitos que quando utilizados se mostram como uma opção viável para melhorar o aproveitamento de área do chip e o fator de qualidade do indutor, comparado com indutor passivo, além de possibilitar o ajuste de parâmetros. Neste trabalho foram estudadas três topologias e abordagens encontradas na literatura para indutores ativos: indutor ativo simples, indutor ativo cascode, indutor ativo com resistência de realimentação. Propomos uma técnica para garantir que o indutor ativo não apresente pólos com parte real positiva, quando conectado a um circuito RC externo, através do cancelamento entre um pólo e um zero. Propomos também uma nova abordagem de projeto para a topologia indutor ativo com resistência de realimentação a qual chamamos de indutor ativo com baixa resistência de realimentação. Para estudo de aplicabilidade foi projetado um LNA (Low Noise Amplifier) utilizando a abordagem de projeto proposta. O amplificador deve atender requisitos de ganho, frequência de operação, impedância de entrada, consumo de potência, figura de ruído além de estabilidade para cargas de saída (pólos com parte real sempre positiva), utilizando o indutor ativo com baixa resistência de realimentação. / Active inductors are circuits that when used prove to be a viable option to improve chip area usage and the inductor\'s quality factor, compared to the passive inductor, while also allowing parameter adjustment. This work studies three topologies and approaches found in literature for active inductors: simple active inductor, cascode active inductor, active inductor with feedback resistance. We propose a technique to guarantee that the active inductor doesn\'t present poles with a positive real part, when connected to an external RC circuit, through cancelling between a pole and a zero. We also propose a new project approach for the topology of the active inductor with feedback resistance which we call low feedback resistance active inductor. To assess the applicability, a LNA (Low Noise Amplifier) was projected using the proposed project approach. The amplifier must meet the requirements regarding gain, operation frequency, input impedance, power consumption, noise figure and also stability for output loads (poles with an always negative real part), using the low feedback resistance active inductor.

A crescente necessidade de produzir circuitos integrados (CIs) cada vez mais miniaturizados para aplicações na faixa de microondas (frequências acima de 1 GHz) com baixo custo de produção e baixo consumo de potênca tem motivado a utilização da tradicional tecnologia Complementary Metal Oxide Semiconductor (CMOS) sobre substrato de silício (Si). Uma aplicação de particular interesse em circuitos integrados operando na faixa de microondas a dos indutores ativos. Rotineiramente, estes indutores ativos são fabricados por meio de processos relativamente custosos como aqueles normalmente envolvidos em tecnologias empregando substrato de arsenato de gálio (GaAs). Por outro lado, novas técnicas de litografia CMOS têm possibilitado a construção de transientes MOSFETs alcançando elevadas frequências de operação. Dessa maneira, o objetivo principal deste trabalho é realizar uma investigação da possibilidade de implementação de indutores ativos operando na faixa de microondas empregando uma tecnologia CMOS convencional sobre substrato de silício. Historicamente, a tecnologia CMOS é atrativa devido às suas características de baixo custo de produção, baixo consumo de potência, alta imunidade aos ruídos e também por oferecer maturidade tecnológica. / The growing need to produce integrated circuits (ICs) increasingly miniaturized for applications in the microwave range (frequencies above 1 GHz) with low cost of production and low consumption power has been stimulating the utilization of traditional technology complementary metal oxide semiconductor (CMOS) on silicon (Si) substrate. One application of particular interest in integrated circuits operating in the microwave range is the active inductors. Ordinarily, these active inductors are fabricated by using relatively expensive technological processes like those usually involved in the gallium arsenide (GaAs) technology. Nonetheless, new techniques of lithography applied to CMOS technology have allowed fabricating MOSFETs transistors reaching high frequencies of operation. In this way, the main goal of this work is realize an investigation of the possibility to implement active inductors operating in the microwave range by using traditional CMOS technology, developed on silicon substrate. Historically, the CMOS technology is attractive by its characteristics of low cost of production, low consumption of power, high immunity to noise and also by offering technological maturity.

Inter-chip input-output (I/O) communication bandwidth demand, which rapidly scaled with integrated circuit scaling, requires high performance I/O links to achieve a per pin data rate as high as multi-Gb/s. The design of high-speed links employing forwarded-clock architecture enables jitter tracking between data and clock from low to high frequencies. Considering the impact of clock to data skew, high frequency sampling clock jitter and data jitter become out of phase at receiver, which reduces the timing margin and limits the data rate. The jitter tracking bandwidth (JTB) between data and clock should be optimized to compensate the clock to data skew. System level analysis shows that the wide tunable range of JTB is needed to compensate different amounts of skews. The implementation of bandpass filtering on forwarded-clock path is able to control the JTB through the controlling of Q. This work introduces a method using bandpass filtering to optimize the JTB in high-speed forwarded-clock transceivers, followed by the implementation of active-inductor-based bandpass filter as clock receiver, which has advantages of low-voltage operation, low power as well as low area consumption. Simulation results shows that the designed filter provides controllable JTB over 40 - 600MHz. The bandpass filter is implemented in IBM 90nm CMOS process.

High performance inductors are playing an increasing role in modern communication systems. Despite the superior performance offered by discrete components, parasitic capacitances from bond pads, board traces and packaging leads reduce the high frequency performance and contribute to the urgency of an integrated solution. Embedded inductors have the potential for significant increase in reliability and performance of the IC. Due to the driving force of CMOS integration and low costs of silicon-based IC fabrication, these inductors lie on a low resistivity silicon substrate, which is a major source of energy loss and limits the frequency response. Therefore, the quality factor of inductors fabricated on silicon continues to be low. The research presented in this thesis investigates amorphous Si and porous Si to improve the resistivity of Si substrates and explores amorphous Si as a structural material for low temperature MEMS fabrication. Planar inductors are built-on undoped amorphous Si in a novel application and a 56% increase in quality factor was measured. Planar inductors are also built-on a porous Si and amorphous Si bilayer and showed 47% improvement. Amorphous Si is also proposed as a low temperature alternative to polysilicon for MEMS devices. Tunable RF MEMS inductors and transformers are fabricated based on an amorphous Si and aluminum bimorph coil that is suspended and warps in a controllable manner. The 3-D displacement is accurately predicted by thermomechanical simulations. The tuning of the devices is achieved by applying a DC voltage and due to joule heating the air gap can be adjusted. A tunable inductor with a 32% tuning range from 5.6 to 8.2 nH and a peak Q of 15 was measured. A transformer with a suspended coil demonstrated a 24% tuning range of the mutual coupling between two stacked windings. The main limitation posed by post-CMOS integration is a strict thermal budget which cannot exceed a critical temperature where impurities can diffuse and materials properties can change. The research carried out in this work accommodates this temperature restriction by limiting the RF fabrication processes to 150°C to facilitate system integration on silicon.

With the rapid growth in the wireless industry there has been increasing demand to make devices with better performance. This means lower power, lower voltage, smaller, and in general more efficient. This has lead to the interest in and necessity for good quality passive components. Good quality passive components make better filters, baluns, voltage controlled oscillators, and matching networks. There has been a lot of work over the last ten years focused on improving the quality of inductors. Typical inductors fabricated on silicon have Q factors of approximately 10. This is because silicon is conductive and therefore acts like a lossy ground plane and develops interfering currents. Improvements that have been attempted include thicker metal layers, thicker dielectric layers, patterned ground shields, as well as using multiple metal layers. These methods, however, still do not improve inductors to the quality of those built on insulating substrates such as glass. The main successful attempt on silicon has been where the inductor coil is released so that it is in the air supported by posts. In some work the inductor coil is raised 50 to 100µm above the underpass by methods like etching or photoresist molding. The suspended inductor approach was applied to an insulating substrate to fabricate and characterize unique suspended inductors and transformers. Inductors were released to have 1µm of air underneath the coil by the use of a release etch. Transformers were made in a similar way except two released layers where used. The top coil, done in plated gold, was released as well as an interconnection layer. Such a small air gap and the transformers with two released metal layers are a couple of the unique features of this thesis work. The devices were characterized up to 20GHz before and after release. An improvement in the peak Q factor (up to 70), as well as in the self-resonance frequency (up to 4GHz higher) was noticed after release. This is expected due to the reduction in parasitics. The results were then compared with simulations and a couple closed form expressions, both of which were able to give a reasonable accuracy. There was also success in getting good high frequency transformers. Even though some good high-Q inductors were fabricated as part of this thesis, there is still further work that can be done. This includes packaging, integration with capacitors, and further optimization.