This thesis presents a novel Resonant Switched-Capacitor Converter with Multiple Resonant Frequencies, abbreviated as MRSCC for both high density and efficiency non-isolated large step-down Intermediate Bus Converter (IBC). Conventional Resonant Switched-Capacitor Converter (RSCC) proposed by Shoyama and its high voltage conversion ratio derivation such as Switched-Tank Converter (STC) by Jiang and li employ half sinusoidal-current charge transfer method between capacitors to achieve high efficiency and density operation by adding a small resonant inductor in series to pure switched-capacitor converter's (SCC) flying capacitor. By operating switching frequency to be the same as its resonant frequency, RSCC achieves zero-current turn off operation, however, this cause RSCC and its derivation suffer from component variation issue for high-volume adoption. Derived from RSCC, MRSCC adds additional high frequency resonant component, operates only during its dead-time, by adding small capacitor in parallel to RSCC's resonant inductor. By operating switching frequency higher than its main resonant frequency, MRSCC utilizes double chopped half-sinusoidal current charge transfer method between capacitors to further improve efficiency. In addition, operating switching frequency consistently higher than its resonant frequency, MRSCC provides high immunity towards component variation, making it and its derivation viable for high-volume adoption. / MS / Following the recent trend, most internet services are moving towards cloud computing. Large data applications and growing popularity of cloud computing require hyperscale data centers and it will continue to grow rapidly in the next few years to keep up with the demand [4]. These cutting-edge data centers will require higher performance multi-core CPU and GPU installations which translates to higher power consumption. From 10MWatts of power, typical data centers deliver only half of this power to the computing load which includes processors, memory and drives. Unfortunately, the rest goes to losses in power conversion, distribution and cooling [5]. Industry members look into increasing backplane voltage from 12V to 48V in order to reduce distribution loss. This thesis proposes a novel Resonant Switched-Capacitor Converter using Multiple Resonant Frequencies to accommodate this increase of backplane voltage.

An ever present trend in the power conversion industry is to get higher performance at a lower cost. In a computer server system, the front-end converter, supplying the load subsystems, is typically a multiple output power supply. The power supply unit is custom designed and its output voltages are fully regulated, so it is not very efficient or cost effective. Most of the load systems in this application are supplied by point-of-load converters (POLs). By leaving the output voltage regulation aspect to POLs, the front-end converter does not need to be a fully regulated, multiple output converter. It can be replaced by a dc-dc transformer (DCX), which is a semi-regulated or unregulated, single output dc-dc converter. A DCX can be made using a modular design to simplify expansion of the system capacity. To realize this concept, the DCX block must have a current sharing feature. The current sharing method for a resonant DCX is discussed in this work. To simplify the system architecture, the current sharing method is based on the droop method, which requires no communication between paralleled units. With this method, the current sharing error is inversely proportional to the droop voltage. In traditional DCX implementations, the droop voltage depends on the resistive voltage drops in the power stage, which is not sufficient to achieve the desired current sharing error. The resonant converter has the inherent characteristic that its conversion gain depends on the load current, so the virtual droop resistance can realized by the resonant tank and the droop voltage can be obtained without incurring conduction loss. An LLC resonant converter is investigated for its droop characteristic. The study shows the required droop voltage is achievable at very high switching frequency. To lower the switching frequency, a notch filter is introduced into the LLC resonant tank to increase the sensitivity of the conversion gain versus the operating frequency. The design of the multi-element resonant tank is discussed. Depending soly on the resonant tank, the droop characteristic is largely varied with the component tolerance in the resonant tank. The current sharing error becomes unacceptable. The active droop control is imposed to make the output regulation characteristic insensitive to the component tolerance. The proposed resonant DCX has simpler circuit structure than the fully regulated resonant converter. Finally simulation and experimental results are presented to verify this concept. / Master of Science

This thesis work provides an insight into the design of Force-amplifying Compliant Mechanisms (FaCMs) that are integrated with micromachined resonant accelerometers to increase their sensitivity. An FaCM, by mechanically amplifying the inertial force, enhances the shift in the resonance frequency of the beams used for sensing the acceleration whose effect causes an axial force on the beams. An extensive study on different configurations of resonators namely, single beam resonator, single-ended tuning fork (SETF), and double-ended tuning fork (DETF), is carried out to gain insights about their resonant behavior. The influence of the boundary conditions on the sensor’s sensitivity emerged from the study. We found that not only the force-amplification factor but also the multi-axial stiffness of the FaCM and proof-mass influence the resonance frequency of the resonator as well as the bandwidth of the modified sensor for certain configurations but not all. Thus, four lumped parameters were identified to quantify the effectiveness of an FaCM. These parameters determine the boundary condition of the sensing beams and also the forces and the moment transmitted to them. Also presented in this work is a computationally efficient model, called the Lumped Parameter Model (LPM) for evaluation of the sensitivity. An analytical expression for the frequency-shift of the sensing resonator beams is obtained by considering the FaCM stiffness parameters as well as the lumped stiffness of the suspension of the inertial mass. Various FaCMs are evaluated and compared to understand how the four lumped parameters influence the sensor’s sensitivity. The FaCMs are synthesized using topology optimization to maximize the net amplification factor with the volume constraint. One of the FaCMs outperforms the lever by a factor of six. Microfabrication of resonant accelerometer coupled with FaCM and comb-drive actuator is carried out using a silicon-on-insulator process. Finally, the selection map technique, a compliant mechanism redesign methodology is used for enhancing the amplification of FaCMs. This technique provides scope for further design improvement in FaCMs for given sensor specifications.

High-frequency and large amplitude current is a driving requirement for applications such as induction heating, wireless power transfer, power amplifier for magnetic resonant imaging, electronic ballasts, and ozone generators. Voltage-fed resonant inverters are normally employed, however, current-fed (CF) resonant inverters are a competitive alternative when the quality factor of the load is significantly high. The input current of a CF resonant inverter is considerably smaller than the output current, which benefits efficiency. A simple, parallel resonant tank is sufficient to create a high-power sinusoidal signal at the output. Additionally, input current is limited at the no-load condition, providing safe operation of the system. Drawbacks of the CF resonant inverter are associated with the implementation of the equivalent current source. A large input inductor is required to create an equivalent dc current source, to reduce power density and the bandwidth of the system. For safety, a switching stage is implemented using bidirectional voltage-blocking switches, which consist of a series connection of a diode and a transistor. The series diode experiences significant conduction loss because of large on-state voltage. The control of the output current amplitude for constant-frequency inverters requires a pre-regulation stage, typically implemented as a cascaded hard-switched dc/dc buck converter. The pre-regulation also reduces the efficiency. In this dissertation, a variety of CF resonant inverters with two input inductors and two grounded switches are investigated for an inductive-load driver with loaded quality factor larger than ten, constant and high-frequency (~500 kHz) operation, high reactive output power (~14 kVA), high bandwidth (~100 kHz), and high efficiency (over 95 %). The implementation of such system required to question the fundamental operation of the CF resonant inverter. The input inductance is reduced by around an order of magnitude, ensuring sufficient bandwidth, and allowing rich harmonic content in the input current. Of particular importance are fundamental and second harmonic components since they influence synchronization of the zero-crossing of the output voltage and the turn-on of the switches. The synchronization occurs at a particular frequency, termed synchronous frequency, and it allows for zero switching loss in the switches, which greatly boosts efficiency. The synchronous conditions were not know prior this work, and the dependence among circuit parameters, input current harmonics, and synchronous frequency are derived for the first time. The series diode of the bidirectional switch can reduce the efficiency of the system to below 90 %, and has to be removed from the system. The detrimental current-spikes can occur if the inverter is not operated in synchronous condition, such as in transients, or during parametric variations of the load coil. The resistance of the load coil has a wide variance, five times or more, while the inductance changes as well by a few percent. To accommodate for non-synchronous conditions, a low-loss current snubber is proposed as a safety measure to replace lossy diodes. The center-piece of the dissertation is the proposal of a two-phase zero-voltage switching buck pre-regulator, as it enables fixed frequency and synchronous operation of the inverter under wide parametric variations of the load. The synchronous operation is controlled by phase-shifting the switching functions of the pre-regulator and inverter. The pre-regulator reduces the dc current in the input inductors, which is a main contributor to current stress and conduction losses in the inverter switches. Total loss of the inverter switches is minimized since no switching loss is present and minimal conduction losses are allowed. The dc current in the input inductors, once seen as a means to transfer power to load, is now contradictory perceived as parasitic, and the power is transferred to the load using a fundamental frequency harmonic! The input current to the resonant tank, previously designed to be a square-wave, now resembles a sine-wave with very rich harmonic content. Additionally, the efficiency of the pre-regulator at heavy-load condition is improved by ensuring ZVS for with an additional inductive tank. The dissertation includes five chapters. The first chapter is an introduction to current-fed resonant inverters, applications, and state-of-the-art means to ensure constant frequency operation under load's parametric variations. The second chapter is dedicated to the optimization of the CF resonant inverter topology with a dc input voltage, two input inductors, and two MOSFETs. The topology is termed as a boost amplifier. If the amplifier operates away from the synchronous frequency, detrimental current spikes will flow though the switches since the series diodes are eliminated. Current spikes reduce the efficiency up to few percent and can create false functioning of the system. Operation at the synchronous frequency is achieved with large, bulky, input inductors, typically around 1-2 mH or higher, when the synchronous frequency follows the resonant frequency of the tank at 500 kHz. The input inductance cannot be reduced arbitrarily to meet the system bandwidth requirement, since the synchronous frequency is increased based on the inductance value. The relationship between the two (input inductance and the synchronous frequency) was unknown prior this work. The synchronous frequency is determined to be a complicated mathematical function of harmonic currents through the input inductors, and it is found using the harmonic decomposition method. As a safety feature, a current snubber is implemented in series with the resonant tank. Snubber utilizes a series inductance of cable connection between the tank and the switching stage, and it is more efficient than the previously employed series diodes. Topology optimization and detailed design procedure are provided with respect to efficiency and system dynamics. The mathematics is verified by a prototype rated at 14 kVA and 1.25 kW. The input inductance is reduced by around an order of magnitude, with the synchronous frequency increase of 2 %. The efficiency of the power amplifier reached 98.5 % and might be improved further with additional optimization. Silicon carbide MOSFETs are employed for their capability to operate efficiently at high frequency, and high temperature. The third chapter is dedicated to the development of the boost amplifier's large signal model using the Generalized State-space Averaging (GSSA) method. The model accurately predicts amplifier's transient and steady-state operation for any type of input voltage source (dc, dc with sinusoidal ripple, pulse-width modulated), and for either synchronous or non-synchronous operating frequency. It overcomes the limitation of the low-frequency model, which works well only for dc voltage-source input and at synchronous frequency. As the measure of accuracy, the zero-crossing of the resonant voltage is predicted with an error less than 2° over a period of synchronous operation, and for a range of interest for input inductance (25 μH – 1000 μH) and loaded-quality factor (10 – 50). The model is validated both in simulation and hardware for start-up transient and steady-state operation. It is then used in the synthesis of modulated output waveforms, including Hann-function and trapezoidal-function envelopes of the output voltage/current. In the fourth chapter, the GSSA model is employed in development of the PWM compensation method that ensures synchronous operation at constant frequency for the wide variation of the load. The boost amplifier is extended with a cascaded pre-regulator whose main purpose is to control the output resonant voltage. The pre-regulator is implemented as two switching half-bridges with same duty-cycle and phase-shift of 180°. The behavior of the cascaded structure is the same as of the buck converter, so the half-bridges are named buck pre-regulators. ZVS operation is ensured by putting an inductive tank between the half-bridges. Each output of half-bridges is connected to each of input inductors of the boost to provide the PWM excitation. Using the GSSA model, the synchronous condition and control laws are derived for the amplifier. Properties of the current harmonics in the input inductors are well examined. It is discovered that the dc harmonic, once used to transfer power, is unwanted (parasitic) since it increases conduction loss in switches of the boost. A better idea is to use the fundamental harmonic for power transfer, since it does not create loss in the switches. Complete elimination of the dc current is not feasible for constant frequency operation of the amplifier since the dc current depends on the load coil's resistance. However, significant mitigation of around 55 % is easily achievable. The proposed method improves significantly the efficiency of both the buck pre-regulator and the boost. Synchronous operation is demonstrated in hardware for fixed switching frequency of 480 kHz, power level up to 750 W, input voltage change from 300 V to 600 V, load coil's resistance change of three times, and load coil's inductance change of 3.5 %. Measured efficiency is around 95 %, with a great room for improvements. Chapter five summarizes key contributions and concludes the dissertation. / Ph. D.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2007. / The helicopter is a prime example of a nonlinear multi-body dynamic system that is subjected to numerous forces and motions to which the system must react. When a helicopter, with a conventionally articulated rotor head, is resting on the ground with its rotor spinning, a condition called ground resonance can develop. Ground resonance is a specific self-excited oscillation of the helicopter and is caused by the interaction between the main rotor blades and the fuselage structure. Inertia forces of the blades perform an out-of-phase lagging motion, which reacts with the elastic landing gear of the helicopter. For certain values of the main rotor angular velocity, the frequency of these inertia forces coincides with a natural vibration frequency of the fuselage structure. If this occurs, the inertia forces of the lagging blades produce oscillations of the fuselage, which then further excite the lagging motion of the blades. This interaction is responsible for an instability of conventionally articulated main rotor helicopters, which is called ground resonance. The ground resonance phenomenon is investigated by means of a classical analytical approach in which the ground resonance equations are derived from Euler-Bernoulli beam theory and verified with results in literature. These equations are required to discuss ground resonance stability in further detail and determine the specific regions in which the phenomenon occurs. These results are incorporated in a simplified numerical model using an elastic multiple-body dynamics analysis program called DYMORE to simulate the South African Rooivalk Combat Support Helicopter. DYMORE is a program that offers nonlinear multi-body dynamic analysis code, using the finite element method, which was specifically developed for helicopter modelling. The complexity of helicopter modelling generally requires large amounts of computing power to ensure reasonable processing time. In order to prevent excessive computational time, the numerical model will be simplified in terms of aerodynamic and structural aspects. The scope of the numerical investigation is, therefore, limited to the ground resonance phenomenon without the effect of aerodynamic forces and representing the fuselage as multi-body beam structures of specified stiffness.

The motivation for this research was to apply methods of vibrations testing in order to determine axial loads in the pin-ended truss members of the Breeding Bam. This method of vibrations testing was necessary in order to determine the in-situ axial loads of the truss members in the bam. Other common methods, such as strain gauges, were not useful for this application. This is because strain gauges can only detect changes in strain and therefore only changes in load. However due to the size and weight of the roof at the Breeding Bam, significant axial loads are produced in the truss members. This in-situ axial load due to the dead load of the roof is a significant portion of any additional loading and cannot be ignored. The ultimate goal of determining the axial loads in the truss members was to develop a model for the roof structure of the bam that accurately predicts axial loads in the truss members over a range of loading conditions. Developing such a model was important in order to make a structural assessment ofthe Breeding Bam's roof structure. In order to determine the axial loads in the truss members, acceleration time histories of the individual truss members were collected using wireless accelerometers provided by MicroStrain of Williston, Vermont. Using the Fourier transform, power spectral densities were produced from the raw acceleration time histories. It was from these plots that the resonant frequencies of the truss members were determined. Knowing the resonant frequencies for a member and the beam vibration equation developed for pin-ended members, the axial load of the truss member were calculated. This process was done for each wrought iron truss member for three separate loading conditions. The purpose of this was to provide enough experimental data so that it could be compared with predictions of several proposed frame models of the bam's roof structure. Ultimately a model was chosen that best predicted the axial loads in the truss members based upon the three loading combinations tested. Using this frame model, an assessment of the bam's roof structure could be made.

The increased power consumption and power density demands of modern technologies have increased the technical requirements of DC/DC and AC/DC power supplies. In this regard, the primary objective of the power supply researcher/engineer is to build energy efficient, high power density converters by reducing the losses and increasing the switching frequency of converters respectively. Operating the converter circuits at higher switching frequencies reduces the size of the passive components such as transformers, inductors, and capacitors, which results in a compact size, weight, and increased power density of the converter. Therefore, the thesis work is focussed on the design, analysis and evaluation of isolated converters operating in the 1 - 5MHz frequency region with the assistance of the latest semi conductor devices, both coreless and core based planar power transformers designed in Mid Sweden University and which are suitable for consumer applications of varying power levels ranging from 1 – 60W. In high frequency converter circuits, since the MOSFET gate driver plays a prominent role, different commercially available MOSFET gate drivers were evaluated in the frequency range of 1 - 5MHz in terms of gate drive power consumption, rise/fall times and electromagnetic interference (EMI) and a suitable driver was proposed. Initially, the research was focused on the design and evaluation of a quasi resonant flyback converter using a multilayered coreless PCB step down transformer in the frequency range of 2.7 – 4MHz up to the power level of 10W. The energy efficiency of this converter is found to be 72 - 84% under zero voltage switching conditions (ZVS). In order to further improve the energy efficiency of the converter in the MHz frequency region, the new material device GaN HEMT was considered. The comparisons were made on a quasi resonant flyback DC-DC converter using both the Si and GaN technology and it was found that an energy efficiency improvement of 8 – 10% was obtained with the GaN device in the frequency range of 3.2 – 5MHz. In order to minimize the gate drive power consumption, switching losses and to increase the frequency of the converter in some applications such as laptop adapters, set top box (STB) etc., a cascode flyback converter using a low voltage GaN HEMT and a high voltage Si MOSFET was designed and evaluated using a multi-layered coreless PCB transformer in the MHz frequency region. Both the simulation and experimental results have shown that, with the assistance of the cascode flyback converter, the switching speeds of the converter can be increased with the benefit of obtaining a significant improvement in the energy efficiency as compared to that for the single switch flyback converter. In order to further maximize the utilization of the transformer, to reduce the voltage stress on MOSFETs and to obtain the maximum power density from the converter circuit, double ended topologies were considered. Due to the lack of high voltage high side gate drivers in the MHz frequency region, a gate drive circuitry utilizing the multi-layered coreless PCB signal transformer was designed and evaluated in both a half-bridge and series resonant converter (SRC). It was found that the gate drive power consumption using this transformer was around 0.66W for the frequency range of 1.5 - v 3.75 MHz. In addition, by using this gate drive circuitry, the maximum energy efficiency of the SRC using multilayered coreless PCB power transformer was found to be 86.5% with an output power of 36.5W in the switching frequency range of 2 – 3MHz. In order to further enhance the energy efficiency of the converter to more than 90%, investigations were carried out by using the multiresonant converter topology (LCC and LLC), novel hybrid core high frequency planar power transformer and the GaN HEMTs. The simulated and experimental results of the designed LCC resonant converter show that it is feasible to obtain higher energy efficiency isolated DC/DC converters in the MHz frequency region. The peak energy efficiency of the LCC converter at 3.5MHz is reported to be 92% using synchronous rectification. Different modulation techniques were implemented to regulate the converter for both line and load variations using a digital controller. In order to realize an AC/DC converter suitable for a laptop adapter application, consideration was given to the low line of the universal input voltage range due to the GaN switch limitation. The energy efficiency of the regulated converter operating in the frequency range of 2.8 – 3.5MHz is reported to be more than 90% with a load power of 45W and an output voltage of 22V dc. In order to determine an efficient power processing method on the secondary side of the converter, a comparison was made between diode rectification and synchronous rectification and optimal rectification was proposed for the converters operating in the MHz frequency range for a given power transfer application. In order to maintain high energy efficiency for a wide load range and to maintain the narrow switching frequency range for the given input voltage specifications, the LLC resonant converter has been designed and evaluated for the adapter application. From the observed results, the energy efficiency of the LLC resonant converter is maintained at a high level for a wide load range as compared to that for the LCC resonant converter. Investigations were also carried out on isolated class E resonant DC-DC converter with the assistance of GaN HEMT and a high performance planar power transformer at the switching frequency of 5MHz. The simulated energy efficiency of the converter for the output power level of 16W is obtained as 88.5% which makes it feasible to utilize the designed isolated converter for various applications that require light weight and low profile converters. In conclusion, the research in this dissertation has addressed various issues related to high frequency isolated converters and has proposed solution by designing highly energy efficient converters to meet the current industrial trends by using coreless and core based planar transformer technologies along with the assistance of GaN HEMTs. With the provided solution, in the near future, it is feasible to realize low profile, high power density DC/DC and AC/DC converters operating in MHz frequency region suitable for various applications. / High Frequency Switch Mode Power Supplies

URBAN RESONANCE explores the connections between architectural design and the multifaceted conceptual links to the idea of resonance in sound. This comparison is made in respect to the fact that a building is somewhat required to resonate with its urban context as well as to orchestrate the resonance relating to systems of people, built form and program, within itself. In using the concept of resonance, it is seen that these three systems, although separately definable, work together in one architectural environment – influencing each other in some way whether beneficial or not. I have chosen these three components of architectural design in response to Johannesburg’s urban and social context, and will explore their integration through celebrating their interstitial spaces of influence and resonance with each other. I seek to allow this idea in design, to produce new meaningful social encounters brought about by the exploration of cross-programming in a building coupled with an architectural response that enforces these social ideas. In the framework of Johannesburg, the question is not necessarily what the systems are but maybe rather how they can be unified and brought together in a building design. Sound is one of the least substantially manipulated and considered parts of building design, yet its relationship to inhabitants and their social connections to each other and space is powerful enough to allow people to perceive their surroundings solely through the things that they hear. URBAN RESONANCE is the explicit exploration of collective social ‘being’ in a building that uses sound and music to unify its programmatic and physical design within the context of Braamfontein, Johannesburg. Keywords:

Neste trabalho, é apresentada uma variação da técnica de espectroscopia por cavidade ressonante tipo ring-down CRDS (do acrônimo em inglês Cavity Ring-Down Spectroscopy) para a obtenção simultânea do espectro de absorção de multicomponentes numa faixa espectral larga do visível. Esta nova técnica se resume no uso do espectro supercontinuum (resultante da irradiação de meios não lineares através de lasers de femtossegundo, ou simplesmente gerada por fontes compactas) como fonte de luz para iluminar a cavidade. Neste contexto são descritas as características dos módulos para a montagem de um MC-SC-CRDS (Multicomponent Supercontinuum Cavity Ring-Down Spectroscopy): os pares de espelhos altamente refletivos, a cavidade ressonante e o sistema de detecção. Alguns problemas relacionados à excitação de multimodos, à luz difusa, ao uso efetivo do intervalo dinâmico de detecção, à baixa resolução do instrumento em resolver linhas estreitas de absorção são situados. Apresentamos os espectros de absorção de H2O (políades 4nu, 4nu + delta ) e O2 (transições proibidas de spin b-X) simultaneamente medidos por essa técnica na faixa do visível, e uma comparação com as linhas de absorção baseadas do banco de dados HITRAN é feita para demonstrar a funcionalidade deste método. / In this work, we present a variation of the technique CRDS (Cavity Ring-Down Spectroscopy) to obtain simultaneously a multicomponent absorption spectrum in a broad visible range. This new approach uses the Supercontinuum (SC) spectrum (resulting from irradiation of nonlinear media by femtosecond lasers, or simply generated by compact sources) as a light source to illuminate the cavity. In this context it is described the features of the modules assembling a MC-SC-CRDS (Multicomponent Supercontinuum Cavity Ring-Down Spectroscopy): a set of high refletivity mirrors, the resonant cavity and the detection system. Some problems related to the multimode excitation, stray light, effective use of the dynamic range of the detector, the poor resolution of the instrument to resolve narrow absorption lines are issued. We present the absorption spectra of H2O (polyads 4nu, 4nu + delta ) and O2 (spin-forbidden b-X branch) measured simultaneously by this technique in the visible range and a comparison with the absorption lines based on HITRAN database is made to demonstrate the functionality of this method.

Calculamos a estrutura eletrônica de heteroestruturas semicondutoras III-V e II-VI na aproximação da função envelope, usando o método k.p, na presença de campos elétricos e magnéticos externos. Para isso desenvolvemos um método numérico baseado na técnica das diferenças finitas e no método da potência inversa para resolvermos a equação de massa efetiva. As principais características desse método são a estabilidade, a rápida convergência e a possibilidade de calcular os estados ligados de poços quânticos com formas arbitrárias na presença de campos elétricos e magnéticos. Investigamos inicialmente o tunelamento ressonante de portadores em um poço quântico duplo assimétrico de GaAs/GaAlAs na presença de um campo elétrico aplicado na direção de crescimento e de um campo magnético aplicado no plano perpendicular. Mostramos como as relações de dispersão de um poço quântico simples podem ser obtidas experimentalmente da curva do campo elétrico versus campo magnético ressonante e como, para determinadas intensidades desses campos, podemos colocar os estados eletrônicos e de buracos em ressonância simultaneamente, isto é, mostramos a possibilidade de termos o tunelamento ressonante de éxcitons. Investigamos também o tunelamento ressonante de portadores em um poço quântico duplo assimétrico de CdTe/CdMnTe na presença de um campo elétrico e de um campo magnético aplicados na direção de crescimento. Mostramos então como podemos obter o tunelamento ressonante de elétrons, buracos e também de éxcitons. Outro problema estudado foi a anisotropia do efeito Zeeman com a direção do campo magnético aplicado em um poço quântico simples de CdTe/CdMnTe. Calculamos os níveis de energia e as transições eletrônicas desse sistema em função da intensidade e da direção do campo magnético. Em todos os casos fazemos uma comparação dos nossos resultados com dados experimentais apresentados na literatura. / The electronic structure of III-V and II-VI semiconductor heterostructures has been calculated in the envelope function approximation using the k.p method in the presence of external electric and magnetic fields. A numerical method based on the technique of the finite differences and in the inverse power method has been developed to solve the effective-mass equation. The main features of this method are stability, fast convergence and ability to calculate bound states of quantum wells with arbitrary shapes including electric and magnetic fields. We investigate the resonant tunneling of carriers in an GaAs/GaAlAs asymmetric double quantum well in the presence of an electric field applied in the growth direction and a magnetic field applied in the perpendicular plane. We show how the dispersion relation of a quantum well can be experimentally determined from the electric versus resonant magnetic fields curves. For certain intensities of these fields, we can make both electrons and holes resonate simultaneously, which leads to resonant tunneling of excitons. The resonant tunneling of carriers has also been investigated in a CdTe/CdMnTe asymmetric double quantum well in the presence of an electric field and a magnetic field applied both in the growth direction. For this system we show that how we can obtain resonant tunneling of electrons, holes and excitons. Finally, we have studied the anisotropy of the Zeeman Effect as a function of the direction of the magnetic field for a CdTe/CdMnTe quantum well. We calculated the energy levels and the electronic transitions of this system as a function of the intensity and the direction of the magnetic field. In all the cases we compare our results with available experimental data.