Silicon carbide power devices

Abstract unavailable please refer to PD

Delay-line stabilized microwave oscillator with frequency control

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The technique whereby the frequency of an oscillator is stabilized using an external feedback network incorporating a delay-line is extended in this thesis to provide digital control of the frequency of the oscillator. Novel circuit techniques have been developed and analysed to permit the implementation of the proposed frequency control technique in a compact format, suitable for implementation in either hybrid or monolithic microwave integrated circuits. Two new microstrip circuits have been introduced, namely the three-port ring discriminator and the single PIN diode phase shifter. Frequency control of an oscillator is achieved by digitally controlling the time delay through phase shifters in the feedback path of the oscillator's stabilization circuit. A new microstrip phase shifting circuit has been developed which has the advantage of requiring only one active switching element to achieve each digital bit of phase change. The circuit has been analysed in detail and results obtained at X-band to support the theoretical predictions. As part of the analysis of this new component, and in order to permit a greater degree of precision in the design of the phase changes, some new microstrip design techniques have been introduced. These have led to a more exact design for coupled line phase shifters and to an equivalent circuit to represent the excess phase in microstrip DC breaks. Delay-line stabilization of an oscillator requires the use of a phase sensitive network, or frequency discriminator. Whilst this could be realised on microstrip using conventional circuitry, by interconnecting two hybrid rings, a new circuit component, namely the three-port ring discriminator, was developed to provide a simpler, more compact solution. A rigorous analysis of the new circuit is presented, and the behaviour verified through measurements over the frequency range 8-12GHz. The new single PIN diode phase shifter has been incorporated in the delay path of a three-port ring discriminator, and used to control the frequency of an oscillator. Results are presented for circuits at X-band which show the degree of frequency stabilization that has been obtained, together with the reduction in oscillator phase noise. In addition, the original concept of delayline switching to control the frequency of an oscillator has been extended to yield a further new circuit, based on a three-port ring with a switchedfeed mechanism; results are presented which verify the operation of this new circuit both theoretically and through practical measurement. New techniques for controlling the frequency of microstrip oscillators have thus been established, both theoretically and through practical measurement, which combine simple methods of frequency selection with inherent low-noise performance

Miniaturised and reconfigurable planar filters for ultra-wideband applications

An increasing demand for electromagnetic spectrum has resulted from the emergence of feature-rich and faster throughputs wireless applications. This necessitates the developments of dynamic reconfigurable or multifunctional systems to better exploit the existing spectrum. Future wireless devices will be expected to communicate over several bands with various other devices in order to fine tune the services they provide to the user. Each band may require a separate RF transceiver and such modern wireless multi-band multi-mode communication systems call for high performance, highly integrated compact modules. Since the Federal Communications Commission (FCC) released the unlicensed frequency band 3.1-10.6 GHz for ultra-wideband (UWB) commercial communications, the development race for commercialising UWB technology has seen a dramatic increase around the world. The aim of this research is to develop reconfigurable planar microwave filters for ultrawideband applications. The project investigates some key design issues of reconfigurable filters, which are being observed constantly in the latest development and realisation of microwave filters. Both analytical and numerical methods are performed to construct a realistic and functional design. Two different types of frequency reconfigurability are investigated in this thesis: discrete (e.g. PIN diode, Optical switch) and continuous (e.g. varactor diode). Using the equivalent circuits and considering the direct coupled filter structure in most cases, several topologies with attractive features are developed for future communication systems. The proposed works may be broadly categorised into three sections as follows. The first section explores a square ring shape close loop resonator along with an opencircuited stub in the symmetry plane. To realise a reconfigurable frequency states within the same spectrum, an innovative approach is developed for this case. An optical or photoconductive switch, comprised of a silicon die activated using near infrared light is investigated as a substitute of PIN diode and performances are evaluated to compare the feasibilities. In addition, a in-band interference rejection technique via externally coupled Tshape resonator is shown. However, it is observed that both structures achieve significant size reductions by utilising the inner part of the resonators. To improve the filter selectivity, a convenient design approach generating a pair of transmission zeros between both passband edges and a single zero in the stop band for harmonic suppression is discussed in the second section. Moreover, the development of notched rejection bands are studied and several novel methods to create a single and multiple notched bands employing the square ring shape structure are proposed. On inspection, it is found that the notch structure can be implemented without deteriorating the filter performances. The discussions are supplemented with detailed design examples which are accompanied by theoretical, simulated and experimental results in order to illustrate the filter development process and showcase practical filter performance. The third section reveals a novel highly compact planar dual-mode resonator with sharp rejections characteristics for UWB applications. A bandwidth reconfiguring technique is demonstrated by splitting its even-mode resonance. Filter structure with the dual-mode resonator is shown to have a relatively wide tuning range, significantly low insertion loss and a constant selectivity along with frequency variations in comparison to similar published works. Finally, the earlier dual-mode structure are modified to realise a dual wideband behaviour. A detail analysis with comprehensive design procedures is outlined and a solution for controlling the frequency bandwidths independently according to the application interest is provided. In line with the previous section, experimental verification is presented to support and supplement the discussions

Reliability analysis of planar and symmetrical & asymmetrical trench discrete SiC Power MOSFETs

Silicon Carbide MOSFETs are shown in research to outperform Silicon counterparts on many performance metrics, including switching rates and power losses. To further improve their performance, trench and double-trench structures have recently been developed. To replace conventional planar SiC MOSFETs, besides the performance parameters which are mostly stated in datasheets, reliability studies under stress are also needed. This thesis presents a comprehensive comparison between 3rd generation trench SiC power MOSFETs, namely symmetrical double-trench and asymmetrical trench with planar SiC power MOSFETs on four aspects of: switching slew rates (dI/dt & dV/dt), crosstalk characteristics, bias temperature instability and power cycling stability.First, the dynamic performance in both 1st quadrant and 3rd quadrant has been eval- uated on the differences in stress by dI/dt & dV/dt and resultant losses. This is key in understanding many other reliability criterions, i.e. severity of crosstalk induced switchings. In the 1st quadrant, the source current and drain-source voltage switching rates at both turn-ON and turn-OFF are measured under a range of test conditions. Both the symmetrical and asymmetrical trench MOSFETs have up to 2 times faster voltage and current slew rates compared with the planar one. They also indicate only slight changes in switching rate with junction temperature. In the 3rd quadrant, the reverse recovery peak current and total reverse recovery charge are measured with respect to junction temper- ature and load current level. Both the symmetrical and asymmetrical trench MOSFETs have less than half of the reverse recovery charge of that of the planar SiC MOSFET.In the evaluation of crosstalk characteristics, peak shoot-through current and induced gate voltage at crosstalk are measured with respect to junction temperature and external gate resistance. With particularly large external gate resistances connected to intentionally induce parasitic turn-ON, the symmetrical double-trench MOSFET is shown to be more prone to crosstalk with 23 A peak shoot-through current measured while it is only 10 A for asymmetrical trench and 4 A for planar MOSFET under similar test conditions. As the temperature increase, the peak shoot-through current drops for the symmetrical double-trench, while constant for the asymmetrical trench and rising for the planar device.Threshold voltage drift is also measured to reflect the degradation happened with bias temperature instability at various junction temperatures, stressing voltages and time periods. Under low-magnitude gate stress (within the range of datasheets) in both positive and negative bias cases, there is more threshold drift observed on the two trench MOSFETs at all junction temperatures than the planar MOSFET. When the stress magnitude is raised, there is less threshold drift observed on the two trench MOSFETs.To evaluate the ruggedness in continuous switchings, the devices are placed under repetitive turn-ON events. The thermal performance under such operation are compared. The asymmetrical trench MOSFET experiences the highest case temperature rise while the least is observed for the planar MOSFET. With an external heatsink equipped to achieve more efficient cooling, the repetitive turn-ON test transforms into the conventional power cycling. In this condition, both the symmetrical and asymmetrical trench MOSFETs fail earlier than the degraded (but not failed) planar MOSFET

Investigation into the use of active frequency selective surfaces to extend the absorption bandwidth of a conventional Salisbury screen absorber

It is well accepted that the absorption bandwidth of a metal back-plane absorber, built with either dielectric or magnetic materials, is inherently narrow. It is also well known that in order to increase the absorption bandwidth, the absorber thickness must be increased through decreasing the permittivity or permeability of its spacer. This improved performance, however, comes at a cost. The absorption bandwidth is increased at the expense of not only an increase of absorber thickness, specially at lower frequencies, but also the yielding of a mechanically weaker structure. The most important implication of the former is that there is a tradeoff between absorber thickness and absorption bandwidth. These two conflicting absorber properties are, however, of equal importance since the optimum absorber is one which has a small thickness as well as a wideband absorption response. This inherent trade-off is due to the fundamental frequency limitations imposed by the constitutive parameters of materials and is more detrimental at microwave frequencies.The aim of the research programme described in this thesis was thus to investigate the use of adaptive complex impedance structures, in the form of active frequency selective surfaces (AFSSs), to extend the absorption bandwidth of a small thickness Salisbury screen absorber, thus addressing directly the aforementioned by minimizing the trade-off that exists between absorber thickness and absorption bandwidth

Technical Design Report for PANDA Electromagnetic Calorimeter (EMC)

This document presents the technical layout and the envisaged performance of the Electromagnetic Calorimeter (EMC) for the PANDA target spectrometer. The EMC has been designed to meet the physics goals of the PANDA experiment. The performance figures are based on extensive prototype tests and radiation hardness studies. The document shows that the EMC is ready for construction up to the front-end electronics interface