Estudio de Inductancias acopladas y su aplicación a convertidores DC/DC en aplicaciones Aeroespaciales

El presente estudio aborda el tema del inductor acoplado, elemento magnético que se emplea sobre todo en los convertidores de potencia PWM. Entendemos por inductor acoplado, un elemento magnético donde se concentran (acoplan),por ejemplo, todas las inductancias de salida de un mismo convertidor multisalida. Para un convertidor con múltiples salidas de topología tipo Buck , el inductor acoplado estaría constituido por cada una de las inductancias que forman parte del filtro de salida, todas ellas arrolladas sobre un mismo núcleo magnético. Este estudio surge ante la necesidad de utilizar un inductor acoplado con siete bobinados, donde su alta complejidad hace que los métodos de diseño y caracterización existentes no sean precisos o incluso fallen. La tesis se puede dividir en dos bloques. El primero, consiste en el análisis de los métodos de caracterización ya existentes, finalizando el estudio con una propuesta de método de caracterización que permite medir la matriz de inductancia, matriz constituida por inductancias propias y mutuas, para inductores acoplados complejos. Teniendo en cuenta que la matriz de acoplamiento se relaciona directamente con la de inductancias, como se mostrará más adelante, los problemas detectados en los métodos clásicos, es que daban como resultado, por ejemplo, una matriz con valores de acoplamiento mayores a la unidad, siendo el valor máximo e ideal igual a $1$. Además, una matriz se habrá medido correctamente, si es semidefinida positiva (SPD), lo que significa que todos sus valores propios son reales y positivos. Bajo esta condición la matriz representa un elemento físico real. La propuesta del método de caracterización es analizado de forma experimental y comparado con un segundo método de medida, serie y oposición. La parte experimental avala la teórica, y además ha permitido establecer los límites del nuevo método propuesto. Fuera de estos límites el error es demasiado grande. La segunda parte de la tesis se centra en el propio estudio del inductor acoplado. Nuevamente, la bibliografía no facilita herramientas para poder conocer tanto el comportamiento del inductor acoplado, como criterios claros de diseño independientes de la aplicación. El estudio propone un modelo matemático que describe todos los fenómenos descritos en muchos de los artículos que se encuentran en la literatura técnica, como por ejemplo: rizados invertidos, cambios de pendiente de forma brusca, rizados de corriente mayores, menores o incluso cero. Se demuestra que un inductor acoplado de forma ideal posee unas inductancias equivalentes que tienden a ser $m$ veces más grandes que la inductancia sin acoplar, siendo $m$ el número de bobinados. Esto significa que el rizado de corriente se verá reducido $m$ veces. Pero el modelo no explica la existencia del rizado de corriente igual a cero como demuestran algunos estudios y pruebas experimentales. Ampliando el estudio se ha podido demostrar que esto sucede si y solo si el inductor acoplado se encuentra desbalanceado. En ese caso aparece una divergencia en el valor de la inductancia que hacen que la inductancia equivalente tienda a infinito. Gracias al modelo matemático se han podido establecer criterios de diseño con el fin de optimizar el inductor acoplado en masa y volumen. Las verificaciones experimentales ratifican la teoría propuesta, haciendo de las expresiones matemáticas, herramientas para poder conocer previamente los valores de inductancias equivalentes que aparecerán en el circuito, permitiendo así optimizar el diseño cumpliendo las características técnicas deseadas. Con todo ello, el presente estudio presenta una serie de recomendaciones para obtener el diseño óptimo de un inductor acoplado, controlar los rizados de corriente y establecer un rizado de corriente próximo a cero en todas menos en una de las salidas. El método de diseño propuesto ha sido verificado de forma experimental. Como ventajas adicionales, el nuevo método de diseño además reduce tanto la masa como el volumen del inductor acoplado comparado con inductancias desacopladas. Estos resultados son, sin duda, interesante en el desarrollo de proyectos aeroespaciales, donde siempre se busca reducir ambas magnitudes. La tesis finaliza respondiendo las cuestiones planteadas y alcanzando los objetivos fijados. Pero es evidente que a lo largo del estudio han surgido nuevas preguntas que serán el punto de partida de futuras investigaciones.The present study is focused in the topic of coupled inductors, which is a magnetic element used in power converter modules (PCMs) and filters, among other electronic circuits. In a PCM, the coupled inductor couples multiple output inductors of a multi-output converter onto the same magnetic core. The study was triggered by the need to design and measure a coupled inductor of seven windings and the lack of information in technical literature to develop this task successfully. The PhD study is divided in two blocks. The first analyses all characterisation methodologies found in technical literature finding out that the existing methodologies are very limited and of little use for seven-windings coupled inductors. Based on the mathematical condition that the inductance matrix of any magnetic element should be positive semi-definite (PSD), the study researched different techniques to assure that the resulting inductance matrix was PSD. Known characterisation techniques failed in this objective, especially when measuring coupled inductors with more than three windings. A new measuring procedure is proposed, that fulfils the needed requirements of measuring coupled inductors with many windings and with high coupling coefficients. The second part of the PhD, is focused on the behaviour of the coupled inductors under PWM signals. Technical literature does not answer all questions that arise when observing the behaviour of the coupled inductor. A generalised and optimal design of coupled inductors is also missing. Therefore, a new generalised set of equations is proposed, that explains the behaviour of all coupled inductors, in particular, zero-ripple-current conditions and inverted current ripple, both corresponding to an infinite inductance and to a negative inductance. The proposed mathematical model demonstrates that the equivalent inductance of the coupled inductor is ideally “m” times larger (“m” is the number of windings coupled together) than the inductance expected, resulting of the number of turns wound on a given core. But due to the unbalanced situation of the coupled inductors, the equivalent inductance of each winding can grow up to infinity and under this condition it is responsible for zero-ripple current. The deduced mathematical model is then used to propose a design procedure of optimal coupled inductors. When designing close to the unbalanced situation, zero-ripple current can be achieved in all but one winding. This technique can also be used to reduce volume and mass of coupled inductors. All mathematical expressions have been verified experimentally in the thesis. This PhD, therefore, has proposed a new general characterisation method of coupled inductors and a new generalised mathematical model of coupled inductors that explains easily zero-ripple current and negative-ripple current. The new model allows also to design optimised coupled inductors

Temperature-Dependent Dynamic on Resistance in Gamma-Irradiated AlGaN/GaN Power HEMTs

Dynamic RON is a key parameter in terms of device reliability and the efficiency of power-switching converters. In this study, commercial off-the-shelf GaN-on-Si power high-electron-mobility transistors (HEMTs) were irradiated using different regimes of accumulative gamma rays with a 60Co source of photon energy (1.33 MeV), while a base temperature of 53 °C and 133 °C during the irradiation test was applied. This test campaign had the objective of investigating how the combination of gamma irradiation and temperature affects dynamic on-resistance (RON) behaviour. The results indicated that gate voltage bias stress affected the degradation of dynamic on-resistance when irradiation was applied, and that temperature was an accelerating factor in dynamic on-resistance degradation. Finally, we obtained a partial reduction in dynamic RON when a total ionising dose of around 140 krad(SiO2) was applied and the base temperature during the irradiation test was not high

Design of Zero-Ripple-Current Coupled Inductors with PWM signals in Continuous Conduction Mode

Coupled inductors are widely used in multiple outputs and interleaved dc-dc converters. Also filters often use coupled inductors as their inductive part. A generalized design procedure is proposed in this article focused on current ripple minimization and applicable to coupled inductors exposed to pulsewidth modulation signals and in continuous conduction mode. The design provides a very large inductance for all windings but one. Compared to other designs, it adapts to the existing magnetic properties of the magnetic device changing only the inductance ratio, simplifying the design and manufacturing process. It is based on the equivalent inductance value and its divergences. The only assumption applied is that the coupling coefficient among all windings is the same, which is an acceptable approximation in many magnetic core architectures. The theoretical results are experimentally verified. Not only almost zero ripple current is achieved, but also mass and volume is reduced compared to noncoupled inductors. This is an additional advantage of coupled inductors in mass and volume critical applications such as aerospace

Optimal LLC Inverter Design with SiC MOSFETs and Phase Shift Control for Induction Heating Applications

This work presents the analysis of a converter based on an LLC resonant output inverter and its optimal design used in induction heating applications. The new optimal design method improves several operating parameters that leads to an optimization of the dimensioning of the components of the converter. Additionally, this converter achieves an output power factor that can be considered optimal since it allows to minimize the reactive power of the resonant circuit components and reduces the rms values of the output current of the inverter and the current of its switching devices in relation to that found in traditional designs. A complete study of the circuit based on classic models is carried out to introduce simple rules for the design of this type of inverter for induction heating applications and to control its output power based on a phase shift system (PS). Since the inverter is made with silicon carbide (SiC) MOSFET transistors, an efficiency greater than 99% is reached. The experimental results were obtained from the test of a 12 kW 20 kHz converter for induction heating application

Mitochondria as sensors and regulators of calcium signalling.

During the past two decades calcium (Ca(2+)) accumulation in energized mitochondria has emerged as a biological process of utmost physiological relevance. Mitochondrial Ca(2+) uptake was shown to control intracellular Ca(2+) signalling, cell metabolism, cell survival and other cell-type specific functions by buffering cytosolic Ca(2+) levels and regulating mitochondrial effectors. Recently, the identity of mitochondrial Ca(2+) transporters has been revealed, opening new perspectives for investigation and molecular intervention