Intentional and Unintentional Light Emission Diode in Power Electronics applications

The phenomenon of light emission in a P-N junction is due to the recombination of electrons and holes in the junction. This light emission phenomenon has been widely used in power electronics applications. Among them, it is worth remembering the use of the light-emitting diode (LED) as a source of light in lighting system applications. In this perspective, LEDs technology has been widely studied as an “Intentional” source of light in Power Electronics. In such a context, a LED light bulb can be assumed as a combination of LED semiconductor materials and a ballast circuit, where the LED driver circuit plays a fundamental role from a power quality point of view. In fact, the driver controls the input current of the LED light bulb that must be compliant with the standards and regulations concerning currents harmonic. Furthermore, the whole lighting system must meet the standards. In this perspective, a sufficient power factor (PF) must be guarantee thanks to a proper design of the lighting system, especially when a very huge number of lamps are used. Hence, the electrical model of each light bulb is fundamental to foresee the effects of different design configurations. In this perspective, my research activity has been firstly focused on providing such a generic circuit model of LED light bulbs which is useful at the design stage of the lighting system to guarantee a high power factor. The proposed circuit model is able to emulate the current drawn of any LED light bulbs, whatever lamp driver is used. The procedure and equations to obtain the components of the equivalent circuit of the LED light bulbs have been provided. It is worth highlighting that such a procedure enables to obtain the model of any LED light bulbs available on the market without the knowledge of its inner components (driver, LED package, and so on). The components in the model of a generic lamp can be obtained by a few simple current measurements to be performed at the lamp terminals. These parameters are the coefficients of the functions obtained by linear or polynomial interpolation of the rms of the current harmonics. The model validation has confirmed the effectiveness and robustness of the proposed solution. In the context of LED light bulbs, my research activity has also focused on the study of the LED drivers topologies. Among the various LED drivers available on the market for integrated lighting applications, the single-stage discontinuous current conduction mode flyback PFC converter has been revealed as the preferred choice in order to achieve a high PF with a good cost-performance ratio. Notwithstanding the use of a suitable control method that enables a high PF, different inherent causes of distortion in the input current, due to the power processing mechanism of the High PF QR flyback converter, have been found. Hence, the different issues due to the correlation of these causes, which are the ringing current, the crossover distortion due to transformer leakage inductance, and crossover distortion due to the input storage capacitor have been widely studied. It is worth noting that the problem of the input current distortion caused by the ringing current has been studied for the first time in a QR flyback. Moreover, the crossover distortion due to the input storage capacitor in the case of a Hi-PF flyback LED driver has never been treated in literature. Finally, some practical design guidelines for mitigating the inherent causes of distortion of the input current in a Hi-PF QR flyback driver for LED applications have been provided. An emerging application of the light emission in a P-N junction is the use of light emission from the body diode of a SiC device for condition monitoring (CM). In this perspective, the LED is an “unintentional” source of light in Power Electronics. More specifically, the reliability of power electronics devices is raising a great interest, especially, in some critical applications. In this perspective, several condition monitoring methods have been widely proposed so far. Among them, the light emission phenomenon from the body diode of a SiC device has been gained more interest in the last decade. In detail, the electroluminescence phenomenon refers to the photoemission from a SiC material and it is related to its intrinsic body diode. The light brightness in the SiC intrinsic body diode depends on the junction temperature, where the light intensity decrease at high temperature. Thus, the electroluminescence phenomenon can be a suitable indicator of the temperature of the SiC MOSFET chip during a general power converter application. In this perspective, my research activity has been firstly focused on the description of several CM methods adopted for power devices. Moreover, the intrusiveness of these condition monitoring methods has been also deeply investigated for the first time. More specifically, three intrusiveness criteria have been considered at the device level, converter operation level, and conversion system level. Then, three figures of merit (FOM) have been accordingly considered and assigned to all the condition monitoring methods by means of a mark based rule. Finally, a unique single-value figure of merit has been obtained from the product of the three figures of merit. An application-based FOM that accounts for the importance of the different intrusiveness criteria in a given application has been also proposed. The use of a single value to compare the different CM methods is the main strength of these overall FOMs but they do not provide a fixed value due to the subjective of the marks. On the other hand, the rank of the CM methods according to an intrusiveness criterion is almost unbiased. Therefore, a strategy to compare the CM methods according to the rank instead of the mark is proposed by borrowing some reasoning from the Pareto optimality. Such an approach has revealed that the CM methods adopting, respectively, Integrated photodiode, IR Camera, Acoustic, and two thermo-sensitive electrical parameters (turn on-off delay time and peak gate current) are the best ones in terms of intrusiveness, and they are equivalent each other from the Pareto optimality point of view. Finally, the Integrated photodiode and the IR Camera CM methods are the less intrusive according to the overall FOM. Therefore, these results have highlighted that the use of the Integrated photodiode is one of the best choices, and considering the SiC MOSFET intrinsic diode as an unintentional diode, a new CM method adopting the Integrated photodiode in a SiC MOSFET module has been devised. More specifically, a non-invasive temperature sensing method for high-voltage SiC MOSFET chips based on the measurement of light emission during reverse conduction has been proposed. The method is based on a compact sensing circuit that can be easily embedded in the package, allowing online temperature estimation. The effectiveness of the circuit has been confirmed by the analysis of a commercial SiC power module. It is worth noting that the proposed temperature sensing strategy is fast, inexpensive, accurate, and of course, non-invasive. By summing up, this work has been focused on the application of “intentional” and “unintentional” light emission diode in Power Electronics. The first, second, and third chapters have been focused on the use of commercial LED light bulbs. More specifically, Chapter 1 describes the power quality issues and the drivers adopted to mitigate them. Chapter 2 reports the analysis of the intrinsic causes of current distortion in high power factor Quasi-Resonant flyback and proposes some mitigation guidelines. Then, in Chapter 3, a circuit model of a generic LED light bulb has been proposed with the aim to emulate the current drawn from a generic LED light bulb. Chapter 4 focuses on the advantages (in terms of intrusiveness) of using unintentional light emission diode for condition monitoring of power devices. Finally, Chapter 5 describes the proposed innovative condition monitoring method based on the integration of a photodiode to correlate the “unintentional” light emission of a SiC material to the temperature of the device.