Contributions to design of dsp controlled pulse width modulated ac-ac converters with new generation semiconductor switches

AC-AC Converters are the circuits designed for providing adjustable output voltages from a fixed or variable input voltage. As AC sources and loads are the main parts of the industry, AC-AC Converter is an important topic. These converters have a wide range of application areas such as lighting control, industrial heating, soft starters for induction machines and speed control of pump or fan motors. Basicly, AC-AC Converters could be grouped in two main topics, which are autotransformers and power electronic converters. Autotransformers are conventional forms of AC-AC Conversion systems. They were invented decades ago from the semiconductor switches. They have been widely used in regulation systems. However, there are some important disadvantages of autotransformer based AC converter systems. Size and weight of those devices are significantly higher than the power electronic converters. As autotransformers are heavy and massive devices, it is impossible to use them in weight-restricted systems such as planes, cars or ships. In voltage regulation manner, the response time of the transformers are quite long when comparing with the power electronic converters. The intertia of those servo motors limits the response time of them as servo motors are used for regulation tap changing. To solve this problem, with developing semiconductor technology, thyristor based solid-state power converters have been invented. Their response time is very short when compared to that of autotransformers. Also, with the other developments in semiconductor technology, their power-range is also increased. Although the thyristor based converters solved the slow response problem, they were causing crucial low frequency harmonics in both output and input sides. Besides, their filter size and weight requirements are very high as they are operating at low frequency levels without changing the utility frequency (due to the nature of thyristor). Several decades ago, power transistors having fast response and high electrical limits are started to be produced. In these days, numerous types of high-power IGBT's and MOSFET's can be found in the market. As control techniques are improved, previously mentioned developments in semiconductor technology made AC-AC PWM Converters a beneficial choice for numerous cases needing less harmonic distortion such as motor drive applications, lighting control, industrial heating, regulators and soft starting systems of alternating current machines. In the last decade, Gallium Nitride (GaN) based new generation power semiconductor switches has been announced with their low conduction and switching losses. They are started to be used in several industrial systems and academical studies. Actually, those switches has been used in high-frequency, low-power systems such as radio applications for decades. From the beginning of 2010's, semiconductor production technologies allowed to develop high-voltage/high-current GaN switches and several companies started to produce commercial new generation GaN based semiconductor switches for high-voltage/high-current applications. Main advantages of these new generation switches can be listed as their low on-state-resistance, low turn-on and turn-off times, low power consumptions and lower gate threshold voltages when compared to state-of-art transistors. As in every aspects of engineering, advantages result in some trade-offs, too. These mentioned properties of new generation power switches demand extra attention on the gate drive circuit and PCB routes. As the response time and threshold voltage of the GaN switches are very low, any unfiltered noise may cause dangerous situations for the converter. Due to that reason, noise margin of the gate drive circuit is very narrow and PCB must be designed carefully. To sum up, the main advantages of PWM Power Converters are their lower input and output harmonics, low size, low weight, high quality outputs and high response time. Also, as these transistors can operate at high switching frequencies, size and weight of the filtering elements are also becomes smaller. Since abovely mentioned properties are making PWM AC-AC Converters a good choice for AC-AC Conversion systems, the advantages and disadvantages of installing GaN switches to the PWM AC-AC converters are needed to be investigated. In this thesis, a pulse width modulated digital signal processor controlled single-phase AC-AC converter was designed by GaN switches. The circuit was designed and analysed both in simulation environment and laboratory. The circuit design was improved according to the simulation results and the optimised circuit was tested practically. The relations of load current, load angle, switching frequency between total converter efficiency and input power factor were obtained. Efficient operational limits and performance for a low power PWM controlled AC-AC converter designed by GaN switches were achieved.