Single phase buck-boost AC-AC converter

Abstract

AC voltage converters are widely used as one of the power electronics systems to control an output ac voltage, where a variable ac voltage is obtained from a fixed ac voltage, for power ranges from few watts up to fraction of megawatts. Phase angle control line commutated voltage controllers and integral cycle control of thyristors have been extensively employed in this type of regulators for many applications. Such techniques offer some advantages as simplicity and the ability of controlling large amount of power economically. However, they suffer from inherent disadvantages such as, retardation of the firing angle causes lagging power factor at the input side especially at large firing angles and high low order harmonic content in both of load and supply sides. Moreover a discontinuity of power flow appears at both input and output sides. Solid-state switch based AC-AC PWM regulator similar to the DC-DC converters have been reported to alleviate the problems. They require a bi-directional freewheeling path across the load. As a result, only controlled freewheeling device could be used on the circuits. Also switches in the circuits require synchronized (current- sensed) switching to attain in phase input current to improve power factor. In this thesis a single phase Buck-Boost AC-AC converter has been proposed where a single bi-directional switch has been used. We use the philosophy of half cycle to half cycle conversion individually and connecting the outputs to the load either in wired OR, or in differential mode. Such conversion has the advantage of input current being in phase with supply voltage without additional sensing and control circuits and the high frequency chopped input current can be made near sinusoidal with small input filter resulting in low input current THD and high power factor. The conversion is achieved by Buck-Boost conversion of each half cycle. Hence the AC-AC switch mode conversion proposed in this thesis provides step up/down output voltage. Also, such conversion is achieved with low input current THD, high input power factor, small input filter and high conversion efficiency.