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Abstract

An AC variable voltage source utilizing pulse width modulation is provided that converts an input of two AC voltage waveforms of the same frequency but opposite phase into a single AC voltage output of the same frequency but with a variable amplitude. The variable voltage source includes a control voltage input that determines the amplitude of the AC voltage output. The two AC input voltage waveform are provided by buck and boost windings of an AC input transformer having a primary winding connected to an AC source. The variable voltage source utilizes phase switch stages each connected to a respective one of the buck or boost windings and including back to back power MOSFETS. Each of the phase switch stages is controlled at a switching frequence much higher than the AC input frequency with the duty cycle of operation of each of the phase switch stages determining the amplitude of the controlled output. In one arrangement, an inductor is provided in series with each of the phase switch stages for operation in a current fed mode. In another arrangement, an inductor is provided in series with the common output of the phase switch stages for operation in a voltage fed mode. For line regulation of an AC line, the control voltage is a feedback signal derived from the AC regulated output. For a proportional controller, the control voltage is a DC voltage proportional to the amplitude of the desired AC output voltage.

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