A sinusoidal PWM series voltage regulator

This article presents low cost pulse width modulated (PWM) ac voltage regulator that utilizes series voltage boost technology. The circuit does not require any kind of synchronization with the source mains. The basic component of the PWM ac regulator design is a series boost transformer in which the primary voltage is controlled by a PWM control circuit. The control circuit is composed of a rectifier bridge and one transistor acting as a switch to energize the primary windings at the precise instant of the ac mains half cycle. The transformer secondary is connected in series with the mains supply. The load is connected across both the supply and the transformer. Voltage sags down to 50 percent have been accommodated for a line voltage of 120 volts and load up to 350 watts in an inexpensive laboratory implementation. The load voltage and current are sinusoidal with minimal harmonic as compared to the conventional phase-controlled competitor. The switching of the modulating devices is at high frequency resulting in the use of simple filtering elements. The device complies with ANSI standard C 84.1-1939 for class A secondary distribution voltage regulation.