Circuits for conditioning AC power supplied to computers are under development. A power conditioner of this type would be an interface between a conventional AC power line and a computer power supply (that is, an AC-to- DC converter) that contains one or more rectifier(s) and inductor(s). Typically, such a power converter is characterized by a power factor <1, and, because it presents a nonlinear load to the power line, it injects currents at harmonics of the power-line frequency back into the power line. These harmonic currents can cause interference with the operations of other electronic equipment as well as overheating of power-line transformers.

The MOSFETs S and S´ Are Switched on and off periodically to effect pulse-width modulation to shape the source current to a purely sinusoidal waveform when presented with a nonlinear load like a computer power supply.

The main purpose served by the power conditioner is to prevent injection of the harmonics into the power line and to bring the power factor up to 1. It would not be necessary to modify either the computer power supply or the AC-power-distribution system. Instead, the power conditioner could be manufactured as a plug-in unit that could simply be inserted between an AC outlet and the computer AC-power plug. Hence, it would be easy to retrofit a previously constructed system with power conditioners.

The effects of power factors

The present developmental power conditioners can be characterized as boost AC-to-AC converters. Like the UPF rectifiers, these power converters utilize current shaping. In particular, they utilize pulse-width modulation as a versatile means of shaping currents to control the flow of power. The modulation in such a power converter is controlled by a combination of an inner average- current loop and an outer voltage-control loop. Together, these control loops maintain a regulated output voltage while forcing the input current to be sinusoidal.

The figure is a simplified schematic diagram of such a power conditioner for a single-phase power line. When the metal oxide semiconductor field-effect transistors (MOSFETs) labeled S are turned on, the magnitude of source current through the boost inductors increases. When the MOSFETs labeled S´ are turned on, the magnitude of the current through the boost inductors decreases. Hence, by suitably modulating the switching of S and S´, the source current can be controlled to be sinusoidal.

This work was done by David Lofftus of GSE Technology Applications and Giri Venkataramanan of Montana State University for Kennedy Space Center. For further information, access the Technical Support Package (TSP) free on-line at  under the Electronic Components & Systems category.