A parallel-connected active filter for the reduction of supply current distortion

The use of switching regulators and power supplies in domestic and light industrial environments is becoming more common. Their popularity derives from a number of desirable characteristics, but a highly undesirable feature, often overlooked by the designer, is the highly nonlinear load imposed by these systems on the supply. For a single load, it is not uncommon to find more than 90% of the power extracted during the central 30° of the supply halfwave. When large numbers of these systems are connected to a common supply (for example, a bank may have literally hundreds of PCs in a dealing room in a single building), significant harmonic distortion can be induced in the supply and thus experienced by other loads. In this paper, we discuss a practical implementation of a waveform correcting circuit; the system stores energy in a capacitor and injects it appropriately into the local supply distribution subsystem, such that the overall load experienced by the external supply is sinusoidal. The system is physically small, and dissipates little power in its own right. It is thus suitable for installation in, for example, an office environment