1992 Volume 112 Issue 4 Pages 402-408
Recently, the influence on power supply systems and electronic instruments of noise from semiconductor circuits such as thyristor control circuits has become an important problem.
In 1976, Kusko et al. proposed the transfilter, which is used both as a transformer and a low-pass filter to eliminate this noise. They proposed a simple equivalent T-type filter circuit having fixed circuit constants and a circuit equation for the transfilter. Then they showed that the transfilter has excellent frequency-cutoff characteristics through the use of the transfer admittance calculated from this equivalent circuit.
However, this transfilter is both a R-L-C-low-pass filter and a leakage transformer having series and parallel ferroresonant circuits. Therefore, if the magnetic flux density in the core exceeds the linear range of magnetizing characteristics and enters the saturation range, the output waveform becomes distorted due to the effect of ferroresonances.
To obtain good filter characteristics, we must keep the ratio of input voltage and output voltage constant. Waveform distortion by the effect of ferroresonances must also be eliminated.
For this transfilter, we have developed an equivalent circuit and characteristics analysis method which considers the effect of magnetic saturation. We have shown that the proposed method has high accuracy, and we have also determined the designing conditions for obtaining a good transfilter.