Improvements in high-performance lowpass filters using coupled windings

The use of mutually coupled windings in lowpass filters is considered. The addition of a small auxilary winding to the main filter choke allows the fundamental ripple component to be `tuned out¿, giving a much improved filter characteristic without the addition of any further components. Filters using this principle have been termed `mutual-type filters¿. The transfer functions and conditions for fundamental ripple suppression have been derived for the two mutual filter configurations. Component current ratings and peak magnetisation ampere-turns have been calculated to improve core and capacitor utilisation and a transient analysis have been carried out. Two design approaches are presented. A method of converting existing L-section filters into mutual filters is suggested by connecting the capacitor or load on to a tapping point on the choke, or by winding a few extra turns on to the choke core. In a 2.3 kVA, 3-pulse rectified mains application, the mutual filters´ output had less than a third of the distortion of that of an equivalent L-section filter. Possible application areas are discussed and it is concluded that big savings in cost, size and weight can be achieved if the new filters are used to smooth rectified mains.