To eliminate noise in a DC power supply a coupled inductor can be used as part of a two-port filter along with a capacitor. One inductor, Ldc carries the DC current due to the DC blocking capacitor in series with the Lac inductor. The Lac inductor carries the AC ripple current. The coupled inductor’s mutual inductance becomes part of the filter equivalent circuit model thus allowing more than one inductance to be integrated into the circuit design. This mutual inductance creates a fourth order filter using only three components.