Patent ID: 8148972

Claim:
A filter circuit comprising: a monitoring circuit that monitors positive winding current and negative winding current flowing respectively through a positive winding and a negative winding of a common-mode choke coil; and an adjusting circuit that a lies an adjustment based on the result of monitoring the positive winding current and the negative winding current in such a manner that balance is achieved between the positive winding current and negative winding current, wherein the monitoring circuit includes first and second current detecting circuits that detect current values of the positive winding current and negative winding current, respectively, wherein the adjusting circuit includes: a differential amplifier that compares the current values detected by the first and second current detecting circuits and produces a signal conforming to the comparison result; and a first current control element and/or a second current control element inserted into a path of the positive winding current and/or a path of the negative winding current of the common-mode choke coil, the first current control element and/or second current control element each receiving the signal from the differential amplifier and varying a value of current flowing through the path into which the current control element is inserted.