To reduce high-frequency leakage current without fail, even at a high switching frequency, by detecting a zero-phase current flowing from each phase winding of a brushless direct-current motor to earth creating a current having a waveform similar to that of the zero-phase current to forcibly cancel out the leakage current from the brushless direct-current motor.
Since high-frequency leakage current I1 flows to a direct current circuit 5, the currents, the magnetic of which is in proportion to zero-phase current, are detected using a zero-phase current detector 31. The output voltage of the zero-phase current detector 31 is class A push-pull-amplified using transistor 21, 33 to create a current I2 for canceling out similar to the zero-phase current in waveform. The current I2 for canceling out has such a polarity as to make the said current flow from earth E to a current canceling-out circuit 30. Based on the polarity and waveform, the high-frequency leakage current I1 is absorbed into the current canceling-out circuit 30 and is thereby forcibly canceled out. This enables reducing the high-frequency leakage current I1 without fail, even if the switching frequency of a switching circuit 11 is high.
SANO TETSUO
MOTOHASHI HIDEAKI
TANAKA HIROYUKI
