DRIVING DEVICE FOR BRUSHLESS MOTOR

PROBLEM TO BE SOLVED: To provide a driving device for a brushless motor that can stably detect switching timing of a power supply mode even in a region where a duty ratio is high on the basis of a voltage (hereinafter a pulse induced voltage) induced in a non-power-supply phase by applying a pulsed voltage to two phases among three phases.SOLUTION: A duty ratio for generating a sufficiently large pulse induced voltage is stored as a fixed value DF in advance, and when a basic duty ratio DB based upon a target rotating speed is larger than the fixed value DF, a duty ratio in timing when a voltage of a non-power-supply phase is detected is set as the fixed value DF. A duty ratio in timing when the voltage of the non-power-supply phase is not detected is set as a value obtained by correcting the basic duty ratio DB by a difference between the basic duty ratio DB and fixed value DF so that the duty ratio is the basic duty ratio on the average. Consequently, a pulse induced voltage which is large enough to determine switching of a power supply mode is generated in the timing when the voltage of the non-power-supply phase is detected so as to stably switch power supply modes.