Patent ID: 7402970

Claim:
An electric power steering device comprising: two series circuits, comprising switching elements of an upper stage and a lower stage, respectively; an H bridge circuit comprising the two series circuits which are connected in parallel with each other; an electric motor for giving an auxiliary steering force to a steering system; a control part for controlling the switching elements on a diagonal line in the upper stage and a diagonal line in the lower stage of the H bridge circuit to be respectively pulse-width modulated by a plurality of differing duty ratios in accordance with a steering torque exerted on the steering system; and an electric power supply source for supplying an electric power to the H bridge circuit; wherein the electric motor is provided between intermediate portions of the two series circuits, the intermediate portions being located between the switching element of the upper stage and the switching element of the lower stage, wherein at a state that when the voltage of the electric power supply source is at least a prescribed voltage, the control part controls the switching elements on the diagonal lines to be pulse-width modulated by the different duty ratios with a frequency exceeding an audible range, and wherein at an abnormal time when the voltage of the electric power supply source is less than the prescribed voltage, the control part controls the switching elements in the upper stage of the switching elements on the diagonal lines to be pulse-width modulated with a frequency within the audible range, and the switching elements in the lower stage are controlled to be turned on.