To provide a motor driving device that calculates and detects a terminal voltage of a capacitor by using a quick-response signal of a current detection circuit to correctly determine the amount of capacitor energy to be controlled for charging and discharging and thereby can improve the effective use of energy, and also to provide a method of detecting a terminal voltage of a capacitor.
A DC-DC converter unit 700 is equipped with a capacitor current detection circuit 702 for detecting a current in a capacitor B, a capacitor peak current detection circuit 701 for detecting a peak current in the capacitor B, and a main circuit DC voltage detection circuit 703 for detecting a terminal voltage of a smoothing capacitor 103. A DC-DC converter control unit 800 calculates and detects the terminal voltage of the capacitor B from the peak current and the inductance of a reactor L or from the peak current, the inductance of the reactor L1, and the terminal voltage of the smoothing capacitor 103.
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