Various embodiments according to the present invention relate to a power conversion device and method, the device comprising: a converter; a capacitor unit including a plurality of capacitors for storing input voltage input thereto; a switch unit connected to the capacitor unit and including a plurality of switches for selectively connecting at least one capacitor among the plurality of capacitors to the converter; and a controller connected to the capacitor unit and the switch unit, wherein the controller determines at least one capacitor satisfying a specified condition, among the plurality of capacitors, sets at least one switch among the plurality of switches to be turned on, the at least one switch corresponding to the at least one capacitor, and sets at least another switch among the plurality of switches except for the at least one switch, to be turned off, so that the at least one capacitor and the converter are electrically connected and configured to allow at least partial voltage of the input voltage, stored in the at least one capacitor, to be supplied to the converter. Therefore, a power conversion device, disposed on a circuit on which the plurality of capacitors (or cells) is connected in series to a power supply unit, can establish selective connection by using the switch unit, so as to adjust input voltage and provide the adjusted input voltage to the converter, and can reduce switching loss of the power conversion device. Various other embodiments are possible.