A power converting device including a pulse width modulation circuit, a switch unit, a power output unit and a voltage start unit is provided. The pulse width modulation circuit increases a start voltage in a soft start mode and is operated under the start voltage to generate a pulse width modulation signal. The switch unit is for receiving an input voltage, and forming a charge path and a discharge path alternately according to the pulse width modulation signal. The power output unit converts the input voltage to a core voltage in accordance with the charge path and the discharge path. The voltage start unit is for detecting the start voltage, and for transmitting a control signal to interrupt the formation of the discharge path when the start voltage is smaller than the core voltage.