An inrush current prevention circuit for a DC-DC converter is provided and in preferred aspects comprises a switching element that transforms an input voltage by being switched on and off and outputs the transformed voltage. A filter filtrates the outputted voltage, transformed via the switching element, and outputs the filtrated voltage as an output voltage. A reference voltage generator generates a reference voltage. An error amplifier compares the reference voltage and output voltage and outputs an error signal. A Pulse Width Modulation (PWM) signal generator generates a PWM signal to switch on and off the switching element according to the error signal. An on-off circuit either transmits or isolates the PWM signal to the switching element. An Electronic Control Unit (ECU) controls the on-off circuit. Preferred systems of the invention can prevent an inrush current immediately following power input or during reactivation of the DC-DC converter.