A structure of a fly-back power converting apparatus is disclosed. The structure includes a power transistor, a current detector, a pulse width modulation (PWM) signal generator and a current limiter. The power transistor is coupled to an input voltage and receives a PWM signal. The current detector detects a current output from the power transistor and generates a detecting voltage according to the current. The PWM signal generator generates the PWM signal according to a comparing result by comparing the detecting voltage and a standard voltage. The current limiter generates the standard voltage according to a turn-on time of the power transistor.