A half bridge switching dc-dc converter an input dc voltage to an output dc voltage. The converter includes a switching circuit for receiving the input dc voltage and generating a periodic square wave voltage oscillating from a high value corresponding to the input dc voltage to a low value corresponding to a reference voltage. The periodic square wave voltage oscillates at a main frequency with a main duty cycle equal to about 50% when the converter operates in a steady state. The converter further includes a conversion circuit for providing the output dc voltage from the square wave voltage based on the main frequency and on the main duty cycle. The converter still further comprises a switching control circuit controlling the switching circuit for temporarily varying the main duty cycle during at least one period of the square wave after a power on of the converter.