In a start-up mode (MS), at a start instant (t3, t13), a power converter (11) starts operating. After a predetermined period of time (T2, T12) from the start instant (t3, t13) it is checked whether the output
voltage (Vo) of the power converter (11) has a desired value or not. If not, a protection mode (M3) is entered, otherwise the normal operating mode (M2) is entered. During the normal operating mode (M2), it is checked whether a drop in the output
voltage (Vo) occurs. If yes, firstly, the power converter (11) is brought into the offmode (M1). Secondly, the power converter (11) is brought into the start-up mode (MS). If, after the restart, the output
voltage (Vo) has the desired value, the earlier detected drop of the output voltage (Vo) was most likely due to a drop of the mains voltage (Vm) at the input of the power converter (11) and the normal operating mode (M2) can be resumed. If, after the restart, the output voltage (Vo) does not have the desired value, the earlier detected drop of the output voltage (Vo) was most likely due to a failure and the protection mode (M3) is entered. Now, the power converter (11) is not allowed to enter the start-up mode (MS) at all, or only after a relatively long period of time.