Abstract: A buck converter circuit comprising a pre-charging switch, a first switching element, a second switching element, a third switching element, a fourth switching element, a flying capacitor, a voltage supply node, a first flying capacitor node, a second flying capacitor node, a switch node and a control system. The voltage supply node, the pre-charging switch and the first flying capacitor node are sequentially connected in series. The voltage supply node, the first switching element and the first flying capacitor node are sequentially connected in series. The first flying capacitor node, the flying capacitor, the second flying capacitor node and the fourth switching element are sequentially connected in series. The first flying capacitor node, the second switching element, the switch node, the third switching element and the second flying capacitor node are sequentially connected in series. The control system is configured for executing a pre-charging process and a cyclic converting process.

Abstract: A converter arrangement connects to a DC source or load and to a plurality of AC terminals. Each converter arrangement includes a DC part with two capacitors arranged in parallel to the DC source or DC load and a neutral DC point between them. At least one AC leg is connected to a neutral AC point via an AC leg capacitor. A pre-charge unit is arranged at each leg of the AC terminal. The method includes opening a decoupling switch and all second AC switches; loading the pre-charge unit; sequentially closing each second AC switch; measuring at each closing step a voltage between each AC leg of the second converter arrangement; and determining the stuck AC switch of the second converter arrangement based on the measured voltage.