The invention relates to a voltage converting circuit (2) having:--an input terminal (11) for connection to a varying voltage or current source, and an output terminal (12) with a supply voltage,--an inductive element (5) being with a first terminal connected to the input terminal and with a second terminal to a capacitive element (8) and to the output terminal (12), the capacitive element (8) being with one terminal connected to a reference voltage (9),--a first switch (6) being with a first terminal connected to the second terminal of the inductive element (5), and with a second terminal to the reference voltage (9), the first switch (6) comprising a gate electrode (31) for opening and closing of the switch when a control voltage of a first level is applied to the gate,--an oscillator (13) comprising a power input (14) connected to the output terminal (12), a start-up output (15) connected to the gate electrode (31) of the first switch (6) for supplying a pulse-like signal to the gate electrode (31) and an oscillator output (16), and--a second switch (7) connected in parallel with the first switch (6), the second switch having a gate electrode (32) connected to the oscillator output (16), wherein the first switch (6) remains opened when the supply voltage at the output terminal (12) reaches a predetermined level, and the second switch (7) is operated by the oscillator output (16) at a second voltage level that is higher than the first voltage level.