An isolated fly-buck converter provided for converting an input voltage (Vin) to an output voltage (VS) comprises (i) on a primary side, a primary winding (X1) and a non-isolated buck (CP) connected in series, and a pair of switches (Q1, Q2), wherein the switches are switchable between a forward phase, in which the primary winding and the non-isolated buck are connected to the input voltage, and a fly-buck phase, in which the primary winding and the non-isolated buck are disconnected from the input voltage and are connected to one another in a closed circuit; and (ii) on a secondary side, a secondary winding (X2) coupled to the primary winding, and a first capacitive element (CS) connected over the secondary winding and a first rectifying element (D1) connected to the secondary winding to prevent current from being flown through the secondary winding during the forward phase, wherein the output voltage is achieved as the voltage over the first capacitive element. A supplementary circuitry is provided at the secondary side by aid of which the input voltage can be measured on the secondary side, the supplementary circuitry comprising a second capacitive element (CF) and a second rectifying element (D2) connected in series, wherein the supplementary circuitry is connected over the second wiring such that the second rectifying element prevents current from being flown through the supplementary circuitry during the fly-buck phase, wherein the sum of the voltage over the second capacitive element and the output voltage is indicative of the input voltage.