The multilevel converters have advantages of low voltage stresses of the switches and possible reduction of size of the passive components. Input-series output-series (ISOS) configuration of the converter enable the utilization of low voltage rating switches in converters with high input voltage and high output voltage in applications that requires galvanic isolation. While multilevel topology offers many features, it also necessitates a balance control of the input capacitors. The usage of flying capacitor for voltage balancing is addressed in this paper. The paper discusses design of input-series and output-series resonant converter for discharge lamp power supply. Zero-voltage switching (ZVS) is achieved for each main switch without any auxiliary circuit. First of all the operation of the converter is analyzed on the simulation model and after that the presented converter are verified on experimental prototype with 3 phase AC input voltage and 3 kW output power.