Resonant converter and resonant conversion circuit system

A resonant converter comprises a first conversion circuit, a resonant transformation circuit, a second transformation circuit and a switching circuit. The resonant transformation circuit is coupled to the plurality of first bridge arm units of the first conversion circuit and is used for converting the first voltage into a second voltage. The second conversion circuit is coupled to the resonant transformation circuit and comprises a plurality of second bridge arm units and a plurality of multi-level switch elements. The switching circuit is coupled to the second conversion circuit, and is used for selectively changing the connection positions of the plurality of second bridge arm units, so that the second conversion circuit converts a second voltage into a first direct current through the plurality of second bridge arm units instead of the plurality of multi-level switch elements. Or enabling the second conversion circuit to convert the second voltage into the second direct current through the plurality of se