Three-level, constant-frequency, soft-switched isolated converters provide zero-voltage-switching (ZVS) conditions for the turn-on of all primary switches over a wide range of input voltage and output load. These converters achieve ZVS with the minimum duty cycle loss and circulating current, which optimizes the conversion efficiency. The ZVS of the primary switches is achieved by the energy stored in an inductor on the primary side of the isolation transformer. The inductor and transformer are arranged so that a change in the phase shift between the outer and inner pair of switches of the series connection of four switches changes the volt-second product on the windings of the transformer and the windings of the inductor in opposite directions. In some embodiments the primary-side inductor is coupled inductor with two windings, whereas in the other embodiments the inductor has only one winding.