A dc-dc converter can include a plurality of switches, a piezoelectric resonator (PR) for power stage energy storage, and a means for controlling one or more switching sequences. The switches operate in accordance with the switching sequences to transfer energy from the input to the output via the PR while providing low-loss resonant soft-charging of the PR's capacitance. The switching sequences include: connected stages in which a first and second PR terminals are both connected to one of the input, the output, or the other PR terminal; and open stages in which at least one of the first or second PR terminal is not connected by a closed switch to one of the input, the output, or the other PR terminal.