High-efficiency dc-dc step up converters, based on the synchronous Three Levels Neutral Point Clamped configuration were built and investigated. The baseline circuit based on hard-switched, fast high-current MOSFETs provided high efficiency levels on a wide range of operating conditions. Several approaches to further improve performance were investigated. Very high efficiencies were experimentally demonstrated by an hard switched configuration where the switches consisted of a suitable arrangement of very-low RDSon and very fast MOSFET transistors.