A transformer-coupled buck-boost DC-DC power converter is disclosed. An active clamp circuit is provided to form a resonant circuit with the transformer to control the slew rate of the secondary current and allow the secondary switch to be turned ON at conditions of zero voltage and relatively low current. The characteristic resonant period of the active clamp transformer circuit may be less than the minimum converter operating period. A winding of the transformer is shunted during a clamp phase to retain energy in the transformer. ZVS phases are provided to reduce switching losses when switches in the converter are turned ON. An energy-storage phase may be varied to control the amount of energy stored per operating cycle. An input-storage phase may transfer energy to the clamp capacitor during a series of converter operating cycles and transfer said energy to the secondary during different converter operating cycles.