A DC-DC power converter has an auxiliary tank cascaded to share an efficiency tank's inductor, capacitor, and transformer. Switching transistors pump the auxiliary tank at startup to provide a boost current. The switching frequency is reduced in steps and the voltage gain and power of the converter sensed until the voltage gain matches a voltage gain calculated for the efficiency tank. Then tank switchover occurs and transistors to the efficiency tank are pumped with the last switching frequency used by the auxiliary tank, and the auxiliary tank is not pumped. Since the voltage gains before and after tank switchover are equal, no output voltage deviation or current spike occurs. A voltage sag or failure switches back to the auxiliary tank at a switching frequency determined by a dynamic contour line where the voltage gains of the two tanks are equal for the current power state.