Many power converters operate with an input voltage that has a normal range of voltage that is quite narrow, for example, commercial ac voltage or from a regulated upstream power converter. To accommodate transient and abnormal conditions, power converters are often designed for a wide range of input voltage, which seriously compromises their efficiency at nominal voltage. This invention teaches a power converter that is optimized for the normal operating voltage range. A variable dc-dc transformer and a buck derived modulator are used in series. The buck circuit is most efficient at high duty cycles, and for normal input voltage, the buck converter may be saturated at 100 percent duty cycle. The duty cycle of the buck converter is reduced to accommodate over voltage transients. Efficiency is compromised, but the duration is short, so that is acceptable. For normal and under voltage conditions, the effective turns ratio of the variable dc-dc transformer is modulated to effect control. At normal input voltage, the effective turns ratio is nearly maximized, for optimum efficiency. At lower voltages, the efficiency is compromised, but the duration is short, so that is acceptable.