1. A method of converting power from a source at a source voltage, VS, via a converter input at an input voltage, VIN, for delivery to a load via a converter output at a rectified output voltage, VOUT, where a current drawn by the load, IL, may vary over a normal operating range from a minimum load current, IL-MIN, to a maximum load current, IL-MAX, the method comprising:providing a transformer;
forming a resonant circuit including the transformer having a characteristic resonant frequency and a characteristic resonant period;
providing output circuitry connected to the transformer for delivering the rectified output voltage to the load;
providing input circuitry including two or more primary switches connected to drive the resonant circuit;
providing a switch controller to operate the two or more primary switches in a series of converter operating cycles to provide an essentially fixed voltage transformation ratio, K=VOUT/VIN, at a load current, each converter operating cycle is characterized by (a) two power transfer intervals of essentially equal duration, each interval having a duration, TPTI, less than the characteristic resonant period, during which one or more of the primary switches are ON, a resonant current at the characteristic resonant frequency and a magnetizing current flowing through a winding of the transformer, and power is transferred from the converter input to the converter output via the transformer; and (b) two energy-recycling intervals, each having a duration during which the two or more primary switches are OFF and currents in the converter are used to charge and discharge capacitances in the converter;
establishing a predetermined full duration, TPTI-FULL, approximately equal one half of the characteristic resonant period, for each power transfer interval for conditions in which the load current, IL, is greater than or equal to a first predetermined threshold, IL1;
establishing a predetermined minimum duration, TPTI-MIN, for each power transfer interval for conditions in which the load current, IL, is less than or equal to a second predetermined threshold, IL2; and
adjusting the duration of each power transfer interval, TPTI, from the predetermined full duration, TPTI-FULL, to the predetermined minimum, TPTI-MIN, as a function of variations in the load current, IL, between the first threshold, IL1, and the second threshold, IL2.