Patent ID: 7561446

Claim:
Apparatus for converting power from an input source at an input voltage for delivery to a load at an output voltage, the apparatus comprising, a transformer having a primary winding and a secondary winding, secondary circuit elements connected to the secondary winding adapted to deliver power to the load at the output voltage, active clamp circuitry connected to the primary winding and including a clamp switch and a clamp capacitor, a plurality of primary switches connected to the primary winding, a switch controller adapted to operate the clamp switch and the primary switches in a series of converter operating cycles, each converter operating cycle comprising the following phases: (a) an energy-storage phase during which the primary winding is connected to the input source, the energy-storage phase being characterized by a transfer of energy from the input source to the transformer, wherein an average value of primary current flowing in the primary winding has a polarity and the average is taken over the duration of the energy-storage phase, (b) an energy-transfer phase characterized by a transfer of energy from the transformer to the load, and (c) a clamp phase during which the primary winding of the transformer is clamped, the clamp phase being characterized by essentially zero voltage across said primary winding and an average value of current flowing in the primary winding, the average value of current having a polarity, wherein the average is taken over the duration of the clamp phase and the polarity is opposite of the polarity of the average value of primary current during the energy-storage phase immediately preceding the clamp phase.