Patent ID: 7402956

Claim:
A ballast for powering at least one discharge lamp, comprising: an inverter having output connections adapted for coupling to the at least one discharge lamp; and a switching converter for providing a substantially direct current (DC) rail voltage, comprising: first and second input terminals for receiving an input voltage; first and second output terminals coupled to the inverter, wherein the DC rail voltage is provided between the first and second output terminals; a transformer having a primary winding and a secondary winding; a transistor having a gate terminal, a drain terminal, and a source terminal, wherein the drain terminal is coupled to the primary winding of the transformer and the source terminal is coupled to circuit ground; and a control circuit coupled to the transistor and operable to provide substantially zero voltage switching (ZVS) of the transistor, the control circuit comprising: a first connection coupled to the drain terminal of the transistor, a second connection coupled to the gate terminal of the transistor, a third connection coupled to the secondary winding of the transformer, and a fourth connection coupled to the first output terminal; a drive circuit coupled to the first and second connections, and operable to provide an output signal for turning the transistor on and off; a turn-on synchronization circuit coupled to the first connection and to the drive circuit, wherein the turn-on synchronization circuit is operable to monitor a voltage between the drain and source terminals of the transistor and, in response to the voltage between the drain and source terminals approaching zero, to direct the drive circuit to turn the transistor on; a current mirror circuit coupled to the first connection and to the drive circuit, wherein the current mirror circuit is operable to monitor a current through the primary winding of the transformer and, in response to the current through the primary winding reaching a predetermined peak level, to direct the drive circuit to turn the transistor off; a rail voltage regulator circuit coupled to the fourth connection and to the drive circuit, wherein the rail voltage regulator circuit is operable to monitor the rail voltage and to control the output signal of the drive circuit so as to maintain the rail voltage at a first predetermined level; and a lamp current regulator circuit coupled to the third connection and to the drive circuit, wherein lamp current regulator circuit is operable to monitor the output current of the switching converter and to control the output signal of the drive circuit so as to maintain the output current at a second predetermined level.