High side driver technique for miniature cold cathode fluorescent lamp

Abstract

A circuit for supplying power to a fluorescent lamp comprising a buck regulator with a high side drive. A dc battery is coupled to a drain of a first transistor. A source of the first transistor is coupled to an inverter for powering the lamp. A first control signal is coupled to a primary winding of a transformer. A first terminal of a secondary winding of the transformer is coupled to the anode of a diode. The cathode of the diode is coupled to the gate of the first transistor. A second terminal of the secondary winding of the transformer is coupled to the source of first transistor. The first control signal is activated to bias the first transistor through the transformer by charging the gate to a voltage higher than the control voltage due to the transformer turns ratio. A diode is coupled to capture a charge on the gate. A second transistor is coupled to the gate of the first transistor to drain the captured charge to ground, turning off the first transistor when a second control signal is activated. A third transistor is coupled to ground the source of the first transistor when a third control signal is activated. The lamp brightness is regulated by varying a duty cycle of the buck regulator synchronized to the inverter. The first transistor is turned on when two inverter transistors change state. After a time determined by a lamp current feedback signal, the first transistor is turned off and the third transistor is turned on. The third transistor stays on until the inverter transistors change state again, then the first transistor is turned on again.