Electronic ballast for fluorescent lamps with phase-locked loop control scheme

This paper proposes a phase-locked loop (PLL) control scheme for the electronic ballast employing a parallel-serial resonant tank (PSRT), in which the resonant frequency of the circuit is continuously tracked, in order to drive the fluorescent lamps even with low equivalent resistance at steady-state. Compared with the conventional PLL control of the electronic ballasts for cold cathode fluorescent lamps, the proposed PLL control scheme, by sensing the phase signals at the both ends of the resonant inductor, is able to have resonant frequency tracked for the electronic ballasts, where the equivalent resistance of the fluorescent lamps is even equal to hundreds of ohms or lower at steady-state. By employing the current regulator circuit, the proposed PLL controller regulates the values of the lamp's current by adjusting the operating frequency from the resonant frequency as a reference frequency point, the lamp's current is accurately controlled. The analysis of the proposed PLL control scheme will be presented in this paper. Finally, the implementation of a prototype circuit is provided to demonstrate the performance of the proposed PLL control scheme by associating with a current regulator circuit.