Analysis of instabilities in piezoelectric transformers driving cold cathode fluorescent lamps

This paper analyzes the behavior of power supplies for cold cathode fluorescent lamps (CCFL) employing piezoelectric transformers (PTs). It has been demonstrated that the non-linear equivalent lamp resistance interacts with the PT that behaves like a high-Q resonant circuit, giving rise to instabilities in the audible frequency range [Spiazzi, G. et al. 2004]. Moreover, when the PTs are driven by a voltage-fed inverter and its DC supply voltage is used to control the lamp voltage, the relation between the control variable and the lamp current can become nonmonotonic, depending also on the coupling network used between the inverter and the PT. This paper extends the analysis reported in Spiazzi, G. et al. (2004) by considering different coupling networks as well as considering the inverter switching frequency as the control variable for controlling the lamp current. SPICE simulations and analytical models are employed to explain the unstable behaviors observed in different experimental prototypes.