Analysis of large-signal and small-signal instabilities in piezoelectric transformers driving cold cathode fluorescent lamps

This paper investigates the behavior of voltage-fed piezoelectric transformers (PTs) driving cold cathode fluorescent lamps (CCFLs). It is shown that the nonlinear equivalent lamp resistance can have a dramatic impact on the overall system performance, causing possible instabilities both at large and small signal level. The effects of the variable load resistance, with a negative small-signal value, are enhanced by the PT, that behaves like a high-Q resonant circuit, and by the presence of the coupling inductor, usually placed between the half-bridge inverter and the PT to ensure zero-voltage commutation of the inverter switches. Such instabilities are investigated considering a constant frequency inverter where the lamp current is controlled by varying the DC-link voltage. SPICE simulations and analytical models are employed to explain the unstable behaviors observed in different experimental prototypes.