Cold cathode fluorescent lamps driven by piezoelectric transformers: Stability conditions and thermal effect

The envelope impedance (EI) concept was used to develop a systematic and simple approach for studying the conditions for stability of piezoelectric transformers (PT) based ballasts for cold cathode fluorescent lamps (CCFL) systems and the implications of the thermal operating conditions on such systems. The envelope analysis of the CCFL, which was verified experimentally, revealed that the magnitude of the negative EI decreases with temperature. This might cause the system to become unstable at low temperatures due to the fact that the loop gain of the PT-CCFL system encircled the (-1, 0) point in the Nyquist plane. In such cases, the PT-CCFL system may enter a quasi-stable oscillatory operation mode in which the current is AM modulated by a low frequency parasitic signal. This phenomenon is analyzed and explained by considering the temperature effect on the EI of the CCFL. The results of this work provide an insight into the stability issue of the PT-CCFL system, and could help in finding remedies to the instability.