Control circuit for piezo transformer based fluorescent lamp power supplies

Abstract

A control circuit for a piezo transformer based power supply for a fluorescent lamp includes driver circuitry generating pulse waveforms for providing excitation to primary inputs of the piezo transformer, and circuitry for regulating lamp current and the voltage across the piezo transformer primary inputs. The frequency of the pulse waveforms is varied in response to the magnitude of lamp current to maintain a predetermined desired lamp current as represented by a current reference signal. The duty cycle of the driver circuitry is varied in response to the magnitude of the voltage across the piezo transformer primary inputs to maintain a predetermined desired piezo primary voltage as represented by a voltage reference signal. The piezo transformer is operated as close to resonance as possible, contributing to greater circuit efficiency. The driver circuitry in the control circuit employs four transistors arranged as a full bridge with respect to the piezo transformer primary inputs. The phase of drive signals supplied to one pair of the transistors is varied with respect to the phase of drive signals supplied to the other pair, thereby varying duty cycle and average voltage of the piezo transformer primary inputs. A controller integrated circuit contains a number of components of the control circuit, enabling its use in a variety of piezo-based power supply applications.