Design method for a self-oscillating electronic ballast with constant lamp current control

In present paper the analysis and design procedure of a high frequency self oscillating electronic ballast with constant lamp current control is presented. The self-oscillating electronic ballasts have two important drawbacks, first, the lamp current varies according to the AC input voltage, causing undesired effects in the lamp brightness; and second, the design method lacks of precision at the very high frequency operation (greater than 200 kHz) due to the Cgs parasitic capacitance. The ballast is modelled by using the descriptive function method and the extended Nyquist criterion; the model of the ballast includes the control circuit and takes into account the high frequency effect of the Mosfet parasitic capacitance Cgs. Finally, experimental results of a 38 W self-oscillating electronic ballast prototype with constant-lamp-current control for HID lamps are presented to validate the analysis and the proposed methodology. The switching frequency is selected to 2MHz in order to avoid the acoustic resonance problem.