Electronic ballast

A high power factor, gas discharge lamp ballast comprising

• means for full wave rectifying low frequency ac line voltage,
• switching power conversion means connected to said ac voltage rectifying means for generating at the output a second low frequency rectified sine wave voltage at the same frequency of said low frequency ac voltage,
• a rectified sinusoidal voltage unfolding means which has as input said second low frequency rectified line wave voltage and generates unrectified output low frequency sine wave voltage needed to drive said gas discharge lamp,
• said switching power conversion means comprising a dc-to-dc switching power converter operating in discontinuous conduction mode and with no line frequency energy storage,
• switching control means for operating said switching power conversion stage at a fixed switching frequency and at a constant duty ratio in a mode of control selected from open-loop and closed-loop control of the current in said gas discharge lamp,
• said unfolding means comprising an arrangement of controlled switches having one of said controlled switches between each of the two input terminals and each of the two output terminals, allowing to connect input terminals to output terminals with direct or inverse polarity,
• control mean for operating said controlled switches of said unfolding means for reversing polarity of connection between said input terminals and said output terminals at each zero crossing of said ac line voltage thereby converting said second low frequency rectified sine wave into said output low frequency sine wave,
• whereby said switching power conversion stage operating in discontinuous conduction mode draws a current from said ac voltage rectifying means whose average very closely follows instantaneous line voltage for near unity power factor operation while operating at a fixed switching frequency and a constant duty ratio, and provides a high output impedance to said gas discharge lamp guaranteeing stable lamp operation.