HPS electronic ballast with modulation index control

Acoustic resonance in HPS lamps can be avoided by using a controlled PWM inverter and third harmonic injection. The crest factor can also be reduced by harmonic injection. This paper proposes to use of the modulation index adjustment as a method of eliminating the transformer at the output of the ballast as well as maintaining the lamp nominal power during its lifetime. An ignitor for high-intensity discharge lamps based on forced oscillation via an external periodic voltage square wave supplied by a digitally controlled inverter is also presented.