A new control method for a low frequency inverter of MHD lamp ballasts with a synchronous rectifier

In this paper, in order to improve the commutation characteristics of the low frequency bridge type ballasts, a new control method is proposed. It is characterized by the concept that the PWM controller modulates the output current with a square wave current reference and the inverting current rises monotonically. This method prevents the inverter from having an overshoot and a ringing problem on the inverted lamp current. As a result, the ZVS condition is maintained just after inverting and the restrike voltage of the lamp is minimized. Also, The principle of the proposed control is explained with an open loop description and the loss analysis of the improved inverter is performed. The new control scheme is adopted to a feedback controller and the operation is verified with the experimental results of a prototype 70 W ballast