Design of a hybrid ballast with magnetic shunts: application to control of HID lamps

A procedure to design a lag ballast with magnetic shunts for HID lamps is proposed. A bidirectional semiconductor switch with phase angle control, connected in parallel to one of the ballast coils, acts on lamp power. The static switch control signal is provided by a power feedback loop that actuates on lamp current, compensating variations of the main voltage and/or lamp parameters. This system saves energy and extends lamp life. Measurements on a 70 W high-pressure sodium (HPS) lamp prototype ballast are presented. The dependence of the inductances on the magnetic circuit parameters is emphasized. Power factor and total harmonic distortion of line current are comparable to a conventional lag ballast.