Discharge lamp operating circuit

Claim 1

A discharge lamp operating circuit comprising: a switching circuit including first and second switch circuits connected in parallel with each other, said first switch circuit having series-connected first and second switches in which one end of said first switch and one end of said second switch are connected to each other, said second switch circuit having series-connected third and fourth switches in which one end of said third switch and one end of said fourth switch are connected to each other; a discharge lamp including a first heating type of electrode connected between a central connection point of said first switch circuit and a central connection point of said second switch circuit and a second heating type of electrode connected between a second end of said second switch and a second end of said fourth switch; a ballast condenser connected to said discharge lamp; a DC power source connected between second ends of said first and third switches and said second ends of said second and fourth switches; a switch control circuit for controlling turn-on and turn-off of said first to fourth switches to effect heating, starting and maintaining operation of said discharge lamp at a high frequency a capacitance of said ballast condenser being selected so that a time required for charging and discharging said ballast condenser is longer than a time scheduled for charging and discharging said ballast condenser in one period of said high frequency; and a switching frequency f 0 at the time of heating of said discharge lamp being selected to be smaller than a switching frequency f at the time of maintaining operation of said discharge lamp wherein said switching frequencies f 0 and f satisfies a relation of 2≦f/f 0 ≦10.