Patent ID: 6288501
Filing Date: 2001-09-11
Classification: H05B,Y02B

Abstract:
A ballast for a discharge lamp comprising:a voltage converter providing a DC power from a DC source voltage, said voltage converter including a switching element and an energy storing element having an inductance, said switching element operating to repetitively switch the DC source voltage in order to store energy into said energy storing element;a power converter receiving said energy and converting it into an operating power for driving said discharge lamp;a controller which provides a command value and turns on and off said switching element for varying time periods in accordance with the command value in order to regulate the output of said voltage converter for generating a necessary power to said discharge lamp;said controller giving a variable OFF-period in which said switching element is turned off and a variable ON-period in which said switching element is turned on,said controller providing a minimum OFF-end limit and a forced maximum OFF-end limit for terminating said variable OFF-period,said controller providing a minimum ON-end limit and a forced maximum ON-end limit for terminating said variable ON-period,said minimum OFF-end limit being defined at a timing when a secondary current flowing from said inductance decreases to zero,said minimum ON-end limit being defined at a timing when a primary current flowing through said switching element or a primary voltage developed across said switching element reaches a level corresponding to said command value,said controller terminating the OFF-period at said minimum OFF-end limit or at said forced maximum OFF-end limit, whichever comes earliersaid controller terminating the ON-period at said minimum ON-end limit or at said forced maximum ON-end limit, whichever comes earlier.