| 摘要 |
A ballast includes a voltage converter providing a DC power from a DC source voltage. The converter includes a switching element and an energy storing element having an inductance. The switching element is controlled to operate for repetitively switching the DC source voltage in order to store energy into the energy storing element. A power converter is included in the ballast to receive the energy and convert it into an operating power for driving the discharge lamp. A controller provides a command value and turns on and off the switching element for variable periods, respectively in accordance with the command value for generating a necessary power to the lamp. The controller gives a variable OFF-period and a variable ON-period to the switching element. The controller gives a minimum OFF-end limit and a forced maximum OFF-end limit for terminating the OFF-period, and also gives a minimum ON-end limit and a forced maximum ON-end limit for terminating the variable ON-period. The minimum OFF-end limit is defined at a timing when a secondary current flowing from the inductance decreases to zero, while the minimum ON-end limit is defined at a timing when a primary current flowing through the switching element reaches the command value. The controller terminates the OFF-period at the minimum OFF-end limit or at the forced maximum OFF-end limit, whichever comes earlier, and terminates the ON-period at the minimum ON-end limit or at said forced maximum ON-end limit, whichever comes earlier.
|
