Patent ID: 6759813
Filing Date: 2004-07-06
Classification: H05B,Y02B

Abstract:
An operating circuit for a discharge lamp (1), the operating circuit comprising:an AC voltage generator (6, 7) operable to generate an AC supply power at a variable frequency (F) for the purpose of igniting and operating the discharge lamp (1); a resonant circuit (9, 10, 11) coupled between the AC voltage generator and the discharge lamp; a control circuit (12) coupled to the AC voltage generator and operable to drive the AC voltage generator, the control circuit being further operable to monitor a supply current in the AC voltage generator and to shut down the AC voltage generator in response to the supply current exceeding a threshold current value; and wherein the AC voltage generator and the control circuit are further operable: (i) to gradually lower the frequency (F) from a starting frequency (Fstart) in order to attempt to ignite the discharge lamp at a frequency at which a resonant peak of the resonant circuit occurs; (ii) if the control circuit shuts down the AC voltage generator, to make a new attempt to ignite the lamp by restarting the AC voltage generator and then gradually lowering the frequency (F), wherein during the new attempt to ignite the lamp the starting frequency (Fstart) is set with reference to the frequency at which shut down occurred; (iii) if the control circuit does not shut down the AC voltage generator, and if the frequency (F) is greater than a prescribed minimum frequency (Fmin), to lower the frequency (F) and continue to attempt to ignite the lamp; (iv) if the control circuit does not shut down the AC voltage generator, and if the frequency is less than or equal to the prescribed minimum frequency (Fmin), to conclude that the lamp has been ignited and to provide regulated current for operating the lamp; (v) to increment a running variable (t) for each attempt to ignite the lamp; and (vi) in response to the running variable (t) reaching a prescribed maximum value, to refrain from any further attempt to ignite the lamp.