Patent ID: 6958580

Claim:
An electronic ballast for a high intensity discharge lamp, said ballast comprising: a power converter which converts an input DC voltage into a lamp power for driving the high intensity discharge lamp; a dimmer which provides a dimmer command of a varying dimming ratio in order to reduce the lamp power for dimming the lamp to a deeper extent as the dimming ratio decreases; a voltage monitor which monitors a lamp voltage across said discharge lamp; a controller which controls said power converter to provide a voltage-wattage output characteristic which gives the lamp power varying with the varying lamp voltage being monitored, said controller receiving said dimmer command to provide said voltage-wattage output characteristic which lowers the lamp power for the given monitored lamp voltage as said dimming ratio decreases; said voltage-wattage output characteristic defining an effective constant wattage control (ECWC) voltage range within which said power converter is controlled to give a roughly constant lamp power while allowing only a small lamp power error from a maximum lamp power when the monitored lamp voltage varies between a lower bound and an upper bound of said ECWC voltage range, wherein said controller operates to modify said voltage-wattage output characteristic in such a manner as to lower the lower bound of said ECWC voltage range as said dimming ratio decreases down from 100% at which a rated power is supplied to the discharge lamp.