Solid state ballast for high intensity discharge lamps

摘要

A solid state ballast includes a pulse width modulation (PWM) circuit to maintain substantially constant power to a discharge lamp over its lifetime. The PWM circuit is powered from a high voltage (HV) DC source and is referenced to the source voltage level. Lamp power is maintained substantially constant provided the HVDC source maintains at least a normal lamp operating voltage level and is operated at a reducing power levels as the HVDC source voltage drops below the normal lamp operating level. Lamp operating power can also be controlled by selecting the PWM reference voltage level. For initial power-up, a bootstrap power supply is provided to power solid state circuitry of the ballast from the HVDC source. The bootstrap power supply is shut-down by operation of a primary low voltage power supply or if operated for more than a few seconds. A lamp is driven through a capacitor which is charged to a high voltage for lamp ignition. A relay selectively shorts the capacitor for a high power connection to the lamp. A timer circuit cycles the relay approximately every two seconds and is disabled upon lamp ignition for high power lamp operation.