In the past, there have been a number of efforts to improve the performance of ballasts for fluorescent lamps. One direction such efforts have taken is to utilize electronic ballasts of the type having an input section for power factor and harmonic correction and an output section operating as a current-fed power resonant converter. Active preregulator circuits have been used to accomplish very high power factor and harmonic correction in such input sections. At the same time, the instant-start type of fluorescent lamps continue to be extremely popular, calling for ballasts which are compatible with instant-start lamps.
The use of active preregulators in instant-start applications has led to startup problems in that the integrated circuits used in such active preregulators take appreciable time to attain steady state operating conditions during start-up and can present undesirable operating conditions to the fluorescent lamps when passed through the converter section during start-up transient conditions. For example, one integrated circuit useful in active preregulators typically takes 100 milliseconds up to 500 or 1,000 milliseconds to reach steady state operating conditions. At steady state conditions, the active preregulator provides 170 volts DC output, however, during transient start-up conditions the output is substantially below that. When operating instant-start lamps, this results in the undesirable effect of an unacceptably "preheat" or glow period at low voltage. Instant-start lamps are typically specified to be operated at low initial voltage for no more than 50 milliseconds, because longer "preheat" periods undesirably shorten lamp life due to excessive electrode erosion during such low-voltage "preheat" conditions. This is in addition to undesirable visible phenomena during starting.
The present invention overcomes these difficulties by delaying startup of the power converter section of the ballast until the preregulator section has stabilized, thus providing "instant" starting with electronic ballasts .