Design considerations for optimum ignition and dimming of fluorescent lamps using a resonant inverter operating open loop

The design of dimming electronic ballasts for fluorescent lamps has to be made taking into account that fluorescent lamp life is greatly affected by the starting scenario and by how the filament heating current is varied during dimming operation. Another important aspect is that the ballast should provide a wide stable control range of the output power, some applications require dimming down the output power to less than 20% of the nominal value. One of the most commonly used topologies for driving fluorescent lamps is the resonant half bridge inverter. In this topology, the starting scenario and the stable dimming range depends on both the control parameter selected and the control strategy used. This paper focuses on the LCC series-parallel resonant inverter. Different design and control methodologies are analyzed from the point of view of the starting scenario and the stable control range obtained. Generalized characteristics are obtained to simplify the design of the resonant tank, including operating points where the fundamental approach gives no accurate results. Finally, the theoretical and practical results obtained using the different control strategies are compared.