IC fluorescent dimming considerations

  
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This article provides an overview of fluorescent dimming and an application circuit for low-end small fixture applications ICs where dimming levels below 10% are required and more comprehensive protection features are needed. The electronic ballast circuit block diagram (see Fig. 1) includes the ac-line input voltage (typically 220 Vac/50 Hz or 12
IC fluorescent dimming considerations - schematic

0 Vac/60 Hz), an EMI filter to block circuit generated switching noise, a rectifier and smoothing capacitor for ac to dc conversion, a control IC and half-bridge inverter for dc to high-frequency ac conversion, and the resonant tank circuit to ignite and run the lamp. The additional circuit block required for dimming is also shown that includes a dimming reference signal, a lamp current sensing and feedback signal, and a summing circuit for closed-loop control of the lamp current. The lamp requires a current to preheat the filaments, a high-voltage for ignition, and a high-frequency ac current during running. To fulfill these requirements, the electronic ballast circuit first performs a low-frequency ac/dc conversion at the input, followed by a high-frequency dc-to-ac conversion at the output. The ac mains voltage is full-wave rectified and then peak-charges a capacitor to produce a smooth dc bus voltage. The dc bus voltage is then converted into a high-frequency, 50% duty-cycle, ac square-wave voltage using a standard half-bridge switching circuit. The high-frequency ac square-wave voltage then drives the resonant tank circuit and becomes filtered to produce a sinusoidal current and voltage at the lamp. The complete schematic is shown in Fig. 2. The 220-Vac/50-Hz line input voltage is full-wave rectified (BR1) and then goes through the EMI filter (CF and LF) before being smoothed by the dc bus capacitor (CBUS). The...



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