AN2590

Typical dimming system for incandescent lamps

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the AC input, and the triac is then immediately turned off. It is impossible to adjust the DC

voltage across the storage capacitor and then extend triac firing angle less than 90°.

However the lamp may continue to flicker and remain unstable if the DC voltage is not well

managed.

1.2

Proposed solution

The CFL ballast has been developed based on the L6574D. The lamp power can be

dimmed from below 20% up to 100% (full illumination) by adjusting the triac dimmer.

With the implementation of the PFC solution, the lamp power can be adjusted by switching

the frequency of the driver circuit that corresponds to the illumination level set by the triac

dimmer. The triac can be triggered at every point during the half cycle and conducts

continuously until the end of the half cycle (180°). In addition, a circuit detects the firing

angle of the triac and adjusts lamp power by adjusting the switching frequency of the half

bridge to control lamp power depending on the position of the triac dimmer.

Please note that while the triac dimmer is connected to the capacitive load (CFL), the triac

will not be fired if trigger angle is set lower than 50°. Because bus voltage (the voltage

across the storage capacitor) falls, the system stops operating at an angle less than 40°. For

this reason, the dimmable ballast cannot be operated in full dimming range. A hysteretic

range per half AC cycle was turned on at 50° and turned off at 40°. Figure 5 shows us the

Gate pulse (trigger) and firing angle per half cycle.

Figure 5.

The half AC line cycle of sinuous wave and firing angle