ST AN2590 APPLICATION NOTE

AN2590

Application note

20 W dimmable CFL ballast solution

Introduction

This application note describes the 20 W dimmable CFL ballast solution which can completely replace incandescent lamps in formal dimming systems. The dimmer controlled range is 20% - 100% of full light output without changing the formal triac dimmer.

Compared to the formal CFL solution, this solution not only provides energy savings, but also higher reliability and a much longer operating life.

The solution is based on the high performance ballast driver L6574D, two of the STD4NK50Z Zener protected SuperMESH™ Power MOSFETs, and two of the STTH1L06 turbo 2 ultra fast high voltage rectifiers.

Figure 1. Typical dimmer

Figure 2. Dimming CFL ballast solution

October 2007

Rev 1

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Contents	AN2590

Contents

1	Typical dimming system for incandescent lamps . . . . . . . . . . . . . . . .		. 4
	1.1	Formal CFL ballast and its disadvantages . . . . . . . . . . . . . . . . . . . . . . . . .	4
	1.2	Proposed solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
2	Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
	2.1	Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	2.2	Dimming function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	2.3	Principle of the main circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	2.4	Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
3	Electrical specifications and reference design board . . . . . . . . . . . . .		10
	3.1	Electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
	3.2	PCB layout view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
4	Test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11
5	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14
	5.1	References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
6	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		16

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AN2590	List of figures

List of figures

Figure 1. Typical dimmer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Dimming CFL ballast solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 3. Typical dimming system consisting of incandescent lamp and triac dimmer . . . . . . . . . . . . 4 Figure 4. Block diagram of formal CFL ballast. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 5. The half AC line cycle of sinuous wave and firing angle. . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 6. Schematic of the reference design board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 7. Main circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 8. L1 current per switching period. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 9. L1 current shape. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 10. PCB top-side view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 11. PCB bottom-side view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 12. Full load efficiency vs. AC line input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 13. Triac turn-on time vs. power dissipation to the lamp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 14. AC input voltage and current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 15. L1 voltage and current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 16. Power factor in operating range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 17. Lamp voltage and current at Ton=9 ms, Vlamp=100 V/div, Ilamp= 200 mA/div. . . . . . . . . 13 Figure 18. Lamp voltage and current at Ton=5 ms, Vlamp=100 V/div, Ilamp= 200 mA/div. . . . . . . . . 13 Figure 19. Lamp voltage and current at Ton=3 ms, Vlamp=100 V/div, Ilamp= 200 mA/div. . . . . . . . . 13

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Typical dimming system for incandescent lamps	AN2590

1 Typical dimming system for incandescent lamps

A typical dimming system consists of the lamp(s) and dimmer (Figure 3). The dimmer consists of the triac and a few components, see Figure 1. Dimmers of this type are appropriate for incandescent lamps and are very popular in the market.

Figure 3. Typical dimming system consisting of incandescent lamp and triac dimmer

The triac conducts once it has been triggered and holds latching current. The triac shuts down when the current is less than the holding current. The dimmer works fine with a resistive load. The triac can be triggered at any timing of the sinuous voltage (AC line input), and can be kept in conduction state until reaching zero line voltage. This allows the lamp to be dimmed nearly to 100%.

1.1Formal CFL ballast and its disadvantages

Since the formal CFL is not a resistive load, the ballast has no power factor correction circuit (PFC). If the incandescent lamp (Tungsten filament light bulb) is replaced by a formal CFL (Compact Florescent Lamp) directly, the CFL cannot fully illuminate and may have intermittant blinking or no light at all. Figure 4 shows the block diagram of a formal CFL.

Figure 4. Block diagram of formal CFL ballast

The formal CFL ballast consists of a rectifier, storage capacitor, ballast driver circuit, half bridge and resonant circuit. The source energy stored only to the storage capacitor from the AC input at the timing nears its peak voltage per half cycle. When the formal CFL ballast is connected to the triac dimmer (see Figure 1), the triac conducts only after having been triggered, that is, when the rectified voltage is higher than the voltage across the storage capacitor. At this extreme moment the capacitor can be charged with the same peak from

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AN2590	Typical dimming system for incandescent lamps

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.

1.2Proposed 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

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Board description	AN2590

2 Board description

2.1Application circuit

The reference design board is made according to the schematic shown in Figure 6. It includes an LC filter, bridge rectifier, single stage PFC with high frequency ballast driver and resonant circuit.

Figure 6. Schematic of the reference design board

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