Design of Electronic Ballast for Fluorescent Lamp Dimming-EEWORLD

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Abstract: This paper introduces the design of dimming electronic ballast based on fluorescent lamp dedicated current negative feedback dimming control chip KA7543 and power factor correction chip KA7526. The paper focuses on the necessity of soft start and the corresponding technical solutions, as well as the mechanism and method of dimming and power factor correction.

Keywords: electronic ballast; dimming; soft start; power factor

1 Introduction

In 1999, the annual output of fluorescent lamps in China was 378.76 million, of which 83.8 million were T8 fluorescent lamps [1]. Therefore, improving lighting efficiency and quality is of great significance for saving energy and improving working and living conditions. Compared with traditional inductive ballasts, high-frequency electronic ballasts have a series of advantages such as light weight, small size, high efficiency, no flicker, easy dimming and high power factor.

In the 1980s, the market performance of electronic ballasts with discrete components was not satisfactory. The reason was that discrete ballasts had disadvantages such as poor reliability, high price, pollution to the power grid, and difficulty in dimming. In the 1990s, foreign companies launched integrated control chips for fluorescent lamps, and high-frequency dimming electronic ballasts finally made great progress in the market. Such chips include TDA4817, SG3561A, MC34261, UC3852, KA7543[2], etc. After analyzing Philips' HF-R36TLD and BETPLTD's DTK805 dimming electronic ballast products, the two-chip structure (control chip + power factor correction chip) was the mainstream structure. Microlinear's ML4830 and ML4835[3] chips are of one-chip structure, that is, the control chip and power factor correction chip are integrated into one chip. In 2001, Fudan University Microelectronics Co., Ltd. successfully developed the FM2811 fluorescent lamp electronic ballast chip, which is of one-chip structure. This article discusses FairChild's dimming electronic ballast control chip KA7543 and power factor correction chip KA7526.

2 Chip Introduction

2.1 Structure and function of dimming ballast control chip KA7543

KA7543 is packaged in 14-pin DIP or SOP. The pin configuration is shown in Figure 1 and the pin functions are given in Table 1.

Figure 1 KA7543 pinout

KA7543 has functions such as fluorescent lamp preheating soft start, wide range stepless dimming, no-light protection and abnormal protection, undervoltage self-locking, etc. Figure 2 is the internal function block diagram of KA7543.

Figure 2 KA7543 internal function block diagram

2.2 Structure and function of power factor correction chip KA7526

KA7526 is a power factor correction controller (PFC) chip for fluorescent lamps. The active PFC boost converter embedded in KA7526 adopts a multiplier method [4].

Table 1 KA7543 pin function table

Pinout	name	Function
1	OUT1	Driver output 1
2	NC	No definition
3	Vcc	Power supply
4	Cc	Compensation input
5	V5	5V voltage output
6	VfB	Negative feedback input
7	Vab	Abnormal protection input
8	VI	Light detection input
9	Vdm	Dimming control input
10	Cs	Soft start time control input
11	Cdm	Soft dimming control input
12	GND	Grounding
13	NC	No definition
14	OUT2	Driver output 2

A sinusoidal current in phase with the AC input voltage is generated at the power input, which makes the load appear as a pure resistor, that is, the power factor is approximately 1. The pin functions of KA7526 are shown in Table 2.

Table 2 KA7526 pin function table

Pinout	name	Function
1	INV	Error amplifier inverting input
2	EAOUT	Error amplifier output
3	MULT	Multiplier Input
4	CS	PWM comparator input
5	Idet	Zero current detection input
6	GND	Grounding
7	OUT	Power drive output
8	Vcc	Power pin

3 Principle of dimmable electronic ballast

3.1 Circuit Description

As shown in Figure 3, the dimmable electronic ballast can be divided into three parts: a fluorescent lamp dimming control module composed of KA7543 and its peripheral circuits; a power factor correction module composed of KA7526 and its peripheral circuits; and _an_{electromagnetic} interference (EMI) filter composed of L1 , _C1 , C2 , C3 , C4 , V1 _{,
and NTC.}_{V1
is a varistor that suppresses transient voltage, and}_{NTC
is a thermistor that suppresses surge current impact. The filter can}_suppress common-mode and differential-mode interference from the power supply, and at the same time prevent the switching noise generated by the ballast itself from interfering with other electrical appliances.

Figure 3 Dimming electronic ballast circuit diagram

3.2 Soft Start

ZBK74012-90 divides the starting methods of electronic ballasts into two categories: preheating start (soft start) and non-preheating start (hard start). Fluorescent lamps are hot cathodes and belong to thermal electron emission. Hard start causes the lamp to undergo two glow discharge periods. During this period, the space charge around the lamp cathode is very thin, causing the cathode emission material to sputter rapidly, causing the lamp to blacken early and shorten its life. Reference [5] conducted an in-depth study on this, and the test results are shown in Table 3.

Table 3 Hard start and soft start lamp life

Hard boot		Soft Start
Lamp No.	Number of starts	Lamp No.	Number of starts
1	10500	6	16200
2	9100	7	16100
3	7500	8	16470
4	9180	9	23560
5	5600	10	22730
average value	8772	average value	19010

The test data in Table 3 show that the life of soft-start lamps is more than twice as long as that of hard-start lamps on average.

_{When the voltage Vcc} on pin 3 of KA7543 reaches the start threshold voltage (8.5V), the internal UVLO circuit provides reference voltage (Vref = 2V±5%) and bias current for all circuits in the IC . The soft start circuit in the IC starts to charge the soft start capacitor Cs (C22 in Figure 3), and the IC outputs a preheating frequency fpre that is about 30% higher than the normal operating frequency _fnor ( about 30kHz ) _._The highest soft _start frequency is determined _by the voltage on pin 4 ( Cc _{)
. As}_the voltage Vcs on Cs rises linearly, the switching frequency decreases linearly. When the frequency drops to _{the
natural frequency}_f0 of the LC series resonant circuit , the LC circuit generates series resonance and generates a high voltage pulse of about 1kV at both ends of the lamp tube, which breaks down the lamp tube and ignites it. At this time, the output frequency of the IC drops to the operating frequency fnor , _and the entire soft start time is 0.8~1s.

The startup circuit charges the IC through the resistor R _st ( R ₂₁ in Figure 3 ). The value of R _st can be calculated using equation (1).

Rst ₌ (1)

Where: Vin _is the input voltage after rectification;

V _thmax is the maximum startup threshold voltage of the chip KA7543;

Istmax is _the maximum starting current.

In the circuit of Figure 3, V _in = 220V, V _thmax = 10.5V, I _stmax = 0.25mA. It can be concluded that R _st = 1.2MΩ.

The soft start time is determined by the size of the soft start capacitor _Cs . When the start voltage reaches the threshold voltage, a 313nA current source inside the IC charges the soft start capacitor _{Cs until the voltage Vcs reaches 2V. Therefore, the soft start time ts}_can be calculated by formula ( ₂ ).

ts ₌ (2)

If Cs = _0.2μF , then ts = _1.28s .

3.3 Stepless dimming control

The electronic ballast operates in three stages, that is, after power-on, the ballast operates at the preheating frequency ( f _pre ); after the lamp cathode is fully preheated, it enters the starting frequency ( f ₀ ); and after the lamp tube is ignited, it switches to the operating frequency ( f _nor ).

Reference [6] lists and compares the four existing dimming methods for fluorescent lamps, namely input voltage phase control, ballast impedance control, operating frequency control, and applying periodic discontinuous voltage to the lamp. The conclusion is that the operating frequency control method has the best comprehensive indicators. KA7543 uses the operating frequency control method. The internal dimming circuit of KA7543 is located between pin 9 (Vdm), pin 11 (Cdm) and pin 6 (Vfb). The dimming voltage applied to pin 9 ranges from 0 to 2V, with 0V corresponding to full light and 2V corresponding to full dimming. That is, when the dimming control voltage at pin 9 increases, the switching frequency increases, and _{the
impedance}_of L2 and L3 increases, resulting in a decrease in the lamp current and a dimming of the lamp.

FIG4 is an experimental waveform diagram of lamp voltage and current at different brightness.

(a) Minimum output power, voltage 40V/grid, current 20mA/grid

(b) 60% output power, voltage 40V/grid, current 500mA/grid

Figure 4 Lamp voltage and current waveforms at different brightness

4 Conclusion

It is feasible to design a dimmable electronic ballast using a dedicated integrated chip for fluorescent lamps. The technical and economic indicators of the product are significantly better than those of electronic ballasts using discrete components and traditional inductive ballasts. EMI filtering and PFC technology are used to better meet IEC harmonic standards. When working in the 20% to 100% brightness range, the stepless linear dimming works stably, and the number of lamp openings increases by about 20%. When working in the 10% to 20% brightness range, the fluorescent lamp is not stable. The efficiency is measured to increase by about 15%, which has not reached the theoretical 30.2%, and further optimization and improvement are needed.

Reference address：Design of Electronic Ballast for Fluorescent Lamp Dimming

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