Electronic ballast for operating fluorescent lamps over wide temperature range

C.S. Moo, T.F. Lin and Y.C. Chuang

Abstract: A high-power-factor electronic ballast is designed to operate fluorescent lamps over a wide temperature range. The electronic ballast extracts a constant power from the AC line source by an embedded power regulation circuit. In addition, the ballast circuit is able to generate sufficiently high starting voltages at different operating temperatures. The design of the electronic ballast is discussed with an illustrative example and experimental results.

1 Introduction

Nowadays, more and more fluorescent lamps incorporating high-frequency electronic ballasts are found in lighting applications [ I , 21. In some applications fluorescent lamps may be operated over a wide temperature range. For example, outdoor lighting in some areas may be operated from warm weather in summer to a temperature below freezing in winter, and lamps installed in a freezing compartment are operated under very low temperatures in normal conditions, but under room temperature for maintenance.

Temperature change affects not only the lighting performance but also the electrical characteristics of the fluorescent lamp [3-61. The deteriorated light output may be compensated for by installing more lamps. However, variations in electrical characteristics may have detrimental effects on the ballast and lamp life because typical topologies used for electronic ballasts include a load resonant inverter, and hence the fluorescent lamp plays an important role on the circuit performance. For this reason, the temperature effects on the operating characteristics can complicate the design of the electronic ballast. An electronic ballast with circuit parameters intended for operating at one temperature may not work well at another temperature, or in some worse cases may destroy the lamp and the ballast [7-91. A good remedy for this problem is to operate the lamp at the rated power for all possible operating temperatures. Conventionally, this achievement requires a sophisticated control circuit with voltage and current sensors [lo].

To solve this problem, an electronic ballast circuit with a simple control is proposed for operating the fluorescent lamp at an approximately constant power. A buck-boost converter embedded in the ballast circuit is used to regulate the input power at a desired level and hence the lamp

0 IEE, 2002 IEE Proceerlinys online no. 20020737 doi: IO. I049/ip-cpa:20020737 Online publishing date: 20th November 2002. Paper first received 7th December 2000 and in revised Ibi-iii 4th September 2002 C.S. Moo and T.F. Lin are with the Power Electronics Laboratory, Department of Electrical En~neering. National Sun Yat-Sen University. Kaohsiuiig, Taiwan, R.O.C. Y.C. Chuang is with the Department of Electrical Engineering, Kung-Shan Institute or Technology, 949 Da Wan Rd., Yung Kang City, Tainan Hsien. Taiwan, R.O.C.

power. The buck-boost converter also performs the current shaping function leading to a high power factor at the input line.

2 Temperature effects on electrical characteristics

Electrical characteristics of fluorescent lamps depend heavily on the operating power and the ambient tempera- ture. A fluorescent lamp when operated at a high frequency can be regarded as a resistance. The equivalent lamp resistance is significantly affected by lamp arc power and ambient temperature. This implies that the lamp power can be very different when the operating temperature is changed.

Fig. 1 illustrates the temperature effect on the lamp power, which is illustrated by the electrical characteristic

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75.0

62.5 0 0.5 1.0 1.5 2.0 2.5 3.0

equivalent lamp resistance, pu

Fig. 1 Temnper.rrtiire @cts on bmnp power

curves of the lamp with the load lines of the ballast circuit. When operated at the same ambient temperature, the equivalent lamp resistance increases as the lamp power is reduced. On the other hand, for an operating lamp power, the lamp designed for rooin temperature use has the maximum equivalent resistance at a bulb wall temperature of about 38°C [4]. Deviation from this temperature causes a decrease in the equivalent lamp resistance. This effect is more significant when the temperature is decreased. Conventionally, an electronic ballast is designed for a specific temperature. Curves A and B are two load lines of electronic ballasts designed for different temperatures. As illustrated by Curve A, the fluorescent lamp will not be able to produce enough light output at a low temperature when the ballast is with circuit parameters designed for room

IEE Proc. -Electr. Poirer. Appl., Vol. l j0. No. 2, Mweli 2003 153

temperature. On the other hand, as illustrated by curve B, the lamp can be destroyed by over-power at room temperature when it is originally intended for a low temperature of -15°C. If the ballast is able to drive the lamp at a constant power, the load line will become horizontal as illustrated by curve C. In other words, the lamp will be kept at the rated power when the ambient temperature changes.

3 Circuit configuration and operation

Fig. 2 shows the circuit configuration of the proposed electronic ballast. The electronic ballast consists of a bridge- rectifier, a half-bridge load resonant inverter, and a buck- boost converter with DC-link capacitor (&. The power switches of the half-bridge inverter are controlled by a duty- ratio regulation circuit, which is adjusted according to the magnitude of the input voltage. The active power switch SI and the diode D7 are commonly used by the buck-boost converter and resonant circuit. D5 and D6 are used for isolating the DC-link voltage from the input line source. These two diodes conduct the charging and discharging currents for the DC-link capacitor, respectively. A small low-pass filter is used at the input line for removing high frequency current harmonics.

load resonant inverter ”..,,; --...

CPi

__.__..........-

Fig. 2 Proposed electronic ballast circuit

Fig. 3 shows the theoretical waveforms of the electronic ballast circuit. According to the conducting power switches, operation of the electronic ballast can be illustrated by four operating modes as shown in Fig. 4. In the equivalent circuits K.ec stands for the rectified line voltage. To reduce the losses at switching-on, the inverter is designed to operate with an inductive load. The load resonant circuit is equivalent to a sinusoidal current source Z,.. In this illustrative case, the duty-ratio of the buck-boost converter D is assumed to be 0.5.

In mode I, SI is turned on. The rectified input voltage is applied on the inductor Lh. At this time, the load resonant current is negative. Ds conducts this negative current to charge the DC-link capacitor. When the load current resonates to zero, D8 turns off and Mode I1 is entered.

In mode 11, SI is kept at the on state. Both the inductor current and the positive resonant current flow through SI . The positive resonant current is conducted by D6 to discharge the DC-link capacitor.

Mode I11 begins at the instant when S I is switched off. The resonant current and the inductor current a re then transferred from SI to D7. When the sum of the resonant current and the inductor current goes to zero, mode I11 ends.

In mode IV, S2 is turned on to carry the negative resonant current and the inductor current. The inductor

Fig. 3 Theoretical wawforms

current freewheels through D5 to charge the DC-link capacitor.

4 High-power-factor

The electronic ballast is supplied from the AC line voltage source:

V, = V, sin (271f,t) ( 1 ) whereA is the line frequency, and V,,, is the peak of the line voltage.

In the practical designj,; is much lower than the operating frequency of the load resonant inverterf,. Under such an assumption, the rectified line voltage V,, can be treated as a constant over a high frequency cycle of the inverter. In modes I and 11, V,,, is applied on the inductor of the

154 IEE Proc.-Electr. Power Appl., Vol. 150, No. 2, March 2003

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, - + - __D

' 0 5 ILb C . d

Fig. 4

buck-boost converter, and the inductor current increases linearly:

Operution modes for proposed circuit

(2) v,,

Z L h ( t ) = --t L h

During modes I11 and IV, the inverse of the DC-link voltage is across the inductor. Hence, the inductor current decreases linearly. The inductor current of the buck-boost converter is designed to operate at discontinuous conduc- tion mode over the entire line frequency cycle. Therefore, the inductor current rises from zero at the beginning of mode I and must decline to zero before the end of mode IV. Then, the peak current in the inductor can be calculated as:

(3) C e c I p ( t ) = -DT, Lh

where To is the high frequency switching period. The inductor current is in a form of high-frequency

pulses. It should be noted that only the rising parts of the

IEE Proc.-Electr. Power Appl., Vol. 150, No. 2, March 2003

current pulses are extracted from the line source. Since the active power switch is operated at a constant frequency with a constant duty-ratio, the peak current of each pulse is proportional to the instantaneous voltage of the line source, and hence is the average current:

Equation (4) indicates that the average current is in phase with the AC line voltage. As a result, a high power factor can be aclueved by removing the hgh-frequency contents by a small filter at the input line terminal.

5 Constant power operation

The average input power can be obtained by taking average of the input power over a line frequency cycle:

Equation (5) reveals that Lh should be small enough to ensure the buck-boost being operated with discontinuous inductor current. This equation also indicates that the electronic ballast extracts a constant input power if the duty-ratio is kept constant for a given input voltage. By deducting the circuit losses from the constant input power, a virtually constant lamp power can be achieved.

In a case where the AC voltage may change, the constant power operation can be achieved by controlling the duty- ratio to be inversely proportional to the magnitude of the input voltage:

This can be easily realised by a simple control circuit. The fluorescent lamp requires a higher starting voltage at

a lower operating temperature. This problem can be solved by increasing the DC-link voltage. The lamp is in a state of open-circuit before starting. Once a sufficient voltage is applied across the lamp, a small glow current flows and then a stable arc current. In spite of the variation in the lamp characteristics, the embedded buck-boost converter extracts a constant power from the AC line source. During the starting period, only a small amount of t h s power is consumed by the ballast. The residual power is then accumulated in the DC-link capacitor, leading to the increase in the DC-link voltage. This, in turn, results in a higher voltage on the lamp. By properly designing the open- circuit voltage gain of the load resonant circuit, a sufficiently hgh starting voltage can be obtained even at a very low temperature.

6 Experimental results

An electronic ballast is designed for driving a fluorescent lamp of 40 W. The input voltage is rated at 110 V, and may vary from 90V to 130V. The line frequency is 60Hz, and the inverter frequency is set at 45 kHz. The duty-ratio of the inverter is set to be 0.5 at the rated input voltage. The maximum inductance of the buck-boost converter Lh is calculated in accordance with (5) to ensure that the inductor current can always be discontinuous for all possible operating conditions. The load resonant circuit is designed to be inductive so that switching losses on the active power

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switches of the inverter can be effectiveIy reduced. The selection of the component values has been discussed in the literature [ l l , 121. The designed circuit parameters are listed in Table 1 .

Table 1: Circuit parameters ~

Input filter inductance, L,

Input filter capacitance, C, DC link capacitance, C,, Series inductance, Ls

Buck-boost inductance, Lb

Series capacitance, Cs Parallel capacitance, C,

10 mH

0.1 pF

100 pF

0.675 mH

0.672 mH

0.56 pF

13.33 nF

Fig. 5 shows the measured waveforms of the relevant voltage and current waveforms. The experiments are carried out at a temperature of 25°C. All waveforms are in agreement with the theoretical predictions in Fig. 3. The currents in SI and D7 are much higher than those in S2 and Dg. The line input voltage and current are shown in Fig. 6. The power factor is 0.98.

Fig. 7 shows the variation of the duty-ratio for achieving constant power operation. The duty-ratio is 0.5 at the rated input voltage. It is increased to 0.62 at a low input voltage of 90V, and decreased to 0.42 at a high input voltage of 130V. When the input voltage is at its lowest level, the inductor current tends to be continuous at the peak. The inductor current is discontinuous at this point as shown in Fig. 8. This ensures that the inductor current is discontin- uous over the entire line frequency cycle for the input voltage range.

Fig. 9 shows the starting transient waveforms when the lamp is operated at a temperature of - 15°C and the input voltage is 90 V. This is the worst case for starting. Once the ballast circuit has been switched on, the DC-link voltage rises up gradually and an open-circuit voltage is applied on the lamp resulting in a small glow current. Afi.er 0.75 seconds, the DC-link voltage is increased up to 430V, the lamp voltage reaches to a level of about 410V. At this point, the lamp is ignited successfully. Then, the arc current starts to flow and the lamp voltage drops abruptly.

Fig. 10 shows the experimental results of the lamp power, the input power factor, and the efficiency of the ballast for a temperature range from -15°C to 60°C. The input power factors are always higher than 0.98 for the entire operating range. An almost constant lamp power can be retained. At a lower temperature, the equivalent lamp resistance becomes smaller, leading to a higher resonant current in the inverter circuit and hence more losses. As a result, the ballast efficiency is as high as 0.88 at room temperature, while it deteriorates to 0.82 at - 15°C. The increase in circuit losses results in a small reduction in lamp power. At the worst case, operating at the temperature of -15OC, the lamp power is slightly reduced to 37.5 W.

7 Conclusions and discussions

An electronic ballast with an embedded buck-boost converter has been proposed for operating fluorescent lamps in a wide temperature range. The lamp power is kept approximately constant over the possible operating range by regulating the input power. When the electronic ballast is supplied from a stable input voltage, the input power remains constant simply by operating the buck-boost

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converter at a fixed duty-ratio. In the case where the input voltage may deviate from its rated value, the input power is regulated by adjusting the duty-ratio of the buck-boost converter. With the duty-ratio control, the dimming capability can also be provided. By designing a proper open-circuit voltage, the electronic ballast is able to generate sufficiently high voltage for starting the fluorescent lamp at very low temperature and low input voltage. In addition, the proposed scheme has the advantage of a high input power factor. Experimental results show that lamp power can be kept almost constant and the electronic ballast is operated with satisfactory performance.

Since fluorescent lamp power is composed of arc power and filament power, the proposed electronic ballast is suitable for driving the lamps both with preheating start and instant start. For the lamps with rapid start, the filament power can vary drastically as the temperature changes. To remedy this problem, the filament power should be separated from the load resonant circuit.

With constant power operation, the DC-link voltage can be very high if the fluorescent lamp fails to start or if a fault occurs in the load inverter circuit. Therefore, over-voltage protection should be included.

8 References

1

2

3

‘IES lighting handbook, reference and application’ (Illuminating Engineering Society of North America, 1995, Sth edn.) HAMMER, E.E.: ‘High frequency characteristics of fluorescent lamps up to 500kHz’, J. Illum. Dzq Soc., 1987, pp. 5 2 4 1 GRAOVAC, M., DAWSON, F.P., FILA, M., and CORMACK, D.E.: ‘Fluorescent lamp cold environment perfonnance improve- ment’. Proceedings of IEEE Industrial Applications Society Annual Meeting, St. Louis, USA, 12-15 Oct. 1998. pp. 215&2163

4 HAMMER, E.E.: ‘Effects of ambient temperature on the perfor- mance of bent tube fluorescent lamps’. fEEE Trans. ffzd Appl., 1989,

5 HAMMER, E.E.: ‘Starting voltage comparisons with various bent tube fluorescent lamps’, J. Illzmn?. Eng Soc., 1990, pp. 2-14

6 LEE, C.R., LIN, Z.T., MOO, C.S., and YEN, H.C.: ‘Temperature effects on high-frequency operating characteristics of fluorescent lamps’. Proccedings of the 19th Symposium on Electrical power engineering. Taipei, Taiwan, 21-22 Nov. 1998, pp. 1154-1 158

7 ALLING, W.R.: ‘Important design parameters for solid-state ballasts’, IEEE Trms., Ind A p / ~ l , 1989, 25, (2), pp. 203-207

8 KAZIMIERCZUK, M.K., and SZARANIEC, W.: ‘Electronic ballast for fluorescent lamps’, IEEE Truns. Power Elec/ron., 1993, 8,

25, (2), pp. 216-223 ’

(4), pp, 386-395 9 COSBY, M.C., and NELMS, R.M.: ‘A resonant inverter for

electronic ballast apolications’, fEEE Trans. Ir7d Electron., 1994, 41, .. (4), pp, 418-425

10 ADAMS, J., RIBARICH, T.J., and RIBARICH, J.: ‘A new control IC for diminable high-freauencv electronic ballasts’. Proceedings of

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IEEE Applied powir e lecbnic Conference, Atlanta, USA, 5-27 Feb. 1999, pp. 713-719 ALONSO, J.M., CALLEJA. A.J., FERRERO, F.J., LOPEZ, E., RIBAS, J., and RICO, M.: ‘Single-stage constant-wattage high- power-factor electronic ballast with dimming capability’. Proceedings of IEEE Power electronics specialists’ Conference, Fukuoka, Japan, 19-22 May 1998, pp. 2021-2027 MOO, C.S., CHENG, H.L., and CHANG, Y.N.: ‘Single-stage high-power-factor electronic ballast with asymmetrical pulse- width-modulation for fluorescent lamps’, IEE Proc., Electr Power Appl., 2001, 148, (2), pp. 125-132

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