HIGH POWER FACTOR ELECTRONIC BALLAST WITH CONSTANT DC LINK VOLTAGE R. 0. Brioschi and J. L. F. Vieira Universidade Federal do Espirito Santo Departamento de Engenharia Elktrica - LEPAC - CP: 01-901 1 29060-970 - Vit6ria - ES - Brazil Phone: 55.27.335.2699 - Fax: 55.27.335.2650 - E-Mail:
[email protected]
Abstract - This paper presents a high power factor electronic ballast, based on a single power processing stage, with constant DC link voltage. The switching frequency is controlled to maintain constant the DC link voltage and the voltage across the switches, independently of changes in the AC input voltage. This control method assures zero voltage switching for the specified AC input voltage range. Besides, with an appropriate design of the fluorescent lamps drive circuit, the lamps current can be kept close to the rated value. The power factor correction stage is formed by a boost converter operating in discontinuous conduction mode, which naturally provides high power factor to the utility line. The fluorescent lamps are driven by an unmodulated sine wave current generated from a LC parallel resonant converter, which operates above the resonant frequency to perform zero voltage switching. Theoretical analysis and experimental results are presented for two series connected 40W fluorescent lamps operating from 127V 2 lo%, 60Hz utility line. The switching frequency is changed from 25kHz to 45kHz to maintain the DC link voltage regulated at 410V. The experimental results confirm the high efficiency and the high power factor of this electronic ballast.
I. INTRODUCTION
Fluorescent lamps are usually preferred to replace the incandescent lamps, because they inherently have longer lifetime and yield higher efficacy [l].However, these lamps require high striking voltage during starting and current limiting control after starting because they have negative impedance characteristics. Traditional magnetic ballasts, operating at line frequency, have been used to solved these problems. In spite of their low cost, these ballasts present flickering, high size and weight, hum and stroboscopic effects [2,3,4]. When operating at high frequency, fluorescent lamps present the following characteristics [I ,2,3,4]: the luminous efficacy increases by about 10% which reduces the energy consumption, flickering as well as stroboscopic effects can be eliminated, and the audible noise fall to unnoticeable levels. To obtain these benefits, as well as smaller size and weight, electronic ballasts are used instead of magnetic ballast. The preferred method to drive the fluorescent lamp is with an unmodulated sine wave current with a minimal ripple content. The current crest factor (Ipeak/jrms) for the operating condition should be as low as possible, not exceeding 1.7 [l]. The LC parallel resonant converter has been attractive for this application, because it ensures a sine
0-7803-3840-5/97/$10.008 1997 IEEE
wave current for the lamp with low crest factor, as well as establishes an appropriated voltage during the ignition process, and maintain a steady-state rated current. A compact high power factor electronic ballast incorporating a LC parallel resonant converter can be obtained when high frequency is used. However, at high frequency soft commutation techniques are recommend to achieve high efficiency [5,6]. The utility line can be more efficiently utilized when high power factor (HPF) with low total harmonic distortion (THD) is achieve. The advantages of the HPF with low THD include reduction in the rms line current and in the line current harmonic distortion [7]. HPF can be obtained using two power processing stages. The first one is a high power factor preregulator stage, which converts the AC input voltage to a DC. The second stage transforms the DC voltage to a high frequency AC voltage to drive the fluorescent lamps. Active power factor correction (PFC) can be performed by a boost preregulator operating in continuous conduction mode, where the inductor boost current must follows a sinusoidal reference waveform. This method provides nearly unity power factor with THD less than 5% [2,4,81. When a boost converter operates as a preregulator stage in discontinuous conduction mode, the input current follows naturally the sinusoidal waveform of the input voltage, providing HPF to the utility line [9]. Two power processing stages increases the final cost, besides reducing the electronic ballast reliability. An interesting option to avoid these problems are the HPF electronic ballasts based on a single power processing stage [5,6,7,10,11,12,13,14,15]. This paper presents a HPF electronic ballast based on a single power processing stage, in which a boost converter operates in discontinuous conduction mode [161. Besides that, by controlling the switching frequency this ballast maintain constant the DC link voltage. A LC parallel resonant converter is used to drive the fluorescent lamps by an unmodulated sine wave current. To obtain zero voltage switching, the electronic ballast operates above the resonant frequency. This electronic ballast has been designed to drive two series connected 40W fluorescent lamps operating from 127V & lo%, 60Hz utility line. To maintain the DC link voltage regulated in 410V the switching frequency is changed from 25kHz to 45kHz.
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11. CIRCUITDESCRIPTION
In order to obtain a simple HPF electronic ballast, the power factor correction stage and the inverter output stage are combined in a single stage. This is accomplished by allowing both stages to share the two switches of the electronic ballast. The DC link voltage and, consequently, the switches voltage are kept in a fixed value, independently of the AC line voltage variations. This is obtained controlling the electronic ballast switching frequency. By employing this control method, zero voltage switching can be ensured for the specified AC input voltage range. In addition, with an appropriate design of the fluorescent lamps drive circuit, the lamps current can be maintained close to the rated value. The complete diagram of this electronic ballast is shown in Fig. 1. Figure l a shows the power stage diagram. As can be seen, the power factor correction stage is formed by a boost converter operating in discontinuous conduction mode. The fluorescent lamps are driven by a LC parallel resonant stage, which operates above the resonant frequency to perform zero voltage switching. The control circuit, shalwn in Fig. lb, is based on a PWM regulator IC (UC 3525). The ballast switching frequency is adjusted by altering the equivalent resistance of the internal oscillator circuit of the UC 3525 IC.
Fig. 2. Lamps circuit using just one DC link capacitor. 111. PRINCIPLE OF OPERATION
This; electronic ballast can be viewed as being composed by two simplified independent converters. The first oiie is obtained when the LC parallel resonant converter is considered as a resistive load to Col and Co2. The resulting converter is the two switches boost converter preregulator stage shown in Fig. 3. S2 G2
'Lb
M2 tb
(1
P
R
y G1
46%) r.., M1
CO
-k 0
l-9
vo
Fig. 3. Two switches boost converter. The mains waveforms of two switches boost converter
are shown in Fig. 4.
4 "Lb
-(Vo -Vin)
..............
.............
.......
.............
VO
I
\ ! L7
Ibm ..............................................
Fig. 1. Electronic ballast diagram: (a) power stage and (b) control circuit.
Ag. 4. Mains waveforms of conventional boost.
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C. Power Factor n e power factor is defined by: PF =
Pin
(9)
.iACcm
Considering that the input voltage does not have harmonic components, the power factor can be given by: Fig. 5. Simplified LC parallel resonant converter.
l[v.RELEVANTANALYSIS The relevant chasacteristics of the electronic ballast are defined by the: input cment, power factor and THD. The main parameters to be determined are: the boost inductance and the resonant parameters.
The proposed electronic ballast power factor as a functions of a, given by equation (lo), is shown in Fig. 6. PF
A. Input Current 0 998
The inductor boost peak current during a switching period, obtained from Fig. 4, is given by: I h m =-
vin.tc
0 996
(1)
0 994
Lh
The linear increasing and decreasing times are given by the following equations:
[ ::) I---
t =-
2.f,
a
0 992
Fig. - 6. Power factor as function of
a.
(2)
D. Total Harmonic Distortion - THD
(3)
Considering unit displacement factor, the THQ can be defined by:
Due to the high frequency input filter, the AC line current should be given by the instantaneous mean value of the DC link current, according to the fol!owing equation 191:
TRD
=PF- L J - - -
(11)
The total harmonic distortion (THD) as a function of a is shown in Fig. 7.
where:
Input Power
'
0.2
0.1
0.3
0.4
0.5
Fig. 7. THD as function of a
The input power is obtained from the following equation:
E. Normalized Output Characteristics ofthe Boost Stage
it
v,, (o).iAC(e ) . m
(6)
The instantaneous mean value of current io, obtained from Fig. 4, is given by:
0
where:
vAC(e) = V,.sen(B)
(7)
vi
(12) 8.f,.L,.(Vo -Vin) By integrating equation (12) in a rectified input voltage Iom
From equations (4) - (7) the following expression results:
p.in =
82
=
Substitution of equation (12) into equation (13), one obtains:
'
sen2 0 V, - sen 0
where:
The: LC parallel resonant circuit can be designed to operate: at the undamped natural frequency f, = f,, selected as the middle point of the operating frequency range. At this frequency, the fundamental components amplitudes of voltage across C, and current through R are, respectively [21: vcp= 2 . V o . Q ~ (22) 7T
-
vo vo =-
(16)
VP fmin- minimum switching frequency The normalized output characteristics of the boost stage as a function of the switching frequency is shown in Fig. 8. As can be seen, by controlling the switching frequency, the DC link voltage V, can be kept constant, independently of the AC input voltage variations.
-
VO 3
2.v0 I, =IC. Z, When the fluorescents lamps are off, they can be considered as an open circuit. Therefore, the quality factor at start up is very high. As shown by equation (22), the voltage across the lamps will be high enough to striking them. Ad steady-state, the LC parallel circuit operates above the resonant frequency (fs> fJ, providing ZVS. The lamps power, obtained from equation (23), is: R.1; -2.R.V: p, =--2 n2.Z?
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V. DESIGNPROCEDURE
24
A electronic ballast prototype has been built to meet the following specifications:
22
A. Inpui! Data:
Io
2 0
1
0.8
1.2
1.4
Fig. 8. Normalized output characteristics of the boost stage as a function of fs/fs,,,,. F. Resonant Parameters
The LC parallel resonant Circuit, shown in Fig. 5, is a second order low-pass filter, which can be described by the following equation [2,191:
- undamped natural frequency
f, =
B. Selection ofthe DCLink voltage:
As the boost stage must operate at discontinuous conduction mode, the voltage V, has to be larger than twice the maximum AC peak voltage Vpax = 195V. In this case, V, = 410V has been selected.
(17)
2nd-
4"
r,
Z, =
- characteristic impedance
CP R
QL = - - quality factor at the natural frequency
z,
id;
f, = f, 1--
, for QL 2 1 - resonant frequency
(")
C. a Parameter, Power Factor and THD
(19)
For the lowest input voltage (Vpk = 160V), that also results in the smallest value for the switching frequency (fmh = 25kHz), a value of a = 0.39 is obtained from equation (5). From Fig. 6 and 7 results in: FP = 0.996, THD = 8.23%.
(20)
The LC parallel circuit is fed by a high frequency square wave voltage source Vsw of magnitude V,. Its fundamental
component,
obtained
from
the
- AC input voltage: 127V rms 2 lo%, 60Hz; - output power: Po= 72W; - switching frequency 25kHz c f, < 45kHz; - fluorescent lamp rated current: I,, = 0.35A; - fluorescent lamp ignition voltage: Vig= 800V; - efficiency: 2 90% .
D. Boost Inductance
Fourier
analysis, is given by: 2.v, v1 =-.sen(2n.f8.t:)
From Fig. 8, for a normalized current To = 0.81and from equation (20) for: ,f = 25kHz, IomT= P,N, = 0.17A and V, = 160V, it can be obtained: Lb = 1.2mH.
n
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TBK Stop: 250kS/s
E. Resonant Parameters
8 Acqs
0 I
'l"l---l
I
The lamps equivalent resistance is R =Po/(I,)2= 653 rR. From equation (24) results: 2, = 527.3rR. For f, = f, = 35kHz (mean operation frequency), and using equations (22) and (23) one obtains: C, = 8.64nF, L, = 2.4mH. Equations (19) e (20) results in: QL=1.24 and f, = 20.7kHz. VI. EXPERIMENTAL.RESULTS
A electronic ballast prototype has been built to meet the input data specifications. The diagram is shown in Fig. 1, whose the main parameters and components are the following:
Fig. 10. Rectified AC input voltage vin (lOOV/div) and the boost inductor current iLb(lA/div); Time scale: 2ms/div. TeKStOp 100MS16
- Lb = 1,3 mH, 140 turns on core EE 30/14, IP6; - L, = 2,43 mH, 112 turns on core EE 30/7, IP6; - Lf = 1 mH, 60 turns on core EE 20/10, IP6; - Pulse transformer, 15/15/15 turns on core EE 20/10, IP6; - Input diode rectifier bridge, D1 - D4, 1N4004; - Fast diode, D5: SK3GF04 (Semikron); - M1, M2: IRF 840 (International Rectifier) -
18 Acqs
0
1
Cpl = CP2= 10 nF/630V
Experimental waveforms have been obtained for: VAC = 127V, IAC = 0.632A, f, = 37kHz and V, = 410V. The AC input current and voltage, which demonstrate the high power factor of this electronic ballast, are shown in Fig. 9. The rectified input voltage Vin and the boost inductor current, presenting an 120Hz envelope, are shown in Fig.10. The MOSFETs commutations showing the zero voltage switching can be seen in Fig. 11. Fig. 12 shows the resonant current iLr and the high frequency voltage VMl. The fluorescents lamps voltage V, and the resonant current iLr are shown in Fig 13. The LC parallel resonant currents as a function of the rms input voltage are shown in Fig. 14. The experimentally obtained characteristics were: q = 92%, (b) PF = 0.99 and THD = 10%. Fig. 11. MOSFETs commutations: (a) vM1 (lOOV/div) and iM1(OSNdiv), (b) vM2(lOOV/div) and iM2(lA/div); Time TQK stop' 25OkS/s 7 Acqr I T 1 scale: 5ps/div. Tek Stop lOOMS/s
7Aqs
I
2
1
Fig. 9. Low frequency voltages and currents: input voltage vAC(SOV/div) and input current iAc(OSNdiv); Time scale: 2ms/div.
Fig. 12. Resonant current ih (OSNdiv) and the high frequency voltage: VMl (lOOV/div);Time scale: 5ps/div.
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ACKNOWLEDGMENT
The: authors would like to express their gratitude to "Thornton Inpec Eletronica" Itda. for contributing the magnetic core for this project.
REFERENCES [l] Edw,ud E. Hammer and Terry K. McGowan, "Characteristics of various F40 Fluorescent Systems at 60Hz and High frequency", IEEE: Transactions on Industry Applications, V0l.IA-21, No. 1, JanuarylFebruary 1985, pp. 11-16. [2] M. K. Kazimierczuk and W. Szaraniek, "Electronic Ballast for Fhiorescent Lamps", IEEETransactions on Power Electronics, vol. 8, No. 4, October 1993,pp. 386-395. [3] E.C. Nho, K.H. Jee and G.H. Cho, "New Soft-Switching for High Efficiency Electronic Ballast with Simple Structure", Int. Journal of Electronics, Vo1.71, No.3, 1991, pp. 529-542. [4] Laszlo Laskai and Ira J. Pitel, "Discharge Lamp Ballasting", IEEE PESC'95, Tutorial 2,1995, Atlanta, GA, USA. [SI J.L. Freitas Vieira, Mckcio A. C6 and Lucian0 D. Zorzal, "High Power Factor Electronic Ballast Based on a Single Power Processing Stage", IEEE-PESC Proc., 1995, pp. 687-693. [6] Mkcio A. C6, Domingos S.L. Simonetti and J.L. Freitas Vieira, "High Power Factor Electronic Ballast Operating at Critical Conduction Mode", IEEEPESC Proc., 1996, pp.. [7] Ed Deng and Slobodan Cuk, "Single Stage, High Power Factor, Lamp 1994, pp. 441-449. BdliW", IEEEAPEC ROC., [8] Jim Spangler and Anup KBehera, "Power Factor Correction Used for Fluorescent Lamp Ballast", IEEE-IASProc., 1991, pp.18361841. [9] Kwang-Hwa Liu and Yung-Lin Lin, "Current Waveform Distortion In Power Factor Correction Circuits Employing Discontinuous-Mode Boost Converters", IEEE-PESC Proc., 1989, pp.825-829. [lo] I. Takahashi, "Power Factor Improvement of a Diode Rectifier Circuit", IEEEIAS Annual Meet. Proc., 1990, pp.1289- 1294. [ 1I] Lasvio Laskai, Presad Enjeti and Ira J. Pitel, "A Unity Power Factor Electionic Ballast for Metal Halide Lamps", IEEE-APEC Proc., 1994, pp. 31 -37. [12] C m e l o Licitra, h i g i Malesani, Giogio Spiazzi, Paolo Tenti and Antonio Testa, "Single-Ended Soft-SwitchingElectronic Ballast with Unit Power Factor", IEEE-APECProc., 1991, pp. 953-957. [13] Ed Deng and Slobodan Cuk, "Single Switch, Unit Power Factor, Lamp Ballasts", IEEEAPEC Proc.,1995, pp. 670-676. [14] Cecilio Blanco, Marcos Alonso, Emilio U p e g Antonio Calleja and Manuel Rico, "A Single Stage Fluorescent Lamp Ballast with High Power Factor", IEEE-APECProc., 1996, pp. 616-621. [15] Wei Chen Fred C. Lee and Tokushi Yamauchi, ''AIImproved Charge Pump Electronic Ballast with Low THD and Low Crest Factor", IEEE-APECProc., 1996, pp. 622-627. [16] Peter N. Wood, "Electronic Ballasts Using the Cost-Saving IR2155 Driver", Application Notes AN-955, Intemauonal Rectifier, 1994. [I71 M.I. Mahmoud, "Design Parameters for High frequency Series Resonance Energy Converters Used as Fluorescent Lamp Electronic Ballast", EPE ROC., 1989, pp. 367-371. [le] Peter N. Wood, "High Frequency Discharge Lamp Ballasts Using Power MOSFETs, IGBT's and High Voltage Monolithic Drivers", PCI hoc.,1989, pp.307-325. [I91 T. H. Yu, H. M., Huang and T. F. Wu, "Self Excited Half-Bridge Series Resonance Parallel Loaded fluorescent Lamp Electronic Ballast", IEEEAPEC Proc.; 1989,pp. 657-664.
-
Fig. 13 - Resonant current iLr(O.SA/div) and fluorescents lamps voltage: vm (lOOV/div); Time scale: Spddiv.
V*,(V)
o 4160 - ,110 -
Fig. 14 - LC parallel resonant currents as a function of the rms input voltage: ILr - resonant inductor current, Ia- fluorescents lamps current and Icp - parallel capacitor current.
VII. CONCLUSION A high power factor electronic ballast, based on a single power processing stage, with constant DC link voltage was introduced in this paper. By controlling the switching frequency DC link voltage and voltage across the switches are maintained constant, independently of changes in the AC input voltage. This contriol method ensures zero voltage switching for the specified AC input voltage range. Besides, with an appropriate design of the resonant converter, the lamps current js kept close to the rated value. A simple HPF electronic ballast has been obtained when the power factor correction stage and the inverter output stage were combined in a single stage. This was accomplished by allowing both stages to share the two switches of the electronic ballast. The power factor correction stage is formed by a boost converter operating in discontinuous conduction mode. This operation mode ensures that the input current naturally follows the sinusoidal waveform of the input voltage. The fluorescent lamps are driven by an unmodulated sine wave current generated from a LC paralllel resonant converter, which operates above the resonant frequency to perform zero voltage switching. Theoretical analysis and experimental results are presented for two series connected 40W fluorescent lamps operating firom 127V k lo%, 6OHz utility line. The switching frequency is changed from 25kHz to
45kHz to maintain the DC link voltage regulated at 410V. The experimental results confirm the high efficiency and the high power factor of this t:lectronic ballast.
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