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A 1.2 kW Electronic Ballast for Multiple Lam ps, with D imm ing Capabilityand High-Power-FactorRoger Gules and Ivo Barbi

Federal University of Santa Catarina -UFSCPower E lectronic Institute- NEPP.O. Box 5 119 - 88010-970- Florian6polis-SC-Brazil

Euripedes Martins SimdesLupaquai- ndustria e Comercio Ltda.AV. Eng. AntBnio Francisco de Paula Souza,1771,13043-540 - Campinas - SP - BrazilPhone: +55-48-33 1-9204 - Fax: +55-48-234-5422 Phone: 019-230-4655 Fax:019-230-7893E-mail: roger0 ,ineu.ufsc .br Internet: httD://www.ineD.ufsc.br E-mail: luDacluai@,correionet.co m.brAbstract- A high-frequency lighting system for fluorescentlamp is introduced in this paper. The characteristics of the

proposed lighting system are high performance, high powerfactor with low input current harmonic distortion, dimmingcapability, low current lamps crest factor, and lower cost whencompared with the traditional system with similarcharacteristics. The use of this system in place of theconventional electrom agnetic ballast results in lower pow erconsumption and reduced maintenance cost, because theelectronic system has higher efficiency and longer lamp lifetimethan the electromagnetic ballast. In comparison with theconventional electronic ballast, the proposed system presents asignificant reduction of cost. This reduction becomes moremeaningful with the gro wing of the lamp number.The description of the lighting system, design procedure andexperimental results taken from a 1.2 kW prototype a re includedin the paper.

I. INTRODUCTIONNowadays, the fluorescent lamp is one of the mostpopular lighting systems due to the high luminous efficiency,energy-saving and long lamp lifetime. However, thefluorescent lamps present a negative incremental impedanceand must be operated in series with a current limiting device

named ballast. The most traditional is the electromagneticballast, composed by a large inductor that presents greatsimplicity and low cost. However it must operate at linefrequency (50/60Hz), resulting in high volume and weight,audible noise, flicker as well as low efici enc y.The high-frequency electronic ballast has been widelyemployed s a solution for these problems. This ballastcommonly uses a rectifier stage and a high-frequency inverterwith a resonan t filter that transforms the switching converteroutput characteristic from a voltage source into a currentsource, thus ensuring stable lamp operation. To satisfy theregulations for harmonic current emissions from electronicequipment, and to maintain high pow er quality, a high-power-factor AC-DC converter can replace the rectifier stage.Dimming feature in the fluorescent lamps can also beimplemented with electronic ballast, allowing to control thelamps luminous flux and to optimize the energy delivered tothe lamp, thus providing an important energy saving.However, all these advantages reached with the use ofelectronic ballast have as a consequence an increase in thelighting system cost when compared with the traditionalelectromagnetic ballast. Though the energy savingscompensate the increase of system cost, the higher initial cost

is a problem for the implementation of high efficiencylighting system.In order to make the high performance electronic ballastmore competitive, a high efficiency and low cost high-frequency lighting system is proposed in this paper.11. PROPOSED IGHTING YSTEM

The proposed lighting system is composed by a high-power-factor rectifier that supplies a high-frequency zero-voltage-switching (ZVS) inverter. The output voltage of theAC-DC converter is regulated, maintaining the power of thelamps constant, even for large variations in the utility voltage.The DC voltage has a low ripple, resulting a low currentlamps crest factor. The inverter works with imposedswitching frequency, constant duty-cycle and ZVScommutation. The lighting control is accomplished changingthe inverter switching frequency. The inv erter output voltageis conducted to the resonant filters that provide the lampignition voltage and stable operation after the ign ition.

I

I II 1Fig.1. High-frequency lighting system.

A . High-Power-Factor RectifierThe topology used in this stage is the SEPIC converteroperating in continuous conduction mode (CCM) andaverage-current-control [11. With the SEPIC converter high-power-factor (HPF) and the same DC output voltage that aconventional diodes bridge with capacitive filter, is obtained.Thus, it is possible to use a full configuration of the lightingsystem with HPF, or a low cost configuration with a diodesbridge and capacitive filter rectifier, without changes in thefollowing stages (inverter and resonant filters). The powercircuit and the control block diagram of the S EPIC converter,used as power factor corrector in CCM , is shown in Fig. 2.

0-7803-5160-6/99/ 10.00 1999IEEE. 720

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The control o f the high-power-factor rectifier is based onthe average-current-mode using the dedicated integratedcircuit U C3854.

I; UC3854II

VOLTAGEII I. -_I______-______---________________iFig. 2. High-Power-Factor Rectifier.

With this technique it is possible to implement a verysimple control circuit for unitary power-factor in CCMoperation and also to provide over-current protection.The SEPIC converter does not need limitation of inrushcurrent, because the low value of capacitor C1 between thesource and the cap acitive filter CO, imits this current.The main current and voltage waveforms of the SEPICconverter in CCM are presented in Fig. 3 and Fig. 4respectively.

II tD T TFig.3. Main current waveforms.

t-vo - - - - -

-Vin+VoFig.4.Main voltage waveforms.

B. High-Frequency InverterA half-bridge high-frequency inverter with zero-voltage-switching and constant duty-ratio forms the second stage sshown in Fig. 5 Each fluorescent lamp is connected to a

small high-frequency resonant filter, shown in Fig. 7. Theseries capacitor (Cs) of the resonant filter blocks the DCcomponent of the output voltage.

VD

FigS. High-frequency inverter.The main inve rter waveforms are prese nted in Fig. 6.

II I\ / - s1vs 1

, i D 1 , v ,I II s2vs2

iD2Fig.6 . Main inverter waveforms.

The half-bridge inverter operates with a switchingfrequency above of the resonant filter frequency; thus theoutput current is lagged in relation to the output voltage,allowin g zero-voltage-switching.C. High-Frequency Resonant Filters

The fluorescent lamp can be considered as an open circuitbefore its ignition. After reac hing the ignition voltage and thelamp ignition, the lamp beha ves as a nega tive resistance.The high-fre quency LCC reso nant filter show n in Fig. 7has the function of to reach the ignition voltage and to allow astable lamp operation at stea dy state.s Lr

Fig.7. High-frequency resonant filter.The resonant frequency before the lamp ignition is:

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1FO =cs+ c p

With the inverter switching frequency equal to the resonantfrequency, the voltage across the lamp increases until theignition voltage to be reached. However, if a problem occursin the lamp ignition, the voltage and current in the resonantcircuit can reach dangerous values. With an imposedswitching frequency lightly different o f resonant frequency, itis possible to control the maximum values of voltage andcurrent in the resonant circuit, allowing a safe operation sshown in (2) and (3) [2].

iLr t ) = 2 . c o s o r - t ) - c o s o s . t ) ) 2).vDc os

L r . o s 2 - o r 2 ).S VDCV C p t ) = L r . w r 2 - o s 2 ~ s . c e q 3)

c s . c pc s - pWhere: wr = 2.z.Fo s = 2.n. Ceq =

ig. 8 shows the lamp voltage and resonant inductorcurrent during the starting process, considering a switchingfrequency f )different of the resonant frequency (Fo).

Fig.8. Lamp voltage and resonant current in the lamp ignition.After the ignition, the resonant circuit limits the current atnominal lamp value. A t steady state, the resonant frequency islower than the switching frequency, and the resonant filterbecomes a current source, providing soft commutation in theinverter.If the lamp presents the rectification effect, where only onefilament can emit thermal electrons, the block capacitorvoltage (VCS) reaches the inverters DC inpu t voltage (VDC)and the resonance ceases. Using a resonant filter for eachlamp, a failure in a single lamp would not affect the normaloperation of the o thers lamps. Therefore, in case of failure ina lamp, the resonant filter provides safe operation of thesystem.The filament preheating can be obtained using a PTC(Positive Temperature Co efficient) thermistor in parallel withthe capacitor Cp. The PTC resistance and the lamp voltageare low until the PTC to reach its reference temperature.When this temperature is reached, the PT C resistance is high

and its influence in the resonant circuit becomes null, thenthe lamp ignition voltage is reached.The resonant filter can be considered as a high-frequencyballast, and different lamp power and d ifferent resonant filterconnected with the inverter may be used.D. ighting ControlThe lighting control is accomplished by the variation ofthe inverter switching frequency. With the increase of theswitching frequency, the resonant filter gain changes,dimming all the lamps at the sam e time. Another effect of thehigher switching frequency is that the resonant filter becamemore inductive, so ZVS commutation is obtained even withlow power in the lamps.

111. SIMPLIFIEDESIGN XAMPLEThe input data for a design example of lightingInput AC voltage:DC Bus voltage:Inverter output power:Lamp Power: P L = 6 0 WIgnition voltage:Switching frequency:

A. High-Power-Factor RectiJier1 Lin and Lm inductors: The inductor Lin is calculatedby the maximum input current ripple. Limiting the currentripple to 17 of input peak current, the inductance is equalto:

system are:VAC = 220 VVDC = 300 VPo = 1200 W

Rh4S lamp vol tage : V L= l l 0 VVig = 1100 Vf = 30 kHz

Lin = 0.4.- vAcpk = 0 . 4 . 220*J1 = 2.85mH ( 5 )U f 1 . 4 5 7 . 3 0 - 1 0 3The same value is adopted fo r Lm.

2) Output capacitor CO: The output capacitor iscalculated by the maximum output voltage ripple. Limitingthe voltage ripple to 3 of the nominal value, the outputcapacitance is equal to:( 6 )= 590pF200-oC O = 2 ne 120 VDC AVO - 2 .n. 20.300 9

CO = 680pF3) Capacitor CI: The capacitor C1 is calculated by:

= 2.2p 7)1

2 .n 2 103y .2.85mH-2 .II. . 103y .Lml =B. Resonant FilterI Series Capacitor C s: The series capacitor is defined by:

PL 60VL I10IL = 0.5454A

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15.0.5454 = 290nF- 9)1 5 . Lc s = F)2 . .f 1 5 0 -2 X . 3 0 . IO3Cs = 330 nF/4OOV

2) Parallel Capacitor Cp: Considering a resonant currentin the start five times the lamp current and an ignition voltageof 1100V, the equivalent resonant filter capacitance will be:10)13.15nF

. I L 5 0.5454w = . r =x f Vzg ~ 2 ~ ~ ~ 3 0 ~ 1 0 ~ ~ 1 1 0 0 0The capacitor Cp is calculated by:

CeqaCs 14 .47 .10-9 .330 .10-9 = 13.7nFs- C e q 3 3 0 . 0-9- 14.47. I 0-9

Cp=1 3 nF/1.6kV1 1p = - - -

3) Resonant inductor Lr: The resonant inductor iscalculate d by:

5.0.5454 - 2 .R . 30-IO3 . 3.5.10-9.30013)r= 5.0 .5454. (2 . n.30 10 )2.13.5.

IV. EXPERIMENTALESULTSTo verify the practical aspects of the proposed lightingsystem, a laboratory prototype was built and tested, followingthe simp lified design example.Fig. 9 shows the complete circuit of the high-power-factorrectifier. The power sw itch is a high-speed IGBT. The controland command is implemented with the UC3854, and an

auxiliary source supplies this circuit.

A non-dissipative snubber is used between the outputdiode and the switch S1, minimizing the turn-on losses, andimproving the efficiency in 2 .Fig. 10 shows the complete circuit of the high-frequencyinverter. In this stage two h alf-bridge inverters connected inparallel with the DC Bus voltage are used. The two half-bridge inverters have the same command circuit. Half of thelamps are connected in each half-bridge.An over current circuit is used to protection of theinverters. If a short-circuit in the inverter output to occur, thecurrent protection will be operate and all the lamps will beturned off.The lighting control is accomplished changing manuallythe resistance of SG3525 oscillator.The MOSFET body diodes were used for anti-paralleldiodes.N d I p . I b c Snubbcr_a IJOaolW

sI IRPc4HPsow

_ ~ _ _ _ - _ _ - - - _ _ - -Fig.9.High-power-factor ectifier.

6

LPc3s25 33' 4.11; II

Fig.10. High-frequency inverter.

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Fig.] 1. Input voltage and current at full load.(lOOV/div - 5Ndiv - 2ms/div)Fig12 presents the same waveforms at minimal load(30 ).

Fig. 12. Input voltage and current at minimal load (30 )The SEPIC turn-on com mutatio n is presented in Fig. 13,

1 OOV/div - 2Ndiv 2mddiv).This figure shows the low turn-on losses.

T e k Stop: 250MS/s 214 A W St :>

a

Ch4 5 0 0 A C kFig. 13. S1 Turn-on com mutation 1 OOV/div - 5Ndiv - 2OOnddiv).

Fig.13 shows the SEPICturn-off commutation. Theefficiency of the high-power-factor rectifier at fil l loa d is94 .TBK Stoo: 25OMS/s 35 AUYS

Fig.14. SI Turn-off commu tation (IOOV/div - SNdiv OOnddiv).Fig 15shows the ZVS comm utation in the half-bridgeinverter at full load.

Tek Stop: 100MS/s 316 AC9S 1

c2 F r e q32 80S2kHZ

P

Fig.15. Half-bridge commutation ( MI ) at full load(lOOV/div - 2 N d i v - 5 p div).The ZVS commutation in the half-bridge inverter atminimal load (30 ) is shown in Fig. 16.

c 2 Freq4 2 8984kHZ

Fig.16.Half-bridge commutation (M I) a t minimal load(IOOV/div - 2Ndiv - 5 p div).Fig 17 shows the output voltage and output current in onehalf-bridge inverter at full load.

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Tek Stop: 100MS/5 26 AC45

CZ Freq32 8QQZkHZ

connected in series, so the lamp power does not change withthe cable length. A 60 meters cable was tested and the lamppower was the same obtained with the resonant filter close tothe inverter.Fig 19 presents the lamp startup. The resonant filterimplemented does not use a filament preheating device.

Tek Stop: SOOkS/s S ACQS I

Fig.16. Output voltage and output currentin the Half-bridge at nominal loadIOOV/div - SA/div Sp div).The lamp voltage and resonant inductor current at fullload are presented in Fig 17.

Fig.17. Lamp voltage and resonant inductor current at full loadIOOV/div-O.SA/div - IOp/div).The lamp voltage and resonant inductor current atminimal load are presented in Fig 18.

Fig.18. Lamp voltage and resonant inductor current at minimal loadlOOV/div O.SA/div - Sp div).The cable connecting the resonant filter to the inverter hasa small indu ctance in relation to the resonant inductance (Lr)

Fig.19. Lamp voltage and lamp filament current in the startuplOOV/div- 2Ndiv - 20 0p div).Fig. 20 shows the current in the resonant filter using twolow resistances instead of the lam p filaments, in order to testa fail in the start of the lamp. In Fig. 20 it can be observedthat the resonant current is limited to a constant value,protecting the components of the resonant circuit. Thedifferences between the w aveforms presented in Fig. 20 andthe theoretical waveforms presented in Fig. 8occur due to thecables and filaments resistance, which result in a resonant

filter with low quality factor.The lamp power variation as function of the switchingfrequency is presented in Fig. 21. Fig. 22 shows the variationof the RMS resonant inductor current and the RMS lampfilament current as hncti on of the switching frequency.Tek Stop: 25 OMWS 155 ACqS

Fig.20. Cp capacitor voltage and resonant inductor current, using two lowvalue resistors in place of the lamp filamentslOOOV/div-2A/div-lOps /div).

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