Light Emission of Electroluminescent Lamps Under Different Operating Conditions

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 5, SEPTEMBER/OCTOBER 2013 2361

Light Emission of Electroluminescent LampsUnder Different Operating Conditions

Walter Kaiser, Member, IEEE, Ricardo Paulino Marques, and Alexander Fernández Correa

Abstract—Electroluminescent (EL) lamps are flat solid-state acdriven devices in which light is generated by impact excitation tooptical energies of luminescent centers in a Mn-, Cu-, or Tb-dopedZnS phosphor layer by high-energy electrons and radiant decay tothe ground state of these luminescent centers. They are useful inapplications which require diffuse low-level uniform illumination.This paper presents measurements of light emission and relatedspectra of EL lamps as a function of operation frequency andsinusoidal voltage amplitude. The excitation of other voltage wave-forms is also analyzed, and parameters of the equivalent electricalcircuit are presented.

Index Terms—Electroluminescence, electroluminescent (EL)devices, luminescence.

I. INTRODUCTION

E LECTROLUMINESCENCE is a nonthermal generationof light from luminescent materials by the application of

an electric voltage.With respect to the voltage applied, we can distinguish two

classes of electroluminescent (EL) devices [1]: low-field elec-troluminescent devices like LEDs where light is generated byelectron–hole recombination when a dc voltage of a few voltsis applied to a p-n junction and high-field electroluminescentdevices in which light is generated by the impact excitationof a light emitting center (luminescent center) by high-energyelectrons accelerated through an external electric field, whichis also known as the Destriau effect. Prof. Destriau, working in1936 in the laboratories of Madame Curie in Paris, published areport on the emission of light from zinc sulfide (ZnS) powderswhen excited by an electric field [2].

Based on this high-field electroluminescent phenomenon, ELdevices are categorized according to the phosphor configura-tion and drive voltage in the following: ac or dc thin-filmEL and ac or dc powder EL devices [1], [5]. Thin-film-ac-electroluminescent devices are used as monochrome andcolor displays [3]. They are particularly useful in applications

Manuscript received November 6, 2012; accepted January 2, 2013. Date ofpublication April 26, 2013; date of current version September 16, 2013. Paper2013-ILDC-005, presented at the 2012 IEEE Industry Applications SocietyAnnual Meeting, Las Vegas, NV, USA, October 7–11, and approved for pub-lication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by theIndustrial Lighting and Display Committee of the IEEE Industry ApplicationsSociety.

W. Kaiser and R. P. Marques are with the Universidade de São Paulo, SãoPaulo 05508-030, Brazil (e-mail: [email protected]; [email protected]).

A. F. Correa is with the Instituto Superior Politécnico José AntonioEcheverría, Havana, Cuba (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIA.2013.2260315

where full color is not required but where ruggedness, speed,brightness, high contrast, and a wide angle of vision areneeded.

The focus of this paper is the plastic ac powder EL devices,known as EL lamps. The light emitted is essentially uniformand of low level across the surface area (“lit area”) of itsoptically transparent electrode.

EL lamps are special devices designed for specific appli-cations. They have very low luminous efficiency (typically2.5 lm/W), but the advantage that they offer over traditionallighting sources is their ability to provide even illuminationover a flat area. They are the preferred lamp in relativelylow-intensity situations where it is important to obtain evenglow over an entire area. Moreover, EL lamps provide uniformlight emission independent of the viewing angle, and they donot need a light guide as LEDs to achieve light uniformity.They are lightweight (0.05 g/cm2) and shockproof, have lowvolume, can be fabricated on flexible substrates, and have alow power consumption (15 mW/cm2) [7]. They are a goodchoice for applications as backlight for small liquid crys-tal displays and keypads, wristwatches, luminous switch sur-rounds, avionics instrumentation, automotive dashboards, andthe like.

Keypads of portable devices require a means of backlightingthat permits the user to identify the individual keys. The thinform factor and flexibility allow the EL lamp to be placedjust underneath the keypad, and the uniform light providesthe keypad with a more pleasing appearance. Also, EL lampscan be easily cut to fit any frame, even a very irregular one.EL lamps can be compared with organic light emitting diodes(OLEDs), but the latter are more expensive.

The EL devices are usually driven by sinusoidal voltages ofmore than 100 V peak. Frequencies of several hundreds of hertzto few kilohertz are generally recommended.

This paper analyzes EL lamp emission with respect tovoltage waveform, amplitude, and frequency, shows spectralalterations as a function of frequency and voltage, and suggestsa procedure to obtain parameters of the equivalent electricalcircuit for inverter design.

This paper is organized as follows. Section II describesin more detail the structure of the EL lamp. Section III de-scribes the physics behind the light emission. Aging effectsare presented in Section IV. Electrical modeling and drivingconditions are discussed in Section V. Experimental results oflight emission at different driving conditions are presented anddiscussed in Section VI. Final considerations are presented inSection VII.

0093-9994 © 2013 IEEE

2362 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 5, SEPTEMBER/OCTOBER 2013

Fig. 1. AC powder EL lamp structure.

II. EL LAMP STRUCTURE

The EL lamp whose structure is shown in Fig. 1 behaveslike a flat capacitor. The central layer (50–100 μm) consistsof a suitably doped ZnS phosphor dispersed in a binder andis sandwiched between front and rear electrodes. The phosphoremits light when a large enough electric field is applied throughit. The required field level is on the order of 108 V/m.

Because of this high field level, any imperfection in the thin-film stack may produce a short circuit that causes a destructiveamount of energy to be dissipated if the phosphor were directlyin contact to the electrodes. Therefore, current-limiting layers(insulators) are needed to form a reliable device structure. Otherimportant properties of the insulator layer are adhesion, filmstress, chemical stability, high electric breakdown, low dielec-tric loss, and small number of pinholes to prevent moisturepenetration.

The phosphor layer is screen printed over the front electrodeusing a cellulose-derived organic binder. A white powder ofhigh dielectric constant, for instance, barium titanate, also in anorganic binder, is incorporated between the phosphor layer andthe back electrode [5]. At least one of these electrodes shouldbe transparent to permit viewing of the emitted light.

In commercial devices, the transparent front electrode ismade of polyester or polycarbonate material having a thicknessof about 0.3 mm, which receives a 0.5-μm conducting coatingof indium tin oxide (abbreviated ITO) [6]. Typical ITO containsfrom 80% to 95% In2O3 and SnO2. The ITO sheet or surfaceresistance at that thickness is about 60 Ω/sq, which means thata 6 × 6 square will have the same resistance between any twoedges as a 3 × 3 square.

The rear electrode consists of a conductive sheet depositedover a plastic foil or a ceramic backplane. The conductivepart can be an aluminum foil that provides good electricalconductivity and also enhances the luminance by reflectingback toward the viewer most of the light that would otherwisebe lost to the rear of the display.

III. EL EMISSION MECHANISMS

One of the current trends in solid-state physics is findingmeans for efficient light production by passing electrical currentthrough solids. In order to convert electric energy from theapplied voltage into radiation, three steps have to be considered:excitation by the applied field, energy transport to the lumines-cent center, and emission from this center.

Fig. 2. EL voltage (100 V/div) and current (1 A/div) waveforms and timeresolved light (5 mV/div) time scale 2.5 ms/div.

Allen [14] suggests that light emission in high-field ELdevices occurs by impact excitation processes in analogy withgas discharge lamps. This requires a source of electrons or holesand a region in which they can be accelerated by the electricfield. In a solid, the charge carriers can lose energy by excitinglattice vibrations and the emission of phonons. Therefore, ahigh electric field is necessary so that the rate of energy gainfrom the field substantially exceeds the rate of loss to phonons.Since the path length in a solid is small, the luminescent centerconcentrations should be high, and a large impact cross sectionis required [14], [15].

If the external voltage applied to the EL device is below athreshold value Vth, only displacement current flows withoutEL emission.

When the external voltage level exceeds Vth, a dissipativecurrent in phase with the voltage flows through the phosphorlayer. Electrons are ejected from the insulating layer/phosphorlayer interface by high-field-assisted tunneling. These electronsare accelerated and gain enough kinetic energy to excite lumi-nescent centers and the host lattice. The high-energy electrons,also called hot electrons, excite luminescent centers throughimpact excitation mechanism. EL emission occurs when theelectrons in the excited states of luminescent centers makeradiant transitions to the ground state. The hot electrons travelthrough the phosphor layer and are finally trapped at the phos-phor layer/insulator layer interface on the anode side, buildingup a polarization field. When the polarity of the ac voltage isreversed, the same process takes place in the opposite directionin the phosphor layer.

A polarization effect can be visualized when the EL lamp isdriven by a pulse train. The light output at each pulse is stronglyaffected by the polarity of the preceding voltage pulse. Fig. 2shows a pulsed voltage excitation with three positive pulsesfollowed by three negative pulses with a repetition frequencyof 100 Hz. The duty cycle of each pulse sequence is 10%.The first voltage pulse establishes a strong electric field thataccelerates the electrons within the phosphor layer, excitingMn luminescent centers. These electrons accumulate at theinterface between phosphor and dielectric layers, remainingthere for a rather long period of time, even after the electricfield is removed. If the polarity of the next pulse is the same as

KAISER et al.: LIGHT EMISSION OF EL LAMPS UNDER DIFFERENT OPERATING CONDITIONS 2363

Fig. 3. EL emission mechanism.

that of the preceding pulse, the effective electric field across thephosphor layer is lowered by the superposition of the previouscounter polarization, and the light output is reduced. However,if the polarity of the next pulse is inverted, the effective electricfield is enhanced by additive polarization, and the light outputincreases.

In EL lamps, the emission in ZnS begins in the form of apair of small bright spots. With the increase in electric field,the spots elongate, forming comet-shaped emissive regions asshown in Fig. 3 [1].

The explanation of this phenomenon is that ZnS powdersare fired at high temperatures, having a hexagonal structure,and subsequent cooling transforms them into a cubic structure.The copper sulfide forms conductive thin long needle-shapedprecipitates, and heterojunctions are formed between theseCuxS needle precipitates and zinc sulfite powder. These needlesconcentrate the applied electric field at their tips, producing alocal field enhancement.

The electroluminescent phosphor generally comprises ZnS(zinc sulphide) as a phosphor base material, at least one kindof activator selected from copper and manganese, and at leastone kind of coactivator selected from chlorine, bromine, iodine,and aluminum [10]. The emission colors can be controlled byadding different kinds of luminescent centers. A combinationof Cu and Cl (ZnS:Cu, Cl) gives either blue or green emission,depending on the relative amount of Cl. The combination of Cuand Al (ZnS:Cu, Al) gives green, and the addition of Mn to Cuand Cl (ZnS:Cu, Cl, Mn) gives yellow emission [1], [4], [9].

Manganese is present as an impurity in the ZnS lattice,occupying the zinc site. ZnS is an important II–VI semicon-ductor and suitable for use as a host material for a variety ofdopants because of its wide band gap. Luminescent efficiencyenhancement is obtained by doping ZnS nanocrystals with Mn.These are explained by the interaction of the sp electron holeof the host (ZnS) and the 3d electrons of the impurity (Mn)under the condition of the quantum confinement for the spstates [11]. The yellow emission originates from an internaltransition of the Mn+2 ions. The emission band extends fromthe green to the red region, peaking at 580 nm [15]. Differentcolors can be obtained using trivalent lanthanide ions as co-activators [9], [18].

White light can be obtained by a mixture of blue, bluegreen, and orange phosphors [12], [13]. There are also organic-dye-dispersed inorganic EL panels using ZnS phosphor layersdoped by Coumarin6 (an organic fluorescent dye) [19], [20].Very high luminance levels up to 9120 cd/m2 were obtainedin experiments with organic-dye-dispersed inorganic EL panelsdriven at 240 V [19].

IV. LAMP AGING

EL lamps do not fail to emit light, and therefore, theytechnically have no end of life. The degradation of the de-vice appears by the decay of luminance, voltage increase inthe constant-current mode, and growth of nonemissive area(dark spots) [18]. The decrease of luminance is approximatelyexponential, more rapidly early and extremely slow later intime, and is greatly affected by how the phosphors are poweredand by the ambient temperature in which the lamps are beingoperated.

The lifetime-determining factors of EL devices are the qual-ity of the base phosphor and the way that the lamp is manufac-tured. One of the most important factors is the thoroughness ofthe drying process [8]. The peeling of thin films and the forma-tion of very small dielectric breakdown holes are contributorsto EL lamp degradation.

These small breakdown holes with approximately 30 μm indiameter result from local defects in the structure during thefirst stage of the drive annealing process, and their number doesnot increase with time [1].

Like other electrical capacitors, the presence of moisture inany EL device causes early breakdown during use. Peeling iscaused by moisture in the phosphor layer/insulating interface.During EL drive operation, water ionization is caused by hot-electron impact in the phosphor layer, generating H+ and OH−

ions. The H + ions will combine at the interface to ion gas,and the gas pressure lifts the insulating layer from the phosphorlayer, leading to peeling [1]. EL devices carefully manufacturedto avoid initial problems due to the local breakdown of thedielectric layers and protected from contamination of watervapor are reported to have very long life (> 2000 h) [6]. Thelifetime is defined as the time required for the luminance todecrease to half its initial value.

Even in the absence of humidity, the phosphor itself grad-ually loses its efficiency over time, and the lamp brightnessgradually decreases [7], [22]. Fischer [15], [16] proposed thatthe tips of the copper sulfide imperfection lines, which causethe local field, can be blunted by the drift of copper ions orby the attraction of copper ions from the adjacent host crystal,and EL brightness decreases as the result of local field reduc-tion. Sato et al. [21] suggest that sulfur vacancy and crystalquality are major factors of the deterioration and that deterio-rated EL cell brightness could be recovered by adequate heattreatment.

Another interesting phenomenon in EL devices is a posi-tive shift of the luminance–voltage characteristic curve withoperation. The (L–V) curve shifts to higher voltages withincreasing operation time without changing the curve shape.The L–V curve settles asymptotically to a final curve aftera certain operation time (about 80 h), and this shift is notan indication of degradation but of stabilization due to agingeffect [5].

The useful working life also depends on the voltage andfrequency, and the intensity of emission will decrease rapidlyfrom panels operated at very high voltages and frequencies.A typical life characteristic of ac EL panels is presented in[5] and [8].


Fig. 4. EL lamp voltage and current waveforms and time-resolved lightemission at 1 kHz.

V. EL DRIVING

Due to its structure (two conductive electrodes with a dielec-tric layer between them), the electroluminescent lamp function-ally behaves as a capacitor. The electrical field induced by theapplied voltage thus decreases as charge accumulates on one ofthe electrodes and the light emission is attenuated (see Fig. 4).Therefore, ac voltage must be applied to the electrodes tomaintain continuous illumination. The small capacitance valuesof most EL lamps require a high ac voltage and/or frequency todeliver enough power to produce the desired amount of lightoutput required. Typically, ac voltages of a few hundreds ofvolts in the frequency range of 200 Hz–1 kHz are used to driveEL lamps.

Fig. 4 shows the EL lamp voltage and current waveforms andtime-resolved light emission at 1 kHz. It can be noticed that thelight emission is considerably out of phase with respect to thevoltage.

A. Equivalent Circuit

At a first glance, the extreme nonlinearity associated with thethreshold behavior suggests that a description of the electricalcharacteristics of the EL lamp by impedance seems to beunsuitable since the impedance concept is strictly applicable tolinear systems.

At voltages below the threshold level, the thin-film phosphorbehaves as a capacitor with capacitance CPl. Above the thresh-old level, a dissipative current responsible for light emissionflows through the phosphor layer, and the total device capac-itance shifts to a second capacitance value CDl which is thecapacitance of the dielectric layer taken alone. The luminanceof the EL device is proportional to the power consumed inthis “resistive branch,” and the proportionality constant is theluminous efficiency η (in lumens per watt) which must beobtained experimentally.

Fig. 5. EL lamp equivalent electric model.

Fig. 6. EL lamp equivalent load for inverter design.

It has been verified experimentally [1] that the electric fieldof the phosphor layer is kept constant. As a consequence, theexcess voltage drops on the insulating layers, avoiding thedielectric breakdown of the EL device. Therefore, for volt-ages above the threshold voltage, the clamping action of thephosphor layer is similar to the action of a Zener diode [24].Therefore, a good approximation for an ac equivalent circuitwould be two capacitors connected in series, with one of themshunted by back-to-back Zener diodes as shown in Fig. 5.

For the purpose of inverter design, the simple two-componentcircuit shown in Fig. 6 closely emulates the “impedance” of theEL lamp and can be used as an equivalent load for the inverter.

In our measurements, we used a Glow Hut [34] A5(150 mm × 120 mm) Aqua (blue green) EL panel. Themeasured capacitance value in darkness is Cdark = 122.8 nF,and the capacitance value when illuminated by the fluorescentlamps on the laboratory ceiling is Cilm = 160.4 nF.

However, the capacitance measurement of an EL lamp ona RCL meter can produce erroneous readings because mostimpedance bridges operate at 1 kHz and apply less than 1 Vto the device being tested [8]. The best method to determinethe equivalent circuit parameters is to record EL lamp voltageand current at a fixed frequency when fed by an inverter atnominal conditions. With power removed from the inverter,the EL lamps should be replaced by an adjustable resistor andan adjustable capacitor connected in a parallel configuration.With power applied to the inverter, with the same dc inputvoltage as when it was powering the EL lamp, the resistanceand capacitance should be adjusted to produce exactly thesame output voltage and frequency recorded when the inverterwas connected to the EL lamp. There is only one uniquecombination of resistance and capacitance values which willproduce exactly the same voltage and frequency.

Adopting this procedure, the EL lamp was excited witha sinusoidal voltage of 100-V peak value at 100 Hz. Therecorded lamp voltage and current are shown in Fig. 7 (red line).In the sequence, the lamp was substituted by a capacitor ofC = 124.1 nF in parallel with a resistor of 31 kΩ, and theresultant waveform is shown in Fig. 7 (blue trace).

The lamp and the RC model current waveforms matchperfectly as shown in the zoom view depicted in Fig. 8. The


Fig. 7. Voltage and current for EL lamp and equivalent circuit.

Fig. 8. Zoom view of current for EL lamp and equivalent circuit.

black trace current waveform was obtained with a capacitor of161.2 nF and a resistor of 31 kΩ. The correct value for theequivalent capacitance is the one measured in complete darkconditions.

B. Inverter for EL Lamps

There are several sophisticated EL lamp drivers described inthe literature [24]–[33]. However, low-cost inverters can be im-plemented using a simple “ringing choke” type oscillator. Theinverter and EL lamp capacitance form a resonating oscillator,which allows the voltage and frequency applied to the EL lampto change as the overall impedance of the EL lamp changes.

Thermal characteristic changes in EL lamps are basically off-setting. When the ambient temperature is high, the impedanceof the EL lamp is lower, and more output current is drawn from

the inverter to produce a higher brightness. Since the outputcurrent capacity of the “ringing-choke” inverter is limited, theinverters’ output voltage and frequency will drop, producingan almost constant current at the EL lamp. At low ambienttemperatures, the impedance of the EL lamp is higher, allowingthe inverter output voltage and frequency to increase, producingan almost constant current through the EL lamp.

C. Noise Emission

An EL lamp operating in open air will emanate a tone andharmonics centered on the lamp operating frequency, producingan associated buzz or hum. Specifically, the front electrode andrear electrode vibrate due to the fact that they are alternatelyattracted to each other (when an electric field is present) andrelaxed (when the electric field is not present). Due to this vibra-tion, air is displaced as the volume between the two electrodesis alternately compressed and then returned to normal. Thisvibration occurs at twice the frequency of the ac power sourcesince the compression occurs on both the positive and negativevoltage excursions of the ac input. This noise is particularlyproblematic in applications such as mobile telephones, whichare held close to the user ear, interfering with the operability ofthe device.

There are several noise-cancelling drive schemes [29]–[33].The noisiest waveform is a square wave, and the quietestwaveform has been assumed to be a sine wave. Records of ELlamp sound emissions are presented in [29], and this referencesuggests a special waveform that is quieter than a sine wave.

Another interesting method to substantially cancel this noise[32] is to drive independently two adjacent EL panel regions,each comprising a pair of electrodes. The lamp driver shouldbe configured to alternately charge the electrodes of the first ELpanel region while simultaneously discharging the electrodesof the second EL one. If the electrodes of the first EL panelregion are compressed at a rate which is substantially equal tothe rate at which the electrodes of the second EL panel regionare decompressed, then the noise produced by the air displacedbetween the electrodes is virtually cancelled.

VI. MEASUREMENTS OF EL LAMP LIGHT EMISSION

AT DIFFERENT OPERATING CONDITIONS

In our setup, the luminance measurements were made witha Tektronix J17 photometer, and a calibrated silicon photocellwas used in the time-resolved light emission measurements.All measurements were performed on a Glow Hut [34] A5(150 mm × 120 mm) Aqua (blue green) EL panel. Fig. 9 showspictures of the EL lamp in ON and OFF states.

Initially, the EL lamp was kept operating during 100 h untilthe threshold voltage Vth reached a stable value. All measure-ments were performed with the EL lamp in a dark chamber.

A. Light Emission–AC Voltage Characteristics

The Japanese Society for the Promotion of Science proposedsome standard procedures for the measurement and evaluationof EL device characteristics [1]. The threshold voltage Vth


Fig. 9. EL lamp pictures in OFF and ON states.

Fig. 10. Luminance versus voltage peak value for three different waveforms.

is defined as the voltage corresponding to the luminance of1 cd/m2. The voltage values should be indicated in zero-to-peakvalues (Vp), rather than in rms values, since luminance dependson the peak voltage value. The recommended drive waveformsof EL devices are sinusoidal and square pulse waves of alternatepolarity.

Fig. 10 shows the behavior of luminous emission of theEL lamp when excited at 100 Hz by three different voltagewaveforms. It can be noticed that, for a given luminance, thesquare pulse voltage requires a considerably lower peak valuethan the sinusoidal voltage and also has a lower thresholdvoltage. The threshold voltage does not depend on frequencyand is only slightly dependent on temperature.

The representative brightness–voltage characteristics for sinewave excitation at different frequencies are shown in Fig. 11. Itcan be noticed that the threshold voltage does not depend on fre-quency and is only slightly dependent on temperature [8], [24].As expected, the luminance at a given voltage above the thresh-old value increases with frequency.

Fig. 12 presents the EL lamp rms current–voltage character-istics for sine wave excitation at different frequencies. It can benoticed that, at voltages far above the threshold value, the ELlamp current–voltage characteristics become acceptably linearand the parameters of the EL equivalent inverter load remainfairly constant.

Fig. 11. Luminance versus voltage peak value (sine wave).

Fig. 12. Current (rms value) versus sinusoidal voltage (peak value).

B. Light Emission–Frequency Characteristics

The brightness–frequency characteristics at constant sinu-soidal voltage (peak value) excitation are shown in Fig. 13. Itcan be noticed that higher voltage excitation produces a sharperrise in brightness at a given frequency increment.

Fig. 14 shows the EL lamp temperature–frequency character-istic when excited with constant sinusoidal voltage (126 Vp) and


Fig. 13. Luminance versus frequency at constant voltage (sine wave).

Fig. 14. Temperature versus frequency at constant voltage of 126 Vpeak.

ambient temperature of 28 ◦C. It can be noticed that excitationfrequencies above 5 kHz produce a steep temperature increase.In plastic foil EL lamps, the temperature should be kept below35 ◦C in order to avoid bending.

C. Spectrum–Frequency Characteristics at Constant Voltage

Figs. 15 and 16 show the spectral distributions of the lightemitted of the EL lamp excited with constant sinusoidal voltage(90 Vp) for different frequencies.

Fig. 15 is normalized by the spectral peak value, and Fig. 16is normalized by luminance. Fig. 16 clearly shows that thespectral distribution varies slightly with frequency.

Fig. 15. Spectrum–frequency characteristics at constant voltage (90-Vpeak

sine wave) normalized by the spectrum peak value.

Fig. 16. Spectrum–frequency characteristics at constant voltage (90-Vpeak

sine wave) normalized by luminance.

D. Spectrum–Frequency Characteristics at Constant Voltage

Figs. 17 and 18 show the spectral distributions of the lightemitted from the EL lamp excited with sinusoidal voltages ofdifferent amplitudes and constant frequency (1 kHz).

The spectra in Fig. 17 are normalized by the spectral peakvalue, and the spectra in Fig. 18 are normalized by luminance.It can be noticed from Fig. 18 that the spectral distribution doesnot change with the excitation voltage amplitude.

VII. FINAL REMARKS

This report discussed the physics behind electroluminescentplastic lamps. Although sinusoidal voltage excitation is mostcommon, square pulses are an interesting option since a lowerpeak voltage is required to obtain a given luminance. The lightemission of the EL lamp requires a minimum threshold voltagethat is independent of frequency, although different excitationwaveforms may produce different thresholds; in particular, thethreshold for square wave excitation is much lower. Higher


Fig. 17. Spectrum–voltage characteristics at constant frequency (1 kHz)normalized by the spectrum peak value.

Fig. 18. Spectrum–voltage characteristics at constant frequency (1 kHz)normalized by luminance.

voltage excitation produces a sharper rise in brightness at agiven frequency increment. In plastic foil EL lamps, the tem-perature should be kept below 35 ◦C in order to avoid bending,which limits the operation frequency below 5 kHz. The lightoutput spectrum is insensitive to voltage but varies slightlywith frequency. The current–voltage characteristics becomeacceptably linear at voltages far above the threshold value.A good approximation for an ac equivalent circuit would betwo capacitors connected in series, with one of them shuntedby back-to-back Zener diodes. However, for the purpose ofconverter design, a capacitor in parallel with a resistance closelyemulates the “impedance” of the EL lamp and can be used asan equivalent load for the inverter.

Although OLED use is spreading for similar applications,EL lamps are still a very cost-effective good choice due totheir mechanical robustness and flexibility, light weight andlow volume, uniformity of light emission, and the importantcharacteristic of being easily cut to fit any backplane shape,even a most irregular one.

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Walter Kaiser (M’99) received the B.S., M.S., andPh.D. degrees in electrical engineering from theEscola Politécnica of the University of São Paulo,São Paulo, Brazil, in 1980, 1983, and 1989,respectively.

From 1983 to 1989, he was a Development Engi-neer at the Foundation of Engineering and Technol-ogy of the University of São Paulo and a Surveyor fornaval electrical equipment at the American Bureauof Shipping. He joined the Escola Politécnica ofthe University of São Paulo in 1990, where he is

currently an Associate Professor of Power Electronics. His research focus ison static converters applied to discharge lamps.

Ricardo Paulino Marques received the B.S., M.S.,and Ph.D. degrees in electrical engineering fromthe Escola Politécnica of the University of SãoPaulo, São Paulo, Brazil, in 1991, 1994, and 1997,respectively.

He worked as a Consultant on Industrial processesuntil 1997, when he joined the Escola Politécnicaof the University of São Paulo, where he is cur-rently an Assistant Professor of Control Systems andAutomation. His research interests include controltheory (particularly stochastic processes), modeling,

and optimization.

Alexander Fernández Correa received the B.S. de-gree in electronic engineering from the Universidadde Oriente de Santiago de Cuba, Santiago, Cuba, in1992 and the M.S. and Ph.D. degrees in electricalengineering from the Escola Politécnica of the Uni-versity of São Paulo, São Paulo, Brazil, in 2000 and2004, respectively.

From 1992 to 2005, he was a Research Assistantat the Universidad de Oriente de Santiago de Cuba.He joined the Instituto Superior Politécnico JoséAntonio Echeverría de Havana, Havana, Cuba, in

2005, where he is currently an Instructor/Professor of Electric Circuits. Hisresearch focus is on static converters applied to discharge lamps.

Light Emission of Electroluminescent Lamps Under Different Operating Conditions

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