Alternating Current Light Emitting Diode LightingTechnology with Improved Light Flicker
Dong-Won Lee1, Ho-Myoung An2, and Byungcheul Kim3�∗1ACL Co., Ltd., Seoul 153-031, South Korea
2Department of Electronics, Osan University, Osan 447-749, South Korea3Department of Electronic Engineering, Gyeongnam National University of Science and Technology,
Jinju 660-758, South Korea
In this study, we attempted to improve light flickering in an alternating current light emitting diodelight. Percent flicker in an incandescent lamp is 6–14% and that in a magnetically ballasted fluores-cent lamp is 25–40%. The percent flicker of an incandescent lamp can be achieved in an alternatingcurrent light emitting diode light by emitting light before 40 degree in each voltage phase of three-phase alternating current power. However, this percent flicker cannot be realized by placing thelight emitting diode lamp in two phases of the three-phase alternating current power. Percent flickerof a magnetically ballasted fluorescent lamp can be realized if light is emitted before 10 degreein the voltage phase for each phase by placing the light emitting diode lamp in two phases of thethree-phase alternating current power.
Keywords: Alternating Current Light Emitting Diode Lamp, Percent Flicker, Light Flicker,Three-Phase Alternating Current Power, Control of Voltage Phase.
1. INTRODUCTIONThere were many studies on some potential health con-sequences of flickering in light emitting diode (LED)lights.1–10 Flicker is defined as a rapid and repeated changein the brightness of light. A study has indicated that flickerat frequencies above 5.4 KHz in lighting applications mayhave negligible health effects on humans.11 Because ofthe possible health effects, the U.S. Department of Energyis reviewing light flicker in LED lights. Percent flickeris calculated using the instantaneous maximum and mini-mum values and is used as a metric for quantifying lightflicker.12 Percent flicker of traditional light lamps is lessthan 40%;13 however, alternating current (AC) LED lampsshow a percent flicker of 100%.14 An LED does not emitlight at voltages lower than the threshold voltage of theLED because the current does not flow, and an AC LEDlamp emits maximum light with instantaneous AC maxi-mum voltage. Therefore, AC LED lamps have a problem:the brightness of the light fluctuates depending on the time.The study undertaken by Alliance for Solid-State Illu-mination Systems and Technologies (ASSIST) providedan equation for the maximum percent flicker value for agiven frequency that would produce stroboscopic effectsno greater than that from a 60 W incandescent lamp.
∗Author to whom correspondence should be addressed.
According to this equation, the maximum percent flickerfor a lamp operating at 50 Hz AC with 100 Hz flickeris 10% and for that working on 60 Hz AC with 120 Hzflicker is 14%.14 Thus, there is a need to reduce the per-cent flicker of an LED light from 100% to 6–14%, whichis the same as that for incandescent lamps. In this paper,we explain an approach to reduce the percent flicker byreducing the instantaneous difference between the maxi-mum and minimum brightness of the light emitted by anAC LED light.
2. THE LUMINOUS FLUX MODEL FORCALCULATING PERCENT FLICKER
A circuit diagram of a single phase AC LED Lamp isshown in Figure 1, which gives power factor higher than0.95 in almost implementations.An AC LED lamp requires an AC power source, a
rectifier circuit that converts the AC voltage supplied tothe rectified voltage (Vrect), and LEDs. LEDs may alsoinclude several LED blocks. We have used 3 LED blocks(Fig. 1). A parallel switch block and a current-limiting ele-ment (CS2) are also included in an AC LED lamp. Parallelswitches control the number of lit LED blocks by chang-ing the path of the load current according to instantaneousvoltage. That is, the switches control turn-on phase of the
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Lee et al. Alternating Current Light Emitting Diode Lighting Technology with Improved Light Flicker
Fig. 1. Circuit diagram of a single phase AC LED lamp.
voltage applied to a reconfigurable series LED blocks. Theload current is controlled by the current-limiting element.
Figure 2 shows a luminous flux (L) of the LED lampin Figure 1, when the load is composed of 4 LED blocksand the current-limiting element controls the load currentby a sinusoidal wave.
Rectified current is considered as an instantaneous lumi-nous flux because the current flowing through an LED isproportional to the amount of light emitted by the LED.The instantaneous luminous flux is normalized to the lumi-nous flux at a voltage phase of 90 degree. The luminousflux increases from 0 to 8 as increasing the voltage phaseafter 1 LED block is turned on, from 17 to 28 after 2LED blocks are turned on, from 43 to 61 after 3 LEDblocks are turned on, and from 83 to 100 after 4 LEDblocks are turned on. We observed that the total luminousflux increases rapidly when a lighting block is turned onadditionally. At the moment, the current flowing throughthe lighting block increases only a little bit because thecurrent-limiting element controls the load current by asinusoidal wave. The luminous flux doubles from 8 to 17when 2 LED blocks are turned on; increases from 28 to43, about 3/2 times, when 3 LED blocks are turned on;and increases from 61 to 83, about 4/3 times, when 4 LEDblocks are turned on. Therefore, it is confirmed that theluminous flux of the LED lights is well established the-oretically. The percent flicker is still 100% even thoughthe luminous flux is approximated to a triangular wave tocalculate it simply and conveniently.
Fig. 2. Luminous flux of an LED approximated to a triangular wave.
Fig. 3. LED lamps (ACLL) placed on each voltage phase of three-phaseAC power source.
Percent flicker is calculated simply by assuming the rec-tified current flowing through AC LED lights to a trian-gular wave. The value of percent flicker is not affectedbecause the maximum and minimum values of the recti-fied current do not change even if the rectified current isassumed to the triangular wave as a sinusoidal wave or astaircase wave-like sinusoidal wave for resistive load.
3. RESULTS AND DISCUSSION3.1. LED Lamps Placed on Each Phase in
Three-Phase AC PowerFigure 3 shows a circuit diagram of three-phase AC LEDLamp. As shown in Figure 3, the single phase AC LEDlamp (ACLL) which is shown in Figure 1 is placed oneach voltage phase of three-phase AC power source.The voltage waveforms of the rectified three-phase AC
power are shown in Figure 4.The first phase of rectified voltage (R1), which starts
from 0 degree in the voltage phase, shows maximuminstantaneous rectified voltage in 90 degree of the voltagephase. The second phase of rectified voltage (R2), whichstarts from 120 degree in the voltage phase, shows max-imum instantaneous rectified voltage in 30 degree of thevoltage phase.The third phase of rectified voltage (R3), which starts
from 240 degree in the voltage phase, shows maximuminstantaneous rectified voltage in 150 degree of the voltage
Fig. 4. Voltage waveforms of the rectified three-phase AC power.
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Alternating Current Light Emitting Diode Lighting Technology with Improved Light Flicker Lee et al.
Fig. 5. Luminous flux of each phase in the three-phase AC power whenluminous flux is emitted at the voltage phase of 60 degree.
phase. In other words, the instantaneous maximum recti-fied voltage of each phase is shown in the voltage phasesof 30 degree, 90 degree, and 150 degree.Figure 5 shows the light emissions of LED lamps placed
on each phase of the three-phase AC power when luminousflux is emitted at the voltage phase of 60 degree for eachphase.The first phase of rectified voltage (R1), which starts
from 0 degree in the voltage phase, shows maximuminstantaneous rectified voltage in 90 degree of the volt-age phase in Figure 4. Light emission, however, is zeroin 0 degree to 60 degree of the voltage phase, and beginsafter 60 degree of the voltage phase. It increases linearlyand shows a maximum value in 90 degree of the voltagephase, the light emission decreases linearly after 90 degreeof the voltage phase and stays at zero until the next lightemission waveform occurs for the first phase luminous flux(L1) in Figure 5. To summarize, the triangular waveformmodel explaining the first phase of the luminous flux (L1)indicates that light emission started in a delayed phase ofmore than 60 degree than in the phase of rectified voltage.Moreover, light emission was maximal in the maximuminstantaneous voltage phase and it stopped in the advancedphase of more than 60 degree than in the phase of recti-fied voltage. The second and third phases of the luminousflux, L2 and L3 respectively, are based on the same prin-ciple as L1. Percent flicker is calculated to 100% of thetotal waveform by adding the values of all the phases ofluminous flux L1, L2, and L3. Therefore, we conclude that
Fig. 6. Luminous flux of each phase in three-phase AC power whenlight emission begins at the voltage phase of 45 degree.
Fig. 7. Total waveform added up to the light emission in each phase inthe three-phase AC power.
light emission should be started before 60 degree of thevoltage phase for each phase in order to improve percentflicker.Figure 6 shows a triangular wave model with 100% of
percent flicker when light emission is started at 45 degreeof the voltage phase for each phase in three-phase ACpower.Figure 6 shows all the three phases of the luminous flux,
L1, L2, and L3, for the LED lamp. The total waveform(LT ) added up to the light emission in each phase in thethree-phase AC power is shown in Figure 7.The calculated percent flicker of waveform LT is 20%.
LA is the average light emission level over a cycle of thevoltage phase. Therefore, an improved percent flicker of20% can be achieved in three-phase AC LED lighting witha 100% percent flicker LED lighting lamp in single-phaseAC power lighting.Percent flicker calculated at different voltage phases are
shown in Table I.Ang3 is the voltage phase in which light emission
started. Percent flicker is 20%, 11.1%, and 4.8% for 45degree, 40 degree, and 35 degree in the starting phaseof light emission. Percent flicker is especially zero at 30degree in the starting phase of light emission. As the volt-age phase decreases, the percent flicker increases in thevoltage phase lower than 30 degree in the starting phaseof light emission. Therefore, it is preferable to start light
Table I. Percent flicker at voltage phases of luminous flux emittingangle.
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Lee et al. Alternating Current Light Emitting Diode Lighting Technology with Improved Light Flicker
Fig. 8. Luminous flux of the first and second phases in the three-phaseAC power when luminous flux begins to emit in the voltage phase of 30degree
emission at a voltage phase of 40 degree or less consid-ering that percent flicker of an incandescent lamp is from6% to 14%. It is also preferable to start light emissionat the voltage phase of 45 degree or less considering thatthe percent flicker of a magnetically ballasted fluorescentlamp is from 25% to 40%.
The starting phase of light emission is determinedaccording to the number of the first LED block that isturned on as the voltage phase is increased. That is, as thethreshold voltage of the first lit up-LED block decreases,the light emission starting phase becomes faster.
3.2. LED Lamps Placed in Two Phases in theThree-Phase AC Power
Figure 8 shows luminous flux when light emission startedin the voltage phase of 30 degree in the LED lamp placedin 2 phases in the three-phase AC power.
The first phase of rectified voltage (R1), which startedfrom 0 degree in the voltage phase, shows maximum volt-age at 90 degree of the voltage phase in Figure 4. InFigure 8, the luminous flux (L1) shows 0 for 0∼30 degreeof the voltage phase. Light emission started after 30 degreeof the voltage phase, and is increased linearly to get themaximum value at 90 degree of the voltage phase for thefirst phase rectified voltage. Light emission decreases lin-early after 90 degree in the voltage phase and it is atzero until the next cycle of the rectified voltage begins.
Fig. 9. Waveform adding up the luminous flux of the first and secondphases in the three-phase AC power.
Fig. 10. Luminous flux of the first and second phases in the three-phaseAC power when luminous flux begins to emit in the voltage phase of 10degree.
To summarize, the triangular waveform model explainingthe first phase of the luminous flux (L1) indicates thatlight emission started in a delayed phase of more than 30degree than in the phase of rectified voltage. Moreover,light emission maximized in the maximum instantaneousvoltage phase and it stopped in the advanced phase of morethan 30 degree than in the phase of rectified voltage. Thesecond phase of the luminous flux (L2) is based on thesame principle as that of L1.The waveform added up the light emission of two
phases in the three-phase AC power is shown in Figure 9.The added waveform of the L1 and L2 phases is indi-
cated by LT . Percent flicker of LT waveform is calculatedto 100%. LA shows the average light emission between 0degree and 180 degree in the voltage phase. Therefore, itis concluded that light emission should be started before30 degree in the voltage phase for each phase in order toimprove percent flicker.Figure 10 shows a triangular model indicating that light
emission started at 10 degree in the voltage phase for eachphase.
Fig. 11. Waveform adding up the luminous flux of the first phase andthe second phase in the three-phase AC power.
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Table II. Percent flicker at different voltage phases of luminous fluxemitting angles.
Percent flicker is still 100%. In Figure 10, L1 is theluminous flux in the first phase of the LED lamp and L2is the luminous flux of the second phase of the LED lamp.Figure 11 shows the waveform adding up the light emis-
sion of two phases in the three-phase AC power.Percent flicker of the LT waveform is calculated to
42.9%. LA shows the average light emission between 0degree and 180 degree in the voltage phase.Therefore, an improved percent flicker of 42.9% can be
achieved in two-phase AC LED lighting with a 100% per-cent flicker LED lighting lamp in single-phase AC powerlighting. Percent flicker values calculated at different volt-age phases are given in Table II.Ang2 is the voltage phase in which light emission
started. Percent flicker is 60.0%, 50.0%, 42.9%, and 37.5%for 20 degree, 15 degree, 10 degree, and 5 degree in thestarting phase of light emission. It is preferable to startlight emission at a voltage phase of 10 degree or less con-sidering that percent flicker of a magnetically ballastedfluorescent lamp is from 25% to 40%. It is, however, con-firmed that percent flicker of the same level as that in anincandescent lamp cannot be realized by placing an LEDlamp in two phases of a three-phase AC power.
4. CONCLUSIONAn LED does not emit light at a voltage that is less thanits threshold voltage because current does not flow, andan AC LED lamp emits maximum light in instantaneousAC maximum voltage. Therefore, in an AC LED light, thebrightness of the light is uneven and fluctuating dependingon the time. In this paper, an approach to reduce the instan-taneous difference between the maximum and minimumbrightness levels of the light emitted by an AC LED lightlamp was discussed.
Rectified current is supposed to be the same as theextent of instantaneous light emission because currentflowing through an LED lamp is proportional to the emis-sion of light. Percent flicker is easily calculated by assum-ing light emission to a triangular waveform.Percent flicker is 100% in single-phase AC LED light-
ing. Percent flicker of the same level as that in an incan-descent lamp (6%–14%) is realized if light emission startsbefore 40 degree in the voltage phase for each phase, andpercent flicker of the same level as that in a magneticallyballasted fluorescent lamp (25%–40%) is realized if lightemission starts before 45 degree in the voltage phase foreach phase by placing the LED lamp in each phase ofthe three-phase AC power. Moreover, percent flicker of thelevel of a magnetically ballasted fluorescent lamp can berealized. However, the percent flicker of the same level asthat of an incandescent lamp cannot be realized even iflight emission starts before 10 degree in the voltage phasefor each phase by placing the LED lamp in two phases ofthe three-phase AC power.
Acknowledgment: This work was supported byGyeongnam National University of Science and Technol-ogy Grant 2014.
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Received: 4 February 2015. Accepted: 2 April 2015.
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