Abstract. This application note describes powering high current light

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Abstract. This application note describes powering high current light

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AND8137/D High Current Isolated Voltage Drive Application Note
Abstract. This application note describes powering high current light emitting diodes (LEDs) through line isolation transformer transistor current regulator, ensure optimal performance long life. characteristics explained, followed example design illustrate concept. INTRODUCTION Light emitting diodes, called LEDs, have existed many years. LEDs behave similarly normal diodes that they have forward voltage drop associated with forward current. Early LEDs emitted radiation only infrared (IR) spectrum. Later, visible LEDs emerged using various III-V compounds, such aluminum gallium arsenide (AlGaAs). Other colors, such yellow, amber green came shortly thereafter. breakthrough more colors came with blue LED; originally, this silicon carbide. applications these early LEDs were largely limited power displays, because output limited. breakthrough technology opening door wide variety high power illumination applications, which commercially available. This generation utilizes (AlInGaP) substrate emit significantly higher power amber light intensity. Additional colors, such green blue, built Indium-Gallium-Nitrogen (InGaN) substrate soon followed. full color spectrum, including white, possible using proper mixing filtering multiple colors. Today, colors amber, red-orange, typically from AlInGaP substrates, while royal blue, blue, cyan, green white from InGaN substrates. conversion efficiency electrical energy into light energy very important. Today's LEDs vary between percent efficiency. rest energy converted heat. This heat must effectively dissipated, operating junction temperature must maintained between -40°C +125°C. Incandescent lamps, including tungsten-halogen type, have efficiencies only about percent visible light. These emit broad, almost continuous spectrum
Semiconductor Components Industries, LLC, 2003
energy, including only visible light, also ultraviolet (UV) infrared (IR) unusable heat. Technically, only percent incandescent lamp's energy converted directly into heat; surprisingly large amount heat generated them caused radiation being absorbed surrounding area. This heat reflected away from lamp, there lens filter front lamp heat trapped. only practical obtain different colors with incandescent lamps with filter. This case with LEDs. LEDs produce rather narrow spectrum light therefore intrinsically more efficient converting electrical energy particular color than incandescent lamps with filter. There less electrical energy needed same lumen output, filter will attenuate light output substantially. Therefore color LEDs most efficient obtain colored light. White LEDs have same efficiency incandescent lamps, less efficient than fluorescent lamps. white LEDs have particular advantage over most known white light sources; this advantage longer lifetime. Many incandescent lamps rated between hours 2000 hours life. fluorescent lamp including like compact incandescent type offer between 8000 12,000 hours life. these lamps have filaments. greater number `on-off' cycles shorter lamp life filament breakage. White LEDs other hand have filaments thus have this failure mode. LEDs, regardless color, have extremely long lifetime, their current temperature limits exceeded. Lumileds Lighting LLCt published lifetime data stating that after 50,000 hours LEDs will have percent greater original light output. Using engineering rule thumb with data already collected, plotted, semi-log graph paper, LEDs projected have percent greater original light output after 100,000 hours. There 8736 hours normal year 8760 hours during leap year, which equates 8742 hours year. This calculates over years months continuous service with light greater than percent initial output. Remember, order obtain maximum life, LEDs must operated within manufacturer's
October, 2003 Rev.
Publication Order Number: AND8137/D
AND8137/D
specified limits both current diode junction temperature. LEDs should used where extremely long life desired cost lamp replacement very high. Characterization maximum forward current varies with different type, style, manufacturer LEDs. Lumileds specified maximum forward currents differently constructed LEDs. higher current devices have special thermally designed packages transfer heat from junction heat sink. This paper will concentrate circuits using Lumileds devices. same rules apply devices having other current ratings simply scaling current power designs. forward voltage drop varies between 2.50 4.00 rated forward current; Figure This variation material used, AlInGaP InGaN, various manufacturing tolerances. This variation forward voltage drop must taken into account each lamp design. Lumileds sorts their devices according color, intensity, forward voltage drop maximum rated current. device forward voltage characteristics provide better match maximum current than match lower current, Figure
Figure Typical Forward Voltage Different Colors (Courtesy: Lumileds)
Power Watts) product forward voltage multiplied forward current. LEDs rated total wattage calculated taking minimum maximum forward voltage multiplied 0.35 0.350 2.50 0.88 Watts minimum 0.350 4.00 1.40 Watts maximum average, LEDs rated 0.350 (350 mA), considered watt devices. This makes calculation easy first order approximation. Because amount light limited from single LED, multiple LEDs used increase amount light.
LEDs specified their rated current. easy advantageous place LEDs series because LEDs series have same current. Since LEDs current devices, current control system used operate within manufacturer's specifications. LEDs operated parallel. order operate LEDs parallel, devices must matched using forward voltage drop. This matching should occur manufacturer. process keeping proper voltage current through LEDs called ballasting. Ballasting techniques used extensively other lighting applications like fluorescent lamps.
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AND8137/D
Figure Forward Voltage Matching LEDs (Courtesy: Lumileds)
Energy Supply Voltage Variation, Line Power first source considered power line. power line normally varies within five percent stated value. Like other source, variations much greater. line considered vary percent. United States Canada, normal line take values between Vac. There another condition called `brown out' where line voltage drops another percent Vac. `brown out' condition occurs when electrical utility company lowers value voltage generated. This happens under extreme high demand conditions; utility does this keep generating equipment operational within safe operating conditions while still providing some electrical energy customers. Under this condition incandescent lamps operate reduced light output reduced wattage. Most electric motors operate more economical fashion. line voltage variation from normal stated +10/-20% worst-case normal conditions. Many products sold both North America also Europe. Europe there standards: Vac-50 continental mainland Europe Vac-50
which United Kingdom. European Norm (EN) standards Vac-50 test voltage. overcome line voltage issues design switching power supply that operate from high produce constant voltage constant current output. This occurring today with battery chargers computers cellular phones; with only line cord changes. These called universal input, because they operate anywhere through world. Voltage There many applications where voltage considered voltage. following considered voltage applications: Vac, Vac, Vac. voltage obtained from through step-down isolation transformer. Isolation often required outdoor applications. Transistor Constant Current Design easy approach achieve constant current transistors configuration shown Figure
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AND8137/D
flicker does become issue, capacitor placed after bridge rectifier smooth waveform. other hand, capacitor will cost cause more power dissipated LEDs current regulator. this circuit, following items must considered: line variation. Voltage drop across bridge rectifiers. Electrolytic capacitor selection, used. Effective load resistance. Electrolytic capacitor ripple current, applicable. ripple current rating found capacitor manufacturer's data sheets. This topology rectification inside lamp module, eliminating need polarity protection. addition, rectification very economical, using cost effective, axial leaded 1N4004, surface mount MRA4004, diodes lieu single larger bridge rectifier like MDA2504, which must heat sink. Each item discussed follows: Line Variation: normal line fluctuate percent +10/-20 percent worse case conditions. Therefore, transformer secondary vary between 10.8 13.2 normal secondary voltage 12.0 Vac. There many legacy systems transformers where output voltage 12.6 Vac; this benefit. transformer secondary 12.6 lower limit 12.6 11.34 upper limit 12.6 13.86 under normal percent variations. Bridge Rectifier Voltage Drop: typical forward voltage drop silicon diode considered between volts. many cases, maximum forward voltage drop high Electrolytic Capacitor Selection: determined that capacitor necessary, should chosen value farads, working voltage, temperature operation. working voltage lowest standard value above maximum peak rectified line voltage. 12.6 transformer high line, minimum working voltage 13.86 19.6 Vdc. standard capacitor voltage this system electrolytic. There maximum temperature ratings: 85°C 105°C. 105°C devices have longer life higher ripple current ratings. value capacitor farads determined using equation developed Savant [3].
VMAX
(eq.
LED1
LED2
D44H11 MPS2222
Figure Transistor Constant Current Regulator
Referring Figure source that filtered. sets level which current will limited. When current through LEDs, develops voltage across that reaches approximately begins turn turns starts steal base drive away from This turn will cause conduct less current. Conversely, voltage across decreases, will conduct less will provide more base current Consequently will turn harder. this manner, peak current through LEDs will level determined equation:
Iled
(eq.
provides base drive current There must adequate base current supply required collector (LED) current line voltage. Since failure mode LEDs open circuit, zener diodes serve provide conduction path event failure. value zener voltage which above normal forward voltage drop LEDs. This scheme will allow fail permit other LEDs continue operate. Constant Current Supply Driving LEDs from voltage source requires isolation transformer, bridge rectifier, current regulator. Interestingly, bulk capacitor necessarily needed full wave bridge rectifier used. reason this that output full wave bridge circuit haversine waveform effectively twice input frequency. system this becomes waveform. Visible flicker does normally appear above
Value capacitor farad VMAX Peak line voltage Peak-peak capacitor voltage Twice line frequency (120 system) Effective load resistance
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Example System Amber LEDs
resort hotel wants light walkway between parking side entrance with amber colored LEDs. Each light assembly LEDs wired series shown later Figure LEDs type. electrolytic filter capacitor will used. following conditions assumed exist. current will defined derating 10%. maximum forward drop amber LEDs given 3.27 sense resistor voltage described above. line condition Vac. normal transformer output Vac-60 transformer Class which limited There lamp assemblies used project. Design Procedure: First, after algebraically manipulating equation (1), determine value sense resistor Figure
Iled
(eq.
assume current through Irb, will conduct excess approximately calculate value following equation:
(Vpeak 1.4)
(eq.
this case, resistor value calculated Next, assume transistor that drives LEDs, will have current gain, beta, collector current then required base drive current will given equation:
(eq.
This yields required base drive current Therefore must value that will ensure minimum line voltage. ensure some guard band,
Vpeak stepped down peak voltage lowest line voltage. calculate Vpeak, voltage multiplied lowest line voltage Vrms, then stepped down Vpeak would 12.7 first "1.4" term equation comes from bridge diode drops. second "1.4" term from voltage drop from this case, value calculates standard will used. Next, necessary analyze power dissipated various components high line ensure reliable operation. will begin conducting current when rectified voltage (120 haversine) becomes greater than diode drops sense resistor voltage. current will rapidly increase voltage increases until reaches regulated value. During this transition time, saturation. Significant collector-emitter voltage will appear across until regulated current level been achieved comes saturation moves into linear region. this point there will regulated current flowing through while simultaneously having significant collector-emitter voltage resulting power dissipated device. This will start occur when input voltage becomes greater than forward drops, rated current, sense resistor voltage which also point which feedback through obtained control system. this case, with LEDs each 3.27 sense resistor voltage input must exceed 7.24 picture below.
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AND8137/D
Figure Waveforms from Voltage Lumiled Demo Board
highest peak collector-emitter voltage during conduction time will high line Vpeak less forward drop LEDs, regulated current, plus sense resistor voltage calculate time when significant power being dissipated expression full wave rectified waveform used:
Vin(t) Vpeak
(eq.
where Vpeak peak input voltage high line less bridge diode drops (17.26 Solving yields point which will start dissipating power with respect zero point haversine. Assuming symmetry waveform, then multiplied result when subtracted from haversine period (8.33 yields power dissipation time. this example, this time calculates difference between calculated measured times nonlinearity coming saturation observed collector- emitter voltage waveform Figure Since current regulated waveform estimated square wave current given
Icerms Ipeak (ton
(eq.
where Ipeak power dissipation time period haversine (8.33 ms). This calculates collector/LED current peak collector-emitter voltage will input voltage less sense resistor drops. However, collector- emitter voltage will only appear when full conduction, otherwise voltage zero. collector- emitter voltage considered half sinusoid duration, ton, period, voltage then calculated
Vcerms Vpkce (ton
(eq.
where Vpkce peak voltage from collector-emitter Vcerms voltage from collector-emitter this case, voltage 6.03 average power then product voltage current.
Pceav (Vcerms)(Icerms)
(eq.
this example, average power calculates 1.61 heatsink would likely needed. Additionally, collector-emitter voltage rating must greater than maximum input rectified voltage less forward voltage drops. power calculated similar manner.
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AND8137/D
power dissipated sense resistor calculated with well known Irms2R formula. sense resistor this case dissipates 0.16 resistor used. base drive resistor power dissipation calculated same manner. isolation transformer chosen, designed, achieve different goals. First, transformer must provide proper secondary voltage design. This will depend number LEDs application must drive. goal choose transformer with adequate headroom application, much that drives down efficiency. higher output voltage above that which required number LEDs will cause pass transistor, dissipate more power. Secondly, proper rating transformer must observed. Again, like output voltage, enough application, much able supply excessive power event short circuit output. Line isolation transformers generally have significant series resistance their primary winding. reason this that event short circuit output high resistance will drop input voltage that less power delivered secondary. Third, transformer must meet proper safety agency requirements application. This means must provide proper isolation temperature ratings. this application, would need decided many lamp assemblies will driven from single transformer.
Lamp Assembly
Isolation Transformer 1N4004 1N4004
LED1
Rbase
LED2
D44H11 MPS2222
1N4004
1N4004
Rsense
Lamp Assembly
1N4004 1N4004
LED1
Rbase
LED2
D44H11 MPS2222
1N4004
1N4004
Rsense
Figure Multiple Lamp Assemblies Design Example
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AND8137/D
SPICE Simulation Intusoft SPICE simulation created voltage circuit described above. Lumiled models were generated modifying generic diode models simulate proper voltage drop specified current. This accomplished adjusting saturation current, empirical constant, fundamental diode current equation:
Io(evd
(eq.
this equation forward diode drop LED. defined
(eq.
where Boltzmann's constant, absolute temperature, charge electron. Lumiled specification (typical) forward drop amber 2.85 solving diode equation adjusting diode model accordingly, good correlation achieved between model actual circuit seen below.
Figure Intusoft SPICE Simulation Voltage Lumiled Circuit
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AND8137/D
Amber 1N4004 1N4004 Vled
Amber
XFMR
Vce2 Vstepdown 1N4004 1N4004 Iled
Figure SPICE Simulation Schematic Voltage Lumiled Circuit
Jumper JMP2 JMP3 JMP4
Test Point
NOTE: parallel primary windings, insert jumper from Also insert jumper from insert JMP3.
Prov.
LED1
JMP2 JMP3 JMP4 LED2 Collector Voltage JMP1 Test Loop Measuring Current Holes Gauge Wire D44H11 w/Heatsink 1N4004 MPS2222 2.2, 1N4004 1N4004
Conn1
LED3
Tamura 3FD-424
1N4004
Test Point Ground
Figure Schematic Voltage Lumiled Demo Board
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AND8137/D
BILL MATERIALS
Conn1 D1-D4 Heatsink ZD1-ZD3 LED1-LED3 Tamura Semiconductor Keystone Keystone Keystone Lumileds Littlefuse Kemet Vendor Phoenix Contact Semiconductor Semiconductor AAVID Semiconductor Part Number 1715035 1N4004 D44H11 529802b02100 MPS2222 1/4W 2.2, 1/4W 3FD-424 1N5917 5000 5001 5002 LXHL-M*** (*Indicates color) F625-ND (Digi-Key Part Number) C320C104K5R5CA 399-2054-ND (Digi-Key Part Number)
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AND8137/D
Figure Side Foil Voltage Lumiled Demo Board
Figure Bottom Side Foil Voltage Lumiled Demo Board
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AND8137/D
References Lumiled, www.lumiled.com. Luxeon, www.luxeon.com. Savant, Roden, Carpenter, "Electronic Design, Circuits Systems, Ed", Benjamin/ Cummings Publishing, Redwood City, 94065, 1991, ISBN 0-8053-0285-9, 39-43.
Semiconductor registered trademarks Semiconductor Components Industries, (SCILLC). SCILLC reserves right make changes without further notice products herein. SCILLC makes warranty, representation guarantee regarding suitability products particular purpose, does SCILLC assume liability arising application product circuit, specifically disclaims liability, including without limitation special, consequential incidental damages. "Typical" parameters which provided SCILLC data sheets and/or specifications vary different applications actual performance vary over time. operating parameters, including "Typicals" must validated each customer application customer's technical experts. SCILLC does convey license under patent rights rights others. SCILLC products designed, intended, authorized components systems intended surgical implant into body, other applications intended support sustain life, other application which failure SCILLC product could create situation where personal injury death occur. Should Buyer purchase SCILLC products such unintended unauthorized application, Buyer shall indemnify hold SCILLC officers, employees, subsidiaries, affiliates, distributors harmless against claims, costs, damages, expenses, reasonable attorney fees arising directly indirectly, claim personal injury death associated with such unintended unauthorized use, even such claim alleges that SCILLC negligent regarding design manufacture part. SCILLC Equal Opportunity/Affirmative Action Employer. This literature subject applicable copyright laws resale manner.
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Abstract. This application note describes powering high current light

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