Copyright 2000 Rev. 2.0, 2000-06-08 AN-14 CCFL Power Supply

The Datasheet Archive - 100 Million Datasheets from 7500 Manufacturers.

Copyright 2000 Rev. 2.0, 2000-06-08 AN-14 CCFL Power Supply

Datasheet Thumbnail

Top Searches for this datasheet

Copyright 2000 Rev. 2.0, 2000-06-08
AN-14
CCFL Power Supply
CCFL Power Supply Expands Automotive Applications Back Liquid Crystal Displays
George Henry
LinFinity Microelectronics, Microsemi Company, Garden Grove, California
Abstract
backlight inverter features pioneered LinFinity solving major problems automotive back lighting applications. problems their solutions, along with actual laboratory data, presented this paper.
Light Output Lamp Temperature Display -40°C ambiant
Introduction
Light Output (Nits)
Automotive displays must operate over temperature range same time viewable direct sunlight total darkness. This requires light source capable very intense light that also dimmed less than maximum. Recent developments cold cathode fluorescent lamp technology improve light output cold temperature, have higher operating strike voltages than standard lamps. Operating voltages efficacy also quite non-linear over temperature. These load requirements, coupled with eight sixteen volt range battery present considerable challenge CCFL inverter that must able provide flicker free lighting from more than less than Nit.
Lamp Temperature (°C.)
Figure
CCFL Temperature Characteristics
Over Driving Lamps Decreases Warm-up Time Increases Output Cold Temperature Warm time full brightness from ambient exponential function. Figure plot measured lamp brightness versus time inch panel when panel held constant 40°C. lamp this display nominally rated mArms maximum), driven show increase overall light output decreasing rise time light with current overdrive. What would normally minute warm Nits output with nominal rated drive current accomplished less than seconds when driven also clear from figure that over driving dramatically increase absolute light level temperature. this case, overdrive yields three times light output with corresponding rise time third. results overdrive obvious dramatic. concern overdrive will impact lamp life. will take lengthy testing effort prove hypothesis about lamp life temperature overdrive. Just same, simple calculation using data already have give reasonable prediction. Lamp life related major issues: mercury consumption, which directly proportional lamp current level, operating bulb temperature. Lamp life reduced exponentially bulb temperature drops below optimum. estimated about 1/10th long when bulb temperature compared 65°C.
Page
CCFL light output dependent ambient temperature temperature rise lamp itself. Lamp manufacturers usually measure temperature outside surface bulb, midway between electrodes, most applications more convenient mount thermistor closer end. ease measurement more precise correlation actual application, CCFL light output voltage data presented this paper taken with lamps mounted panels designed automotive use. variety lamps with temperature sensors mounted near instead centered between cathodes were used. Therefor, data directly correlate with lamp manufacturer data. Lamps normally rated peak output rise above ambient. lower temperatures light falls rapidly, shown figure below. standard CCFLs, output virtually unusable below degrees, since small amount light they often pinkish purple color, from perfect white light needed good color rendition panel. Recently available `self heating' lamps increase white light output significantly cold temperatures, flatten curve over higher temperature range well. These lamps provide good display quality over wide thermal operating environment automotive displays.
Copyright 2000 2.0, 2000-06-08
AN-14
following discussion estimates probable effect lamp life where goal lamp optimum temperature quickly possible without exceeding overdrive. bulb temperature feed back system assumed that returns lamp current nominal upon reaching optimum bulb temperature. Assuming automobile display must perform 250,000 miles average speeds miles hour, then 10,000 hours lamp life required. CCFLs used applications rated 10,000 hours minimum, with types rated 50,000 hours. lamp turned average once every miles during life, 25,000 times, overdriven seconds each time, would hours overdrive during 10,000 hour life. additional mercury consumed about 1.04% that required sustain 10,000 hours nominal rated current. Life reduction mercury consumption around hours therefor expected. other hand, hours overdrive results saving about hours operation bulb temperature. This because overdriving reduces warm-up time that nominal drive. Since operating reduced temperature largest detrimental effect lamp life, reasonable expect that lamp life automotive applications actually improved simply overdriving turn shown that overdriving 150% rated current detrimental lamp life, long optimum bulb operating temperature exceeded, then ambient light output levels cold, well response times temperatures optimized with simple temperature feed back system.
LIGHT TIME FUNCTION LAMP CURRENT AMPLITUDE NOMINAL LAMP CURRENT RATING
ANELAT
CCFL Power Supply
Driving Self Heating Lamps
Self-heating lamps made using formulations increased pressure. This results higher operating striking voltages higher operating power. order more light display surface; more lamps longer `formed' lamps used. Formed lamps shapes wrap around three edges panel. bends these lamps tend increase voltage requirements make high ratio dimming more difficult. Multiple straight lamps easier drive demand that current lamps made extremely close equal prevent uneven lighting across display. Note figure strong dependency light output current level. this case, +20% change current resulted increase light output. There really only three ways drive lamp system; complete separate inverter each lamp, single inverter drive both lamps, either parallel series. single inverter case desired because lower cost. Driving lamps parallel common, main drawback that lamp current sharing very difficult. series configuration preferred because lamp current sharing inherent. `ballast' impedance capacitor series with each lamp must control lamp currents parallel arrangement. Since lamp impedance negative, e.g., lamp voltage increases current decreases; ballast impedances must very high closely matched insure current matching. Lamp operating voltage must also closely matched must track lamps age. parallel arrangement, inverter current regulation loop only sense lamp currents, therefor cannot help control matching. series connected arrangement, inverter regulates series current that flows through both lamps, therefor plays major part establishing matched currents. Lamp operating voltage this case double single lamp, when using direct drive inverter topology, still greater than lamp voltage added ballast voltage required parallel case. When operated series, lamp voltage matching aging longer critical. Strike voltage series configuration higher than parallel, requires high voltage transformer more robust. Figure three shows measured individual strike voltages lamps series connected arrangement with third plot total strike voltage connected series. good news here that total strike potential generated inverter only about times, rather that twice individual lamp. dynamics striking lamps series totally understood, experimentation shows that lamp will always strike just before other unequal voltage drops around system. suspected that lamp panel parasitic capacitance's facilitate this behavior. result, however, total applied voltage strike lamps series must equal voltage plus strike voltage.
LIGHT (NITS
mArms mArms
mArms
mArms
TIME (SECONDS)
Figure Resistance Wire Heating been popular military applications pre-heat cold cathode fluorescent lamps improve light output, enhance dimming range, reduce strike potential. Resistive wire coiled around lamp effective accomplishing this task, possible substitute self heating lamps. significant problem with resistance wire heating automotive portable applications related their small size. LCD's designed these applications very thin, requiring diameter lamps. Lamp drive electronics must also made small, requiring higher operating frequencies. Wire coiled around bulb creates large parasitic capacitance between lighted lamp chassis ground, making difficult properly drive lamp range required. Resistance wire heating less efficient more expensive, making self-heating lamps preferred technology.
Copyright 2000 2.0, 2000-06-08
Page
AN-14
STRIKE VOLTAGE TEMPERATURE SELF HEATING LAMPS
1600 1500
CCFL Power Supply
Digital dimming techniques have been used military applications number years, until recently have been expensive commercial applications. During last years LinFinity shown digital dimming effective, cost, reliable method achieving high dimming ratios. Over million lamp hours life testing combined with numerous installations commercial monitors automotive displays confirm lamp reliability degraded digital dimming.
BOTH LAMPS SERIES
1400 1300
STRIKE VOLTAGE (VRMS)
1200
LAMP
1100 1000
LAMP
TEMPERATURE (°C)
Techniques Wide Dimming Range
Lamp Current Shaping Dimming ratios 100:1 relatively easy obtain careful lamp current wave shaping. Lamp current frequency usually 70KHz range, while burst frequency range. important when operating very duty cycles, e.g., less than dozen lamp current cycles burst, carefully shape turn lamp current. Providing overshoot first cycles will insure lamp lights instantly over full length light gradients appear display screen. Figures five show optimized lamp current wave shapes duty cycle.
Figure
Wide Range Dimming
moving vehicle quickly drive from direct sunlight very light when entering tunnel, travel through total darkness uninhabited areas night. Dash mounted displays must quickly adjust emitted light these situations prevent temporary night blindness driver. This absolute safety requirement. There ways CCFL. conventional method called analog dimming, where lamp current amplitude adjusted. Fluorescent lamps emit light directly proportional current that flows through them, they have very limited current range. maximum allowed current divided minimum allowed usually ratio three one. upper end, lamp saturate that increasing current longer increases light output. lower end, lamp will begin extinguish from end, resulting severely uneven output along tube. other method digital dimming, where lamp operated optimum current amplitude, switched relatively frequency "burst rate". During each burst, adjustable number higher frequency lamp current cycles permitted. Control number cycles (pulse width modulated) input signal. duty cycle this signal vary from nearly zero 100%, with corresponding lamp output varying from almost light maximum. Figure four shows light output duty cycle over automotive temperature range self-heating lamp. Note that lamp output duty cycle linear compared analog dimming shown figure one.
LIGHT OUTPUT DUTY CYCLE AMBIENT TEMPERATURE
Figure
LIGHT OUTPUT (NITS)
0100
-40°
-20°
DUTY CYCLE
Figure
Copyright 2000 2.0, 2000-06-08
Figure
Page
AN-14
Preventing Flicker Flicker singular symptom many lighting problems therefor difficult diagnose. human detect less than change light intensity when occurs frequencies less than this reason, burst rate must above care must taken insure optical beating less than occur, most importantly, average current lamp during each burst must identical. most predominant cause flicker very high ratio dimming systems unstable cycle counts each burst lamp current. Synchronizing lamp current frequency burst frequency will eliminate this instability. Additionally, these frequencies synchronized video frame frequency, moving horizontal bars caused slow beat frequencies will prevented. methods synchronize available; phase locked loop (PLL) which locks burst lamp frequencies external video sync pulse, down counter which derives burst rate from external video horizontal sync pulse. this later case, video horizontal frequency slow produce high frequency lamp current, higher frequency source, such video data clock used. Both methods eliminate inherent cycle uncertainty resulting from synchronizing unrelated oscillators with flipflop. Some other sources flicker noise analog brightness control input, noise system power supplies, jitter ramps their output pulses.
CCFL Power Supply
Figure seven shows lamp current duty cycle must change over temperature produce constant light output.
fully Digital Dimming Control Scheme
CCFL controllers available today hard pressed exceed dimming ratios 100:1. Auto manufacturers, however, requesting ratios 1000:1. thousand ratios that vary brightness from Nits achieved with fully digital control system that derives burst rate lamp current from single oscillator. Ideally this oscillator phase locked video controllers vertical sync pulse prevent video interference from optical frequency beating. Digital dimming based counting current cycles applied lamp each burst there instantaneous random changes average current. system interface must digital that analog noise introduced, both analog digital dimming techniques simultaneously employed previously described. case analog dimming, will supply final reference voltage single analog circuit used controller, lamp current error amplifier. Digital counters magnitude comparators analog circuit must used control burst duty cycle. conventional implemented with linear voltage ramp comparator must discarded. These have jitter that cause duty cycle instability because they dependent start finish levels voltage ramp, they require brightness control voltage presented comparator.
Dual Dimming Controls
Previous sections have discussed analog digital dimming attributes they relate lamp performance. This sections describes system that simultaneously uses both types dimming. Since analog current amplitude used help overcome slow response light output CCFLs temperature, ideal self heating lamp controller would adjust current amplitude function bulb ambient temperature. Duty cycle, digital dimming, would adjusted function ambient display light output compared with user brightness setting. Lamp output would feed back compare user setting. prevent temporary driver blindness, sensors that detect ambient light would automatically override user input that brightness quickly adjusts changing ambient light conditions.
CONSTANT LIGHT OUTPUT CONTOURS Duty Cycle Ambient Temperature
100.0%
System Specification Considerations
Extreme dimming extreme cold temperature difficult control without very accurate light temperature feed back, necessary considering brief time lamp itself needs heat usual, severe environment systems, designing full specifications over entire operating range will involve trade nominal performance, cost, both. system designer should give full consideration specifications that relieved obtain best overall cost performance ratio.
90.0% Nits 80.0%
70.0%
Duty Cycle
60.0%
50.0% Nits 40.0% Nits
30.0% Nits 20.0% Nits 10.0% Nits Nits Nits
0.0% perature (°C)
Copyright 2000 2.0, 2000-06-08
Figure
Page

Copyright 2000 Rev. 2.0, 2000-06-08 AN-14 CCFL Power Supply

Top Searches for this datasheet

Other recent searches