Application Note 14Issue 2 March 1996

Transistor Considerations for LCD Backlighting

Neil Chadderton

Introduction

LCD Back l ight ing has generatedwidespread interest from many diversedisciplines within the engineeringindustry. This has no doubt been fueledby the t rend to portabi l i ty andparticularly to the enormous growth ofthe computing market. Products such asnotebook, laptop, and palmtop personalcomputers , portable te levisions,viewcams, point of sale terminals,automotive dashboards, avionicsdisplays, metering and instrumentationusually employ an LCD screen, and assuch require a means of backlighting. Todate the most prevalent method hasbeen to use a small cold cathodefluorescent (CCFL) tube that is usuallyintegrated with a reflector/diffuser intothe display unit. The CCFL powerconsumption can account for asignificant portion (up to 50%) of thetotal system requirement. Therefore toachieve marketable advantages inbattery life and re-charge frequency,much attention must be applied to theCCFL power supply, so as to attain thehighest possible conversion efficiency.

This problem has been the focus ofmany electronic component vendors:much research and design effort beinginvested in order to offer system

designers the most at t ract ivecomponents/solutions in terms ofefficiency, cost, weight, and size. Manyof the analog IC companies havepublished application specific reports,and character ised or developedspecifically, integrated circuits for theapplication.

This note acknowledges this work, andwill draw upon such sources andreproduce these vendors circuits whereappropriate (a list of references isincluded in Appendix A) but it is focusedprimarily on the transistor requirements their mode of operation within thebackl ight ing c ircui t , importantparameters, and their impact on thesystem efficiency.

CCFL Lamp Characteristics

An understanding of the requirementsfor the backlighting power supplyshould begin with a description of theload involved. The fluorescent tubepresents a serious challenge to thecircuit designer. Around 1kV is requiredto strike the tube (initiate conduction), atwhich event the tubes gaseous contentsionise and it begins to conduct at a lowersustaining voltage thus a negative

AN14-1

High Efficiency DC to AC Conversion

Application Note 14Issue 2 March 1996

Application Note 14Issue 2 March 1996

resistance characteristic is evident.Other power supply constraints includean intolerance of DC current , asensitivity to waveform crest factor, andRFI criteria.

The curve tracer plots shown in Figures1 and 2 show the negative resistanceregion for two typical CCFL units: thefirst for a 150mm linear, 10mm diameterbacklight tube for a laptop display, andthe other a U tube as produced for acar dashboard display. Referring tofigure 1, the high striking voltage can beseen at 560V and the negative resistanceexcursion to 240V is self evident.Similarly, these values for Figure 2 are1240V and 900V. Note should also bemade of the slope impedance in theconducting state. The power supplymust accommodate this, and in somecases provision made to regulate thelamp current to ensure a long tube life.

For drive waveforms at low frequencies,a fluorescent tube has time to react tothe changing waveform potential, andeffectively re-strikes on each reversal ofthe waveform polarity, (perceived asflicker on line frequency units). At highdrive waveform frequencies, this effectis not apparent, and the lamp can beapproximated to a resistive load. Usualoperating frequencies range from 25 to120kHz, th is being dictated byconsiderat ion of inaudibi l i tyrequirements, converter inductor size,and at the extreme, parasitic andHV-lead-to-ground coupling capacitance.

Basic Operation Of Converter

The drive requirements dictated by theCCFL tubes behaviour and preferredoperating conditions can be achieved bythe resonant push-pull converter shown

in Figure 3. This is also referred to as theRoyer Converter, after G.H. Royer whoproposed the topology in 1954 as apower converter . (Note: Str ict lyspeaking the backlighting converteruses a modified version of the Royerconverter the original used asaturating transformer to define theoperating frequency, and thereforeproduced a squarewave drive

waveform). The circuit looks simple butth is is very decept ive: manycomponents interact, and while thecircuit is capable of operation withwidely varying component values,(useful during development)optimisation is required for each designto achieve the highest possibleefficiencies.

Transistors Q1 and Q2 are alternativelysaturated by the base drive provided bythe feedback winding W4. The basecurrent is defined by resistors R1 and R2.Supply inductor L1 and pr imarycapacitance C1 force the circuit to runs inusoidal ly thereby minimisingharmonic generation and RFI, andproviding the preferred drive waveformto the load. Voltage step-up is achievedby the W1:(W2 + W3) turns ratio. C2 isthe secondary winding ballast capacitor,and effectively sets the tube current.

Prior to the tube striking, or when notube is connected, the operat ingfrequency is set by the resonant parallelc i rcuit comprising the primarycapacitance C1, and the transformersprimary winding W2+W3. Once the tubehas struck, the ballast capacitor C2 plusdist r ibuted tube and parasit iccapacitances are reflected back throughthe transformer, and the operatingfrequency is lowered.

The secondary load can becomedominant in c ircuits with a hightransformer turns ratio, Eg. thosedesigned to operate from very low DCinput voltages.

Each transistors collector is subject to avoltage= 2 x π/2 x VS, (or just π x VS)where VS is the DC input voltage to theconverter. (The π/2 factor being due tothe relationship between average andpeak values for a sinewave, and the x2mult ipl ier being due to the 2:1autotransformer act ion of thetransformers centre-tapped primary).This primary voltage is stepped up bythe transformer turns ratio Ns:Np, to ahigh enough level to reliably strike thetube under all conditions:- startingvol tage is dependent on displayhousing, location of ground planes, tubeage, and ambient temperature.

The basic converter shown in Figure 3 isa valid and useful circuit that has beenutilised for many systems and indeedoffered as a sub-system by severalmanufacturers.

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Figure 1. CCFL Characteristics - 150mm linear;100V/div horizontal, 200µA/div vertical.

Figure 2. CCFL Characteristics - U tube;200V/div horizontal, 1mA/div vertical.

Figure 3.Generalised Royer Converter.

L1

T1

W1

W2 W3

W4

R2R1C3

L1

0V

+V

Q1 Q2

C2

C1

Application Note 14Issue 2 March 1996

Application Note 14Issue 2 March 1996

resistance characteristic is evident.Other power supply constraints includean intolerance of DC current , asensitivity to waveform crest factor, andRFI criteria.

The curve tracer plots shown in Figures1 and 2 show the negative resistanceregion for two typical CCFL units: thefirst for a 150mm linear, 10mm diameterbacklight tube for a laptop display, andthe other a U tube as produced for acar dashboard display. Referring tofigure 1, the high striking voltage can beseen at 560V and the negative resistanceexcursion to 240V is self evident.Similarly, these values for Figure 2 are1240V and 900V. Note should also bemade of the slope impedance in theconducting state. The power supplymust accommodate this, and in somecases provision made to regulate thelamp current to ensure a long tube life.

For drive waveforms at low frequencies,a fluorescent tube has time to react tothe changing waveform potential, andeffectively re-strikes on each reversal ofthe waveform polarity, (perceived asflicker on line frequency units). At highdrive waveform frequencies, this effectis not apparent, and the lamp can beapproximated to a resistive load. Usualoperating frequencies range from 25 to120kHz, th is being dictated byconsiderat ion of inaudibi l i tyrequirements, converter inductor size,and at the extreme, parasitic andHV-lead-to-ground coupling capacitance.

Basic Operation Of Converter

The drive requirements dictated by theCCFL tubes behaviour and preferredoperating conditions can be achieved bythe resonant push-pull converter shown

in Figure 3. This is also referred to as theRoyer Converter, after G.H. Royer whoproposed the topology in 1954 as apower converter . (Note: Str ict lyspeaking the backlighting converteruses a modified version of the Royerconverter the original used asaturating transformer to define theoperating frequency, and thereforeproduced a squarewave drive

waveform). The circuit looks simple butth is is very decept ive: manycomponents interact, and while thecircuit is capable of operation withwidely varying component values,(useful during development)optimisation is required for each designto achieve the highest possibleefficiencies.

Transistors Q1 and Q2 are alternativelysaturated by the base drive provided bythe feedback winding W4. The basecurrent is defined by resistors R1 and R2.Supply inductor L1 and pr imarycapacitance C1 force the circuit to runs inusoidal ly thereby minimisingharmonic generation and RFI, andproviding the preferred drive waveformto the load. Voltage step-up is achievedby the W1:(W2 + W3) turns ratio. C2 isthe secondary winding ballast capacitor,and effectively sets the tube current.

Prior to the tube striking, or when notube is connected, the operat ingfrequency is set by the resonant parallelc i rcuit comprising the primarycapacitance C1, and the transformersprimary winding W2+W3. Once the tubehas struck, the ballast capacitor C2 plusdist r ibuted tube and parasit iccapacitances are reflected back throughthe transformer, and the operatingfrequency is lowered.

The secondary load can becomedominant in c ircuits with a hightransformer turns ratio, Eg. thosedesigned to operate from very low DCinput voltages.

Each transistors collector is subject to avoltage= 2 x π/2 x VS, (or just π x VS)where VS is the DC input voltage to theconverter. (The π/2 factor being due tothe relationship between average andpeak values for a sinewave, and the x2mult ipl ier being due to the 2:1autotransformer act ion of thetransformers centre-tapped primary).This primary voltage is stepped up bythe transformer turns ratio Ns:Np, to ahigh enough level to reliably strike thetube under all conditions:- startingvol tage is dependent on displayhousing, location of ground planes, tubeage, and ambient temperature.

The basic converter shown in Figure 3 isa valid and useful circuit that has beenutilised for many systems and indeedoffered as a sub-system by severalmanufacturers.

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Figure 1. CCFL Characteristics - 150mm linear;100V/div horizontal, 200µA/div vertical.

Figure 2. CCFL Characteristics - U tube;200V/div horizontal, 1mA/div vertical.

Figure 3.Generalised Royer Converter.

L1

T1

W1

W2 W3

W4

R2R1C3

L1

0V

+V

Q1 Q2

C2

C1

Application Note 14Issue 2 March 1996

Application Note 14Issue 2 March 1996

Backlight Converters WithinControl Loops

Variations on the basic topology arepossible, perhaps the most importantbeing to include the converter within acontrol loop. This can be used toregulate the tube current : - thismaximises tube lifetime, ensures aconstant light output as the battery packvoltage decreases, and enablesadjustment of tube brightness. Theusual form of the circuit is to employ aBuck or step-down converter (directlyfrom the battery pack to increaseefficiency) feeding the centre tap of thetransformer, or the emitter current of thetransistors, depending on thecontrollers technology and capability.F igures 4a and 4b show thesearrangements in conceptual form. Thecontroller can monitor the tube current

directly in the secondary, or in somerecent systems, by the primary current.This latter method allows the tube to befully floating thus minimising HV losses.

Figure 5 shows a circuit published byl inear IC manufacturer LINEARTECHNOLOGY CORP. that exhibits asignificant efficiency improvement overprevious designs; primarily due to thechoice of the ZETEX FZT849. It is basedon the Buck converter current fed Royerscheme of Figure 4b, and monitors thelamps current directly by averaging thepositive half cycles of lamp current, andapplying this signal to the controllersfeedback pin. The electrical conversionefficiency using this form of circuit canbe very high, the stated value for Figure5 being 88%. Higher efficiencies up to92% are possible by using largertransformers to reduce copper and corelosses.

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L1

T1

W1

W2 W3

W4

R2R1

C3

0V

+V

Q1 Q2

C2

C1

L1

D1

PWM

Figure 4a.Royer Converter With PWM Control - High Side Current Fed Version.

FZT849

W1 W2

W4

2

3

4

5

79

1

33nF1K

CTX300-4

300uH

2.2uF

10K

560

50K

1/2

BAV99

1/2BAV99

1N5818

1uF

10uF

CCFT

3KV

15pF

LT1172

Vin

Vsw

Vfb

VcGnd

E2

E1

NC

W3

CTX110092-1

0V

+4.5 to 20V

Vin

Gnd

FZT849

5mA MAX

COILTRONICS

Connect to lowestvoltage available

(Vmin=3V)

Figure 5. Linear Technology LCD Backlight Converter.

L1W1

C2

T1

W2 W3

W4

R2R1C3

0V

+V

Q1 Q2

C1

L1

PWM

D1

Figure 4b.Royer Converter With PWM Control - Low Side (or tail) Current Fed Version.

Application Note 14Issue 2 March 1996

Application Note 14Issue 2 March 1996

Backlight Converters WithinControl Loops

Variations on the basic topology arepossible, perhaps the most importantbeing to include the converter within acontrol loop. This can be used toregulate the tube current : - thismaximises tube lifetime, ensures aconstant light output as the battery packvoltage decreases, and enablesadjustment of tube brightness. Theusual form of the circuit is to employ aBuck or step-down converter (directlyfrom the battery pack to increaseefficiency) feeding the centre tap of thetransformer, or the emitter current of thetransistors, depending on thecontrollers technology and capability.F igures 4a and 4b show thesearrangements in conceptual form. Thecontroller can monitor the tube current

directly in the secondary, or in somerecent systems, by the primary current.This latter method allows the tube to befully floating thus minimising HV losses.

Figure 5 shows a circuit published byl inear IC manufacturer LINEARTECHNOLOGY CORP. that exhibits asignificant efficiency improvement overprevious designs; primarily due to thechoice of the ZETEX FZT849. It is basedon the Buck converter current fed Royerscheme of Figure 4b, and monitors thelamps current directly by averaging thepositive half cycles of lamp current, andapplying this signal to the controllersfeedback pin. The electrical conversionefficiency using this form of circuit canbe very high, the stated value for Figure5 being 88%. Higher efficiencies up to92% are possible by using largertransformers to reduce copper and corelosses.

AN14-5AN14-4

L1

T1

W1

W2 W3

W4

R2R1

C3

0V

+V

Q1 Q2

C2

C1

L1

D1

PWM

Figure 4a.Royer Converter With PWM Control - High Side Current Fed Version.

FZT849

W1 W2

W4

2

3

4

5

79

1

33nF1K

CTX300-4

300uH

2.2uF

10K

560

50K

1/2

BAV99

1/2BAV99

1N5818

1uF

10uF

CCFT

3KV

15pF

LT1172

Vin

Vsw

Vfb

VcGnd

E2

E1

NC

W3

CTX110092-1

0V

+4.5 to 20V

Vin

Gnd

FZT849

5mA MAX

COILTRONICS

Connect to lowestvoltage available

(Vmin=3V)

Figure 5. Linear Technology LCD Backlight Converter.

L1W1

C2

T1

W2 W3

W4

R2R1C3

0V

+V

Q1 Q2

C1

L1

PWM

D1

Figure 4b.Royer Converter With PWM Control - Low Side (or tail) Current Fed Version.

Application Note 14Issue 2 March 1996

Application Note 14Issue 2 March 1996

Figure 6 shows Linear Technologyslatest design using the LT1182 and theZetex ZDT1048 dual transistor. TheLT1182 provides a low component countcircuit and contains all control functionsfor the Royer converter, and thecontrol/switch for the LCD contrastconverter within one package. PrimaryRoyer converter current is sensed by theIC, so that the CCFL tube can be operatedin a floating mode thereby decreasinglosses in the secondary circuit. TheFZT849 transistors, or the ZDT1048 dualpackage are preferred options for thisconverter circuit.

Detailed reports on these circuits can befound via the references listed.

Figure 7 shows an oscillograph of thetransistors operating conditions in sucha circuit. The Collector-Emitter voltagepeaks at 28V (less than π x VS due to thelamp load); the Emitter current is almostconstant at 0 .5A (with a r ipplecomponent dependent on the Buckinductor); and the base voltage appearsas a clipped (due the transistors VBE)version of the primary waveform.

Requisite TransistorCharacteristics

The relatively low operating frequency asrequired by the backlighting RoyerConverter (to minimise HV parasiticcapacitance losses), and the ease oftransformer drive, makes this circuitparticularly suitable for bipolar transistor

implementation. This isnt to excludeMOSFET based designs (some ICvendors have specified MOS as this suitstheir technology) but in terms ofequivalent on-resistance and siliconefficiency, the low voltage bipolar devicehas no equal. For example, the ZETEXZTX849 E-Line (TO-92 compatible)transistor exhibits a RCE(sat) of 36mΩ. Thiscan only be matched by a much larger(and expensive) MOSFET die, onlyavailable in TO-220, D-Pak, and similarlarger packages.

The important transistor characteristicsare voltage rating, VCE(sat), and hFE, andare detailed below.

The voltage rating required deservessome thought with respect to thestandard transistor breakdownparameters , as i t is possible toover-specify a device on grounds ofvoltage rating, and thereby incur areduct ion in ef f ic iency due to

unnecessary on-resistance losses. Theprimary breakdown voltage BVCBO, of aplanar bipolar transistor depends on theepitaxial layer - specifically its thicknessand resistivity. The breakdown voltageof most interest to the designer isusual ly that at tained across theCollector-Emitter (C-E) terminals. Thisvalue can vary between the primarybreakdown BVCBO and a much lowervoltage dependent on the state of thebase terminal bias.

[The breakdown mechanism is causedby the avalanche multiplication effect,whereby free electrons can be impartedwith sufficient energy by the reversebias electric field such that any collisionscan lead to ionisation of the latticeatoms. The free electrons thusgenerated are then accelerated by thefield and produce further ionisation. Thismultiplication of free carriers increasesthe reverse current dramatically, and sothe junction effectively clamps theapplied voltage. The base terminal canobviously influence the junction current thereby modulating the voltagerequired for a breakdown condition.]

Figure 8 shows how the breakdowncharacteristic is seen to vary for differentcircuit conditions. The BVCEO rating (orwhen the base is open circuit) allows theCollector-Base (C-B) leakage current ICBO

to be effectively amplified by thetransistor s β thus s igni f icant lyincreasing the leakage component toICEO. Shorting the Base to the Emitter(BVCES) provides a parallel path for theC-B leakage, and so the voltage requiredfor breakdown is higher than the openbase condition. BVCER denotes the casebetween the open and shorted baseoptions:- R indicating an externalbase-emitter resistance, the value of

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Figure 6. Linear Technology Floating Tube LCD Backlight Converter.

Figure 7. Royer Converter Operating Waveforms:VCE 10V/div; IE 0.5A/div; VBE 2V/divrespectively, 2µs/div horizontal .

Application Note 14Issue 2 March 1996

Application Note 14Issue 2 March 1996

Figure 6 shows Linear Technologyslatest design using the LT1182 and theZetex ZDT1048 dual transistor. TheLT1182 provides a low component countcircuit and contains all control functionsfor the Royer converter, and thecontrol/switch for the LCD contrastconverter within one package. PrimaryRoyer converter current is sensed by theIC, so that the CCFL tube can be operatedin a floating mode thereby decreasinglosses in the secondary circuit. TheFZT849 transistors, or the ZDT1048 dualpackage are preferred options for thisconverter circuit.

Detailed reports on these circuits can befound via the references listed.

Figure 7 shows an oscillograph of thetransistors operating conditions in sucha circuit. The Collector-Emitter voltagepeaks at 28V (less than π x VS due to thelamp load); the Emitter current is almostconstant at 0 .5A (with a r ipplecomponent dependent on the Buckinductor); and the base voltage appearsas a clipped (due the transistors VBE)version of the primary waveform.

Requisite TransistorCharacteristics

The relatively low operating frequency asrequired by the backlighting RoyerConverter (to minimise HV parasiticcapacitance losses), and the ease oftransformer drive, makes this circuitparticularly suitable for bipolar transistor

implementation. This isnt to excludeMOSFET based designs (some ICvendors have specified MOS as this suitstheir technology) but in terms ofequivalent on-resistance and siliconefficiency, the low voltage bipolar devicehas no equal. For example, the ZETEXZTX849 E-Line (TO-92 compatible)transistor exhibits a RCE(sat) of 36mΩ. Thiscan only be matched by a much larger(and expensive) MOSFET die, onlyavailable in TO-220, D-Pak, and similarlarger packages.

The important transistor characteristicsare voltage rating, VCE(sat), and hFE, andare detailed below.

The voltage rating required deservessome thought with respect to thestandard transistor breakdownparameters , as i t is possible toover-specify a device on grounds ofvoltage rating, and thereby incur areduct ion in ef f ic iency due to

unnecessary on-resistance losses. Theprimary breakdown voltage BVCBO, of aplanar bipolar transistor depends on theepitaxial layer - specifically its thicknessand resistivity. The breakdown voltageof most interest to the designer isusual ly that at tained across theCollector-Emitter (C-E) terminals. Thisvalue can vary between the primarybreakdown BVCBO and a much lowervoltage dependent on the state of thebase terminal bias.

[The breakdown mechanism is causedby the avalanche multiplication effect,whereby free electrons can be impartedwith sufficient energy by the reversebias electric field such that any collisionscan lead to ionisation of the latticeatoms. The free electrons thusgenerated are then accelerated by thefield and produce further ionisation. Thismultiplication of free carriers increasesthe reverse current dramatically, and sothe junction effectively clamps theapplied voltage. The base terminal canobviously influence the junction current thereby modulating the voltagerequired for a breakdown condition.]

Figure 8 shows how the breakdowncharacteristic is seen to vary for differentcircuit conditions. The BVCEO rating (orwhen the base is open circuit) allows theCollector-Base (C-B) leakage current ICBO

to be effectively amplified by thetransistor s β thus s igni f icant lyincreasing the leakage component toICEO. Shorting the Base to the Emitter(BVCES) provides a parallel path for theC-B leakage, and so the voltage requiredfor breakdown is higher than the openbase condition. BVCER denotes the casebetween the open and shorted baseoptions:- R indicating an externalbase-emitter resistance, the value of

AN14-7AN14-6

Figure 6. Linear Technology Floating Tube LCD Backlight Converter.

Figure 7. Royer Converter Operating Waveforms:VCE 10V/div; IE 0.5A/div; VBE 2V/divrespectively, 2µs/div horizontal .

Application Note 14Issue 2 March 1996

Application Note 14Issue 2 March 1996

which is typically 100 to 10kΩ. BVCEV orBVC E X is a special case where thebase-emitter is reverse biased; this canprovide a better path for the C-B leakage,and so this rating yields a voltage closeto, or coincident with the BVCBO value.

Figure 9 shows a curve tracer view of therelevant breakdown modes of theZTX849 transistor, including a curveshowing the device in the on state.Curves 1 and 2 are virtually coincidentand show BVCBO and BVCES respectively.Curve 3 shows the BVCEV case with anapplied base bias (VEB) of -1V. Curve 4shows BVCEO at approximately 36V.Curve 5 is a BVCE curve, showing how thebreakdown condition is affected by apositive base bias of 0.5V.

The BVCEV rating has particular relevanceto the Royer Converter, as can besurmised from Figure 7. Examination ofthis will show that the transistor onlyexperiences the high C-E voltage whenthe base voltage has been takennegative by the feedback winding, these

events of course being in perfectsynchronism. An expanded view of theC-E and B-E waveforms is shown inFigure 10.

[Note: The voltage applied by thefeedback winding must not exceed theBVEBO of the transistor. This is specifiedat 5V usually, against an actual of 7.5 to8.5V].

The VCE(sat) and hFE parameters have adirect bearing on the circuits electricalconversion efficiency. This is especiallytrue of low voltage battery poweredsystems, due to the high current levelsinvolved. Selection of standard LFamplifier transistors provides far fromideal results; these parts are for generalpurpose linear and non-critical switchinguse only. The high VCE(sat) inherent to theseparts, and low current gain could reducecircuit efficiency to less than 50%. Forexample, the stated VCE(sat) maximummeasured at 500mA, for the FZT849SOT223 transistor, and a LF devicesometimes quoted as a suitable RoyerConverter transistor are 50mV and 0.5Vrespectively. Eg.

VCE(sat)

@IC

IB

FZT849 50mV 0.5A 20mA

BCP56 0.5V 0.5A 50mA

To address the VCE(sat) issue, large powertransistors are occasionally specified.Unfortunately their capacitance, andcharacteristic low base transport factor(a feature of Epitaxial Base devices) canlead to problems with cross-conductionlosses due to long storage and switchingt imes. The current gain is a lsoimportant, as the losses in the base biascan be significant to the overall figure;judicious selection of the bias resistor toensure a minimum VCE(sat) whi lepreventing base overdrive needs toconsider supply variation, maximumlamp current , and t rans is tor hFE

minimum value and range.

For the above reasons, transistorsdesigned and optimised for high currentswitching applications offer the mostcost-effective and efficient solutions.The table presented in Appendix C listsseveral ZETEX transistors that areeminent ly suitable for the Royerconverter. All of these parts offeroutstanding VCE(sat) and high currentperformance for their size, and many areso-called Super-β transistors; therebyhelping to simplify and improve drivecurrent requirements. Figure 11 showsthe VCE(sat) exhibited by the ZTX1048A fora range of forced gain values. Thisdevice is one of the ZTX1050 series oftransistors that employ a scaled upvariant of the highly efficient Matrixgeometry, developed for the ZETEXSuperSOT series. This enables aVCE(sat) performance similar to theZTX850 series at the low to moderatecurrents relevant to this application,though utilising a smaller die, andtherefore providing a cost and possiblya space saving advantage.

AN14-9AN14-8

Constant IB Curves(Normal Operation)

BVCEO

BVCER

BVCESBVCEX BVCBO

VCE - Collector Emitter Voltage0

Figure 8.Voltage Breakdown Modes of BipolarTransistor.

Figure 9. Breakdown modes of the ZTX849 BipolarTransistor.

Figure 10. Royer Converter: VCE and VBE Waveforms5V/ div and 2V/ div respectively.

50mV

100mV

150mV

200mV

250mV

300mV

0

1mA 10mA 100mA 1A 10A

Figure 11.VCE(sat) v IC for the ZTX1048A BipolarTransistor: Forced gains of 10,20,50,100.

Application Note 14Issue 2 March 1996

Application Note 14Issue 2 March 1996

which is typically 100 to 10kΩ. BVCEV orBVC E X is a special case where thebase-emitter is reverse biased; this canprovide a better path for the C-B leakage,and so this rating yields a voltage closeto, or coincident with the BVCBO value.

Figure 9 shows a curve tracer view of therelevant breakdown modes of theZTX849 transistor, including a curveshowing the device in the on state.Curves 1 and 2 are virtually coincidentand show BVCBO and BVCES respectively.Curve 3 shows the BVCEV case with anapplied base bias (VEB) of -1V. Curve 4shows BVCEO at approximately 36V.Curve 5 is a BVCE curve, showing how thebreakdown condition is affected by apositive base bias of 0.5V.

The BVCEV rating has particular relevanceto the Royer Converter, as can besurmised from Figure 7. Examination ofthis will show that the transistor onlyexperiences the high C-E voltage whenthe base voltage has been takennegative by the feedback winding, these

events of course being in perfectsynchronism. An expanded view of theC-E and B-E waveforms is shown inFigure 10.

[Note: The voltage applied by thefeedback winding must not exceed theBVEBO of the transistor. This is specifiedat 5V usually, against an actual of 7.5 to8.5V].

The VCE(sat) and hFE parameters have adirect bearing on the circuits electricalconversion efficiency. This is especiallytrue of low voltage battery poweredsystems, due to the high current levelsinvolved. Selection of standard LFamplifier transistors provides far fromideal results; these parts are for generalpurpose linear and non-critical switchinguse only. The high VCE(sat) inherent to theseparts, and low current gain could reducecircuit efficiency to less than 50%. Forexample, the stated VCE(sat) maximummeasured at 500mA, for the FZT849SOT223 transistor, and a LF devicesometimes quoted as a suitable RoyerConverter transistor are 50mV and 0.5Vrespectively. Eg.

VCE(sat)

@IC

IB

FZT849 50mV 0.5A 20mA

BCP56 0.5V 0.5A 50mA

To address the VCE(sat) issue, large powertransistors are occasionally specified.Unfortunately their capacitance, andcharacteristic low base transport factor(a feature of Epitaxial Base devices) canlead to problems with cross-conductionlosses due to long storage and switchingt imes. The current gain is a lsoimportant, as the losses in the base biascan be significant to the overall figure;judicious selection of the bias resistor toensure a minimum VCE(sat) whi lepreventing base overdrive needs toconsider supply variation, maximumlamp current , and t rans is tor hFE

minimum value and range.

For the above reasons, transistorsdesigned and optimised for high currentswitching applications offer the mostcost-effective and efficient solutions.The table presented in Appendix C listsseveral ZETEX transistors that areeminent ly suitable for the Royerconverter. All of these parts offeroutstanding VCE(sat) and high currentperformance for their size, and many areso-called Super-β transistors; therebyhelping to simplify and improve drivecurrent requirements. Figure 11 showsthe VCE(sat) exhibited by the ZTX1048A fora range of forced gain values. Thisdevice is one of the ZTX1050 series oftransistors that employ a scaled upvariant of the highly efficient Matrixgeometry, developed for the ZETEXSuperSOT series. This enables aVCE(sat) performance similar to theZTX850 series at the low to moderatecurrents relevant to this application,though utilising a smaller die, andtherefore providing a cost and possiblya space saving advantage.

AN14-9AN14-8

Constant IB Curves(Normal Operation)

BVCEO

BVCER

BVCESBVCEX BVCBO

VCE - Collector Emitter Voltage0

Figure 8.Voltage Breakdown Modes of BipolarTransistor.

Figure 9. Breakdown modes of the ZTX849 BipolarTransistor.

Figure 10. Royer Converter: VCE and VBE Waveforms5V/ div and 2V/ div respectively.

50mV

100mV

150mV

200mV

250mV

300mV

0

1mA 10mA 100mA 1A 10A

Figure 11.VCE(sat) v IC for the ZTX1048A BipolarTransistor: Forced gains of 10,20,50,100.

Application Note 14Issue 2 March 1996

Application Note 14Issue 2 March 1996

Package Options

ZETEX can offer a range of packages toallow complete circuit size and layoutoptimisation. Figure 12 illustrates these,from the TO92 compatible E-Linethrough-hole package, to surface mountoptions SOT23, SOT223, and SM-8.

The SM-8 is a dual island, eight leadedpackage that possesses the same bodydimensions as the industry standardSOT223. These attributes allow it toreplace the two Royer Convertertransistors with a single package twochip device, yielding a significant costand space saving.

For example, the 1048A transistor isavailable as an uncommitted dual withinthe SM8 package as the ZDT1048.

Conclusions

The advanced transistor geometries,and optimised processing employed byZETEX leads to a range of transistorsthat are ideally suited to the LCDbacklighting inverter application.Attention has been applied to specifyinga range of devices relevant to, andexhibiting a superior performancewithin the Royer inverter topology.

References

Transistors as On-Off Switches inSaturable Core CircuitsBright, Pittman and Royer.Westinghouse Electric Corp.,Electrical Manufacturing Dec 1954.

Techniques for 92% Efficient LCDIlluminationApplications Note 55 August 1993Jim WilliamsLinear Technology Corp.,

A Fourth Generation of LCD BacklightTechnology - Component andMeasurement Improvements RefinePerformance Application Note 65October 1995Jim WilliamsLinear Technology Corp.

Switching and Linear Power Supply,Power Converter DesignA. PressmanHayden Press.

Appendix A

LT1070, 1170 Series Switching

Regulators

LT1182, 1183 CCFL/LCD Contrast Dual

Switching Regulator

Linear Technology Corporation,

1630 McCarthy Blvd.,

Milpitas, CA 95035-7487

TEL: (408) 432 1900

Linear Technology (UK) Ltd.,TEL:(01276) 677676

Linear Technology KKTokyo, 102 JAPANTEL: 81-3-3237-7891

Appendix B

CCFL Inverter Transformer andInductor Manufacturers

Coiltronics Inc.,TEL: (407) 241-7876(Transformers and inductors)Represented by METL in the UKTEL: 01844-278781

Sumida Electric Co., Ltd.Tokyo 125 JAPANTEL: 03-3607-5111(Inductors)Represented by ACAL Electronics Ltd.,inthe UKTEL: 0344-727272

Sumida Electric (USA) Co., LtdTEL: (708) 956-0666(Transformers and Inductors)

CoilcraftTEL: (708) 639-6400(Inductors)

Coilcraft (UK)TEL: 0181-301-3553

Newport Components Ltd.,TEL: 01908-615232(Inductors)

Pico Electronics Inc.,NY 10552TEL: (914) 699-5514(Inductors)Represented by Ginsbury ElectronicsLtd., in the UKTEL: 01634-290040

AN14-11AN14-10

Figure 12.Package Options.

Application Note 14Issue 2 March 1996

Application Note 14Issue 2 March 1996

Package Options

ZETEX can offer a range of packages toallow complete circuit size and layoutoptimisation. Figure 12 illustrates these,from the TO92 compatible E-Linethrough-hole package, to surface mountoptions SOT23, SOT223, and SM-8.

The SM-8 is a dual island, eight leadedpackage that possesses the same bodydimensions as the industry standardSOT223. These attributes allow it toreplace the two Royer Convertertransistors with a single package twochip device, yielding a significant costand space saving.

For example, the 1048A transistor isavailable as an uncommitted dual withinthe SM8 package as the ZDT1048.

Conclusions

The advanced transistor geometries,and optimised processing employed byZETEX leads to a range of transistorsthat are ideally suited to the LCDbacklighting inverter application.Attention has been applied to specifyinga range of devices relevant to, andexhibiting a superior performancewithin the Royer inverter topology.

References

Transistors as On-Off Switches inSaturable Core CircuitsBright, Pittman and Royer.Westinghouse Electric Corp.,Electrical Manufacturing Dec 1954.

Techniques for 92% Efficient LCDIlluminationApplications Note 55 August 1993Jim WilliamsLinear Technology Corp.,

A Fourth Generation of LCD BacklightTechnology - Component andMeasurement Improvements RefinePerformance Application Note 65October 1995Jim WilliamsLinear Technology Corp.

Switching and Linear Power Supply,Power Converter DesignA. PressmanHayden Press.

Appendix A

LT1070, 1170 Series Switching

Regulators

LT1182, 1183 CCFL/LCD Contrast Dual

Switching Regulator

Linear Technology Corporation,

1630 McCarthy Blvd.,

Milpitas, CA 95035-7487

TEL: (408) 432 1900

Linear Technology (UK) Ltd.,TEL:(01276) 677676

Linear Technology KKTokyo, 102 JAPANTEL: 81-3-3237-7891

Appendix B

CCFL Inverter Transformer andInductor Manufacturers

Coiltronics Inc.,TEL: (407) 241-7876(Transformers and inductors)Represented by METL in the UKTEL: 01844-278781

Sumida Electric Co., Ltd.Tokyo 125 JAPANTEL: 03-3607-5111(Inductors)Represented by ACAL Electronics Ltd.,inthe UKTEL: 0344-727272

Sumida Electric (USA) Co., LtdTEL: (708) 956-0666(Transformers and Inductors)

CoilcraftTEL: (708) 639-6400(Inductors)

Coilcraft (UK)TEL: 0181-301-3553

Newport Components Ltd.,TEL: 01908-615232(Inductors)

Pico Electronics Inc.,NY 10552TEL: (914) 699-5514(Inductors)Represented by Ginsbury ElectronicsLtd., in the UKTEL: 01634-290040

AN14-11AN14-10

Figure 12.Package Options.

Application Note 14Issue 2 March 1996

AN14-12

Appendix C

ZETEX Royer Converter Transistors

Device

BVCEV

*

V

BVCES

/

BVCBO

V

BVEBO

V

IC

(DC)

A

hFE

@

IC / V

ce

A / V

VCE(sat)

V

@

IC / I

B

A / A

Package SurfaceMountOption

ZTX849 _ 80 6 5 100 - 300 1 / 1 25mV typ

50mV Max

0.5 / 0.02 E-Line FZT849(SOT223)

ZTX869 _ 60 6 5 300 min 1 / 1 20mV typ

50mV Max

0.5 / 0.01 E-Line FZT869(SOT223)

ZTX689B _ 50(typ) 5 3 450 min 1 / 2 60mV typ 0.5 / 0.005 E-Line FZT689B(SOT223)

FMMT619

(SuperSOT)

_ 50 5 2 200 min 1 / 2 55mV typ

125mV typ

200mV Max

0.5 / 0.01

1.0 / 0.01

SOT23 -

ZTX1048A 50 50 5 4 300 -

1200

1 / 2 24mV typ

45mV Max

0.5 / 0.02 E-Line ZDT1048(SM-8)

ZTX1049A 80 80 5 4 300 -

1200

1 / 2 35mV typ

60mV Max

0.5 / 0.02 E-Line ZDT1049(SM-8)

* If specified. For those devices that dont include a BVCEV test, the actual value willbe close to the BVCES/BVCBO figure - please refer to text.