ccfl design project

8/8/2019 CCFL design Project

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EXPERT SYSTEMS AND SOLUTIONS

Email: [email protected]

[email protected]

Cell: 9952749533www.researchprojects.info

PAIYANOOR, OMR, CHENNAI

Call For Research Projects Final

year students of B.E in EEE, ECE, EI,

M.E (Power Systems), M.E (Applied

Electronics), M.E (Power Electronics)

Ph.D Electrical and Electronics.

Students can assemble their hardware in our

Research labs. Experts will be guiding theprojects.


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CCFL Backlight

Jerry ShenSenior Application Engineer

[email protected]

Ryan XieApplication Engineer

[email protected]

Half-bridge solution with L6574

MLD System & Application Group Shanghai Lab


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Preface

To meet the explosive growth demand for the LCD computer monitors

and LCD televisions, with board range product portfolio, ST provides

various solutions to CCFL backlighting.

CCFL (Cold Cathode Fluorescent Lamp), which offers high brightness

and high electrical-to-light efficiency, is currently the best light source

for the modern LCD module.

A high performance half-bridge solution with ST control IC L6574will be introduced.


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Contents

CCFL backlighting overview ST Solution

L6574 Half Bridge drive IC

MOSFET choice in ST

Function Display Standby

Soft start

Dimming ( Analog & Digital )

Protection

Efficiency

Conclusion


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CCFL Overview

Low filament temperature

No preheat required

High voltage

Typ 17 Striking V ~ 1000v Typ 17 Running V ~500v

Low current

Several mA

High frequency

30-

70kHz

CCFL Voltage VS. Current

Characteristics ofCCFL

Ignition


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CCFL Overview

Brightness

Current FrequencyTemperature Lamp Size

Load is LCD panel


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Backlighting System Demands

Cost

Standby

Control

DimmingLine

Regulation

Protection

Efficiency

Open Lamp

Over Voltage

Over Circuit

Below 1% for grounded type

Below 6% for float type

Analog 50%-100%

Digital (PWM) 10%-100%

Electrical Efficiency

Photometric Efficiency

Low Power

Consumption

Controlled by MCU

MOSFETDrive IC

Transformer


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Topology Comparison

ArchitectureArchitecture AdvantagesAdvantagesD

isadvantagesD

isadvantages

Royer+Buck Least expensive (due

to bipolar cost)

Cant control frequency tightly(self-

oscillation)

Complex transformer structure

Low efficiency due to 2 power stages

Full bridge Simple transformer

structure

High efficiency

Requires 4MOSFETs

May require p-channel MOSFETs,

which are higher cost

Expensive control IC

Half bridge Requires only 2 n-

channel MOSFETs

Not easy to drive high-side MOSFETs in

parallel for Multi-lamps load

Higher turn ratio may be required

Push-pull Requires only 2 n-

channel MOS

Low transformer

turns ratio

The voltage ofMOS must be twice of

input DC voltage Vin

Low efficiency when the dc supply goes

beyond a 2-to-1 range


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Function Comparison

Function L6574 TSM108 FAN7548 FAN7310 OZ960 OZ9RR

Topology Half -

Bridge

Buck +Royer

push-pull

Buck +Royer

push-pull

Full-bridge Full-

bridge

Push-pull

Operating voltage 10V16V 8V--60V 6V--30V 4.5V--26V 4.7V--7V 4.7V--7V

Switch Action (DC-AC) ZVS ZVS ZVS ZVS ZVS/ZCS ZVS

Soft-start Yes Yes Yes Yes Yes Yes

Analog/Burst dimming Both Both Both Both Burst only Both

Open lamp protection Yes Yes Yes Yes Yes Yes

Line Regulation Yes Yes Yes Yes Yes Yes

Over Voltage protection Yes Yes Yes Yes Yes

ExistingM

odels for 17"

LCD

M

onitor (4L

amps)


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Contents



MOSFET choice in ST


Soft start


Protection

Efficiency Conclusion


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LCD BlockDiagram

OutputRectifier 1

Feedback

Block

PFCControl IC Control IC

MainSwitching

Block

PFCBlock

BridgeRectifier

OutputRectifier 2

MOSFET

600V/ 10 ~ 11A

MOSFET PFC DI ODE

23 ~ 32 600V/15A: 600V/20AOver 42: 600V/30A x2

Schottky Rectifier

60V Series 3A150V Series 15A~ 20A100V Series 20A

+12V

For Audio AMP

Main Controller

Resonant or

Fly back

Converter

PFC Control IC +5V

ForMicom

& Others

Stand-byOutput

INPUTVOLTAGE

(85 ~ 265 VAC)

L6574

HalfBridge

CCFLLoads

CCFL BacklightingOpen Frame Power Supply

Backlighting

Control IC

MOSFET

200V / 3~7A


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L6574Dedicated IC for Lighting

Feature:Feature: High voltage rail up toHigh voltage rail up to 600v600v

Driver current capabilityDriver current capability source 250mA sink 450mAsource 250mA sink 450mA

Under voltage lockoutUnder voltage lockout

Preheat and frequency shift timingPreheat and frequency shift timing

Integrated bootstrap diodeIntegrated bootstrap diode

Integrated Sense op amp for close loop orprotectionIntegrated Sense op amp for close loop orprotection

Application:Application: LCD monitorLCD monitor

LCD TVLCD TV


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STM

OSF

ET for Half BridgeProducts PKG Bvdss

(V)

Id(cont)

(A)

R ds(on)

()

@10v

Vgs(th)

(V)

Typ

STD7NS20T4 (*) DPAK 200 7 0.4 3

STD7NS20-1 IPAK 200 7 0.4 3

STD4NS25T4 DPAK 250 4 0.9 3

STD4NS25-1 IPAK 250 4 1.5 3

STP6NB25 TO-220 250 5 0.9 4

(*) Used onD

emo board


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Schematic for 4 lamps CCFL Application


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Contents



MOSFET choice in ST


Soft start


Protection



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Function DisplaySteady State

L6574 deploys a ZVS operation of

both MOSFETs, minimizing the power

loss in switching.

Lamp current loop control gives

accurate current regulation towards

variable input voltage


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12VQ6

Q7

2

R3

EN

Vcc

R1

R4 1

Function DisplayStandby function

TTL Voltage on-off Control

En on

Vcc

En

Vcc

En

En off

When the system turns to standby mode,

the IC stops working,

reducing the power dissipation to minimum level


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Function DisplaySoft Start

The soft start function is realized by the frequency shift after the ignition.

Very simple and few external components are required.


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* In Grounded Lamp configuration,Analog dimming range is limited by the

thermometer effect

C R

C parasitic

C parasitic C parasitic

C parasiticull high voltage

Function DisplayAnalog Dimming

Q2

Q1

Cdc

Cp

Cb

CCFL

Rsense

-+

Oscillator

Vref

Vcom

L6574

Vbus

The integrated error amplifier

composes a current control loop and

at the mean time provides an easy way

to control the brightness.


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Function DisplayDigital Dimming

Wide range of brightness adjustment. (10%-100%)

Soft Resume in each PWM cycle.


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Q4

R29

DDim1

DDCQ5

DDim2

R31

R32

FB

R30

C23 R27

R28

D14

Feedback

Compensation

Oscillation

Interrupter

MOSFET

Controller


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20% duty circle

50% duty circle

80% duty circle


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Function DisplayProtection

Connecting

Lamps

Simple and reliable,

individually monitoring each

lamp.

Over voltage

Detector

Appropriate time delay

ensures proper working under

low digital dimming condition.


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Contents



MOSFET choice in ST

Function Display

Standby

Soft start


Protection



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panel

The electrical parameters test The optical parameters test

Note: If the input capacitor of the Voltage Probe is around 10pF, you can not trust

the voltage value you tested.

Efficiency


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* Input Voltage=110V, T ambient=25C

LoadIlamp

(mA)

V lamp

(v)

Power

(W)

Lamp 1 6.5 600 3.9

Lamp 2 6.6 600 3.96

Lamp 3 6.3 600 3.78

Lamp 4 6.4 600 3.84

TotalOutput Power 15.48

110v Input Power 18.0

The Electrical Efficiency=Pout/Pin= 86%

Efficiency4 lamps 17 LCDMonitor efficiency measurement


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* Input Power = 18W, T ambient=25C

EfficiencyMajor Component Temperature Rise

After 10 mins

(C)

After 20 mins

(C)

After 30 mins

(C)

MOSFET 37 38 38

L6574 31 32 32

Transformer 38 39 40


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Contents



MOSFET choice in ST

Function Display

Standby

Soft start


Protection



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Conclusion

Complying with all the performance demands, L6574 providesa valuable solution to CCFL backlight.

Standby

Soft start

Analog & Digital dimming

Open lamp, over voltage, short circuit protection

Above 85% efficiency

The load should always be the display panel not single tube, all

the parameters including frequency should be tuned withspecific panel.


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Preheating ofL6574

Appendix

A3.2I,V5.3Vwhere,I

VCT

preth

pri

th

prepreQ!!v!

prepre /s5.1 v! Q

The preheat time (Tpri) and frequency shift time (Tsh) are all set by Cpre.


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Preheating ofL6574

Appendix

V2Vwhere,R

VI,

R

VI ref

pre

ref

max

ign

ref

min !!!

More current,

higher frequency

))t(

Ig

I(II prepre

fsm

maxminosc !


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Preheating ofL6574

Appendix

At the end of preheating time (t=Tpre),

At the end of frequency shifting (t=Tpre+Tsh),

maxminosc III !

minosc II !

Thus, the Tsh can be expressed with following equation

fsm

maxpre

shIg

ICT

v

v!

,kIIet maxfs !

F/s15.0Ktypically,CKTfsprefssh

Q!!


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Preheating ofL6574

Appendix

Setting the time in preheating phase

Setting the frequency in preheating phase

Summary

prepreprepreCF/s5.1CKT v!v! Q

preprefss CF/s15.0CKT v!v! Q

fign

minCR

41.1f

v!

fpreign

rpeign

maxCRR

)RR(41.1f

vv

v!


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400V Input ?

Appendix

Practical Consideration

- Transformer (winding, leakage inductanceetc)

- Ground Isolation (feedback, dimmingetc)

- Lamp Connection (grounded or floating, protectionetc)

- Extra Cost (photo coupler, dc blocking capacitoretc)

An Attractive Proposal

AC Source PFC DC/DCCCFL

Backlight

AudioAmplifier

Micro

Controller

Possible ?


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Thank YouThank You

ccfl design project

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