CCFL Backlight

1
CCFL Backlight
Half-bridge solution with L6574

• Jerry Shen
• Senior Application Engineer
• Jerry.shen_at_st.com
• Ryan Xie
• Application Engineer
• Ryan.xie_at_st.com

MLD System Application Group Shanghai Lab
2
Preface

• 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.

• 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.

• A high performance half-bridge solution with ST
  control IC L6574 will be introduced.

3
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

4
CCFL Overview
Characteristics of CCFL

• 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

Ignition
CCFL Voltage VS. Current
5
CCFL Overview
Brightness
Current
Frequency
Temperature
Lamp Size
Load is LCD panel
6
Backlighting System Demands
Electrical Efficiency Photometric Efficiency
MOSFET Drive IC Transformer
Open Lamp Over Voltage Over Circuit
Low Power Consumption Controlled by MCU
Below 1 for grounded type Below 6 for float type
Analog 50-100 Digital (PWM) 10-100
7
Topology Comparison
8
Function Comparison
Existing Models for 17" LCD Monitor (4Lamps)
9
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

10
LCD Block Diagram
MOSFET 600V/ 10 11A
MOSFET PFC DIODE 23 32
600V/15A 600V/20A Over 42 600V/30A x2
Schottky Rectifier 60V Series 3A 150V Series
15A 20A 100V Series 20A
MOSFET 200V / 37A
Less Power Loss in Rectifier MOSFET
Output Rectifier 1
Main Switching Block
PFC Block
Bridge Rectifier
Half Bridge
CCFL Loads
100v
INPUT VOLTAGE
Output Rectifier 2
(85 265 VAC)
PFC Control IC
Stand-by Output
Control IC
L6574
Feedback Block
CCFL Backlighting
Open Frame Power Supply
Backlighting Control IC
12V For Audio AMP
Main Controller Resonant or Fly back Converter

PFC Control IC
5V For Micom Others
With BCD technology, the Half-bridge input
voltage can be raised up to 600V
11
L6574 Dedicated IC for Lighting

• Feature
• High voltage rail up to 600v
• Driver current capability source 250 mA sink
  450 mA
• Under voltage lockout
• Preheat and frequency shift timing
• Integrated bootstrap diode
• Integrated Sense op amp for close loop or
  protection

• Application
• LCD monitor
• LCD TV

12
ST MOSFET for Half Bridge
() Used on Demo board
13
Schematic for 4 lamps CCFL Application
14
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

15
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

16
Function DisplayStandby function
Vcc
En
En on
TTL Voltage on-off Control
When the system turns to standby mode, the IC
stops working, reducing the power dissipation to
minimum level
Vcc
En
En off
17
Function DisplaySoft Start

• The soft start function is realized by the
  frequency shift after the ignition.
• Very simple and few external components are
  required.

18
Function DisplayAnalog Dimming

• The integrated error amplifier composes a
  current control loop and at the mean time
  provides an easy way to control the brightness.

In Grounded Lamp configuration, Analog
dimming range is limited by the thermometer
effect
19
Function DisplayDigital Dimming

• Wide range of brightness adjustment. (10-100)
• Soft Resume in each PWM cycle.

20
Function DisplayDigital Dimming
Feedback Compensation
Oscillation Interrupter
MOSFET Controller
21
Function DisplayDigital Dimming
20 duty circle
50 duty circle
80 duty circle
22
Function DisplayProtection

• Simple and reliable, individually monitoring
  each lamp.

• Appropriate time delay ensures proper working
  under low digital dimming condition.

Connecting Lamps
Over voltage Detector
23
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

24
Efficiency
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.
25
Efficiency
4 lamps 17? LCD Monitor efficiency measurement
Input Voltage110V, T ambient25C
The Electrical EfficiencyPout/Pin 86
26
Efficiency
Major Component Temperature Rise
Input Power 18W, T ambient25C
27
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

28
Conclusion

• Complying with all the performance demands, L6574
  provides a 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 with specific panel.

29
Appendix
Preheating of L6574

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

30
Appendix
Preheating of L6574
More current, higher frequency
31
Appendix
Preheating of L6574

• At the end of preheating time (tTpre),

• At the end of frequency shifting (tTpreTsh),

Thus, the Tsh can be expressed with
following equation
32
Appendix
Preheating of L6574
Summary

• Setting the time in preheating phase

• Setting the frequency in preheating phase

33
Appendix
400V Input ?

• An Attractive Proposal

Possible ?
AC Source
PFC
DC/DC
CCFL Backlight
Audio Amplifier
Micro Controller

• Practical Consideration - Transformer
  (winding, leakage inductanceetc) - Ground
  Isolation (feedback, dimmingetc) - Lamp
  Connection (grounded or floating,
  protectionetc) - Extra Cost (photo
  coupler, dc blocking capacitoretc)

34
Thank You