1. EXPERT SYSTEMS AND SOLUTIONS
Email: expertsyssol@gmail.com
expertsyssol@yahoo.com
Cell: 9952749533
www.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 the
projects.

2. CCFL Backlight
Half-bridge solution with L6574
Jerry Shen
Senior Application Engineer
Jerry.shen@st.com
Ryan Xie
Application Engineer
Ryan.xie@st.com
MLD System & Application Group Shanghai Lab

3. 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.

4. 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

5. 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 Ignition
 Several mA
 High frequency
 30-70kHz
CCFL Voltage VS. Current

6. CCFL Overview
Brightness
Current Temperature Frequency Lamp Size
Load is LCD panel

7. Backlighting System Demands
MOSFET
Electrical Efficiency
Drive IC
Photometric Efficiency
Transformer
Efficiency Cost
Open Lamp Low Power
Standby
Over Voltage Protection Consumption
Control
Over Circuit Controlled by MCU
Line
Dimming
Regulation
Below 1% for grounded type Analog 50%-100%
Below 6% for float type Digital (PWM) 10%-100%

8. Topology Comparison
Architecture Advantages Disadvantages
Royer+Buck Least expensive (due Can’t control frequency tightly(self-
to bipolar cost) oscillation)
Complex transformer structure
Low efficiency due to 2 power stages
Full bridge Simple transformer Requires 4 MOSFETs
structure May require p-channel MOSFETs,
High efficiency which are higher cost
Expensive control IC
Half bridge Requires only 2 n- Not easy to drive high-side MOSFETs
channel MOSFETs in parallel for Multi-lamps load
Higher turn ratio may be required
Push-pull Requires only 2 n- The voltage of MOS must be twice of
channel MOS input DC voltage Vin
Low transformer Low efficiency when the dc supply goes
turns ratio beyond a 2-to-1 range

9. Function Comparison
Existing Models for 17" LCD Monitor (4Lamps)
Function L6574 TSM108 FAN7548 FAN7310 OZ960 OZ9RR
Topology Half - Buck +Royer Buck +Royer Full-bridge Full- Push-pull
Bridge push-pull push-pull bridge
Operating voltage 10V—16V 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

10. 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

11. LCD Block Diagram
MOSFET PFC DIODE MOSFET Schottky Rectifier MOSFET
23” ~ 32” 600V/15A 600V/ 10 ~ 11A 60V Series 3A 200V / 3~7A
: 600V/20A 150V Series 15A~ 20A
Over 42” 100V Series 20A
: 600V/30A x2
Output
INPUT Main
Bridge PFC Rectifier 1 Half CCFL
VOLTAGE Switching
Rectifier Block Bridge Loads
Block Output
(85 ~ 265 VAC)
Rectifier 2
PFC Stand-by
Control IC Control IC Output L6574
Feedback
Open Frame Power Supply Block CCFL Backlighting
PFC Control IC Main Controller +12V +5V Backlighting
Resonant or Control IC
Fly back For Audio AMP For Micom
Converter & Others

12. 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

13. ST MOSFET for Half Bridge
Products PKG Bvdss Id(cont) R ds(on) V gs(th)
(V) (A) (Ω) (V)
@10v 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 on Demo board

14. 100v Input F1
2
1
Main Structure
1
C2 C3
C4
Lamp2
2
C14
12
16
D
Lamp1
Vcc
Vbo
Q6 R11 Q1
2 15 G C15 OLP1 FB
Rpre HVG
12V Vcc OLP2
R1 7 C11 T1 C12
OPin+ R12
S
14 6 8
OUT
A
6 D5 D6
OPin- IC OV
D1 C5
D
R7 R5 L6574 R18 R19
5 DDim2 C13 D4
R2 OPout R13 Q2
11 G
K
LVG
D2 1 7
Q7
R14 D3 Lamp4
S
R3 8 C18
R6 EN1
EN
9 LP
FB EN2
C1 R4 4
Rig Lamp3
TapR1 C19
Cpre
Gnd
Cf
C10 OLP3 FB
R8 R15
OLP4
R17 C16
3
1
10
T2
R10 6 8
R16
C9 D8 D9
R9 C7 OV
DDim1
C8
C17 R20 R21
D7
1 7
FB DDim1 DDim2
D14
Vcc R30 R32
Open Lamp Protection R28
Digital Dimming
R29
OLP2 Q4 R31 Q5
D10 R22 R23 DDC
OV
OLP1 R27
C23
D12 D13
R26
LP
Q3
OLP3
D11
R25
C20 C21 C22
OLP4
R24
Schematic for 4 lamps CCFL Application

15. 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

16. Function Display
Steady 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

17. Function Display
Standby function
Q6
12V Vcc
R1
Vcc R2
Q7
En R3
En on EN
C1 R4
TTL Voltage on-off Control
Vcc When the system turns to standby mode,
the IC stops working,
En
reducing the power dissipation to minimum level
En off

18. Function Display
Soft Start
 The soft start function is realized by the frequency shift after the ignition.
 Very simple and few external components are required.

19. Function Display
Analog Dimming
Vbus
 The integrated error amplifier
L6574
Vref +
-
Q1
Cdc Cb
composes a current control loop and
Vcom at the mean time provides an easy way
Oscillator
Q2 Cp CCFL to control the brightness.
Rsense
Full high voltage
* In “Grounded Lamp configuration”,
C parasitic C parasitic
Analog dimming range is limited by the C R
“thermometer effect” C parasitic C parasitic

20. Function Display
Digital Dimming
 Wide range of brightness adjustment. (10%-100%)
 “Soft Resume” in each PWM cycle.

21. Function Display
Digital Dimming
FB DDim1 DDim2
D14
R30 R32
R28
R29
Q4 R31 Q5
DDC
C23 R27
Feedback Oscillation MOSFET
Compensation Interrupter Controller

22. Function Display
Digital Dimming
20% duty circle
50% duty circle
80% duty circle

23. Function Display
Protection
 Simple and reliable,
individually monitoring each
lamp.
 Appropriate time delay
ensures proper working under
low digital dimming condition.
Connecting Over voltage
Lamps Detector

24. 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

25. Efficiency
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.

26. Efficiency
4 lamps 17΄ LCD Monitor efficiency measurement
I lamp V lamp Power
Load
(mA) (v) (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
Total Output Power 15.48
110v Input Power 18.0
* Input Voltage=110V, T ambient=25°C
The Electrical Efficiency=Pout/Pin= 86%

27. Efficiency
Major Component Temperature Rise
After 10 mins After 20 mins After 30 mins
(ºC) (ºC) (ºC)
3
MOSFET 38 38
L6574 31 32 32
Transformer 38 39 40
* Input Power = 18W, T ambient=25°C

28. 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

29. 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.

30. Appendix
Preheating of L6574
 The preheat time (Tpri) and frequency shift time (Tsh) are all set by Cpre.
T pre = 1.5 s / µF × C pre
Vth
T pre = C pre × , where Vth = 3.5V , I pre = 2.3 µA
I pri

31. Appendix
Preheating of L6574
More current,
higher frequency
g m I fs
I osc = I min +( I max − ( t −T pre ))
C pre
Vref Vref
I min = , I max = , where V ref = 2V
Rign R pre

32. Appendix
Preheating of L6574
 At the end of preheating time (t=Tpre),
I osc = I min + I max
 At the end of frequency shifting (t=Tpre+Tsh),
I osc = I min
Thus, the Tsh can be expressed with following equation
C pre ×I max
Tsh =
g m ×I fs
Set I fs = kI max ,
Tsh = K fs C pre , typically K fs = 0.15 s / µF

33. Appendix
Preheating of L6574
Summary
 Setting the time in preheating phase
T pre = K pre × C pre = 1.5 s / µF × C pre
Tsh = K fs × C pre = 0.15 s / µF × C pre
 Setting the frequency in preheating phase
1.41
f min =
R ign × C f
1.41 × ( R ign + R rpe )
f max =
R ign × R pre × C f

34. Appendix
400V Input ?
 An Attractive Proposal
Possible ?
CCFL
AC Source PFC DC/DC Backlight
Audio
Amplifier
Micro
Controller
 Practical Consideration
- Transformer (winding, leakage inductance…etc)
- Ground Isolation (feedback, dimming…etc)
- Lamp Connection (grounded or floating, protection…etc)
- Extra Cost (photo coupler, dc blocking capacitor…etc)

35. Thank You