AN2654
Application note
CCFL backlight half-bridge topology based on L6574 and
STD7NS20
Introduction
Cold cathode fluorescent lamps (CCFL) are widely used in the backlighting of television and
PC monitor applications due to their low cost and high efficiency. A 12 V or 24 V DC voltage
is provided for the backlighting inverter to drive the CCFL. In order to improve the efficiency
and performance of the total system both for the AC-DC power supply and inverter,
STMicroelectronics has introduced a high-voltage ballast-driver IC L6574 that provides the
half-bridge solution for CCFL backlighting. Thanks to BCD
™
offline technology, the L6574
can handle high-side voltage rail up to 600 V and consumes less current. Of course, the
power loss of the bridge rectifier is reduced.
Figure 1
shows the typical LCD TV power section block diagram. Different supply voltages
for backlight applications are available on the market.
Table 1
shows the required backlight
power for various sizes of TV display screens. For the same screen size, there are three
different types of supply voltages for CCFL backlight and each consumes a different current.
Using a high-voltage backlight CCFL solution allows greatly reducing the power losses of
the bridge rectifier and the conduction loss of the primary switch.
Figure 1.
Typical LCD TV power section block diagram
Table 1.
Required backlight power and its current consumption vs. supply voltage
Backlight power
(Watts)
24 W
30 W
120 W
144 W
264 W
Current consumption vs. supply voltage (V
lamp
)
12 V
2A
2.5 A
-
-
-
24 V
1A
1.25 A
5A
6A
11 A
100 V
0.24 A
0.3 A
1.2 A
1.44 A
2.64 A
TV display screen
size (inches)
17
19
27
32
42
May 2008
Rev 1
1/26
www.st.com
Contents
AN2654
Contents
1
Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1
1.2
1.3
Reference design board and PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2
Electrical function of the design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1
2.2
2.3
2.4
Enable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4.1
2.4.2
Analog dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Digital dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.5
Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.5.1
2.5.2
2.5.3
Open lamp protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3
Description of the half-bridge driver . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1
3.2
3.3
3.4
3.5
Functions and block diagram of the IC . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
IC pin description in this reference design . . . . . . . . . . . . . . . . . . . . . . . . 18
HVG driver, LVG driver and oscillator frequency . . . . . . . . . . . . . . . . . . . 18
Timing capacitor, oscillator frequency and equations . . . . . . . . . . . . . . . . 19
Figures for F
min
, R
min
and F
start
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Appendix A Resonant tank design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2/26
AN2654
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Typical LCD TV power section block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
STEVAL-ILC001V1 evaluation board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
PCB top-side view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
PCB bottom-side view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Resonant topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Vgs and Id of MOSFET (Q2) in ZVS mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Lamp current control loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Lamp current during zero-voltage switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Vcom and lamp current in soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
100% of full brightness for analog dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
60% of full brightness for analog dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Digital dimming control circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Vgs of low-side MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Vcom waveform in digital dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Lamp current at 20% of full brightness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Lamp current at 80% of full brightness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Open lamp protection circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Open lamp protection time delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Overvoltage detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Block diagram of L6574 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Oscillation frequency vs. high side/low side driving signal . . . . . . . . . . . . . . . . . . . . . . . . . 19
Timing capacitor and oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
F
min
vs Rf
min
at C
f
= 470 pF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
(F
start
- F
min
) vs Rf
start
at C
f
= 470 pF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
(F
start
- F
min
) vs Rfst at C
f
= 470 pF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
(F
start
- F
min
) vs Rf
st
at C
f
= 470 pF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Fmin at different Rfstart vs Cf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Schematic diagram of half-bridge backlight inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Equivalent model of resonant tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Simplified model of resonant tank. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3/26
Board description
AN2654
1
Board description
High-voltage half-bridge zero-voltage switching topology has been selected. In order to
achieve the best performance, the iteration bench work is nearly inevitable, especially
driving with multi-lamps. However, the procedure of transformer design and resonant tank
equations are described in
Appendix A
which provides the designer with a feasible
approach to start the design work.
1.1
Reference design board and PCB layout
Figure 2.
STEVAL-ILC001V1 evaluation board
Figure 3.
PCB top-side view
4/26
AN2654
Figure 4.
PCB bottom-side view
Board description
1.2
Schematic
This CCFL backlighting solution is designed for 17" LCD monitors. This development design
board is able to drive 4 tubes of CCFL. During normal conditions, the L6574 provides a
current loop that is controlled by varying the switching frequency. The input DC bus voltage
has been lifted to 110 V. This reference design provides an open lamp protection,
overvoltage protection, soft-start time setting, and analog and digital dimming functions.
Figure 5
shows the application schematic and
Table 2
gives the bill of materials.
5/26
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