AN2654 Datasheet, PDF (1/26 Pages) STMicroelectronics – CCFL backlight half-bridge topology based on L6574 and STD7NS20

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
TV display screen ^{Backlight power}
size (inches)
(Watts)
Current consumption vs. supply voltage (V_lamp)
12 V
24 V
100 V
17
24 W
2A
1A
0.24 A
19
30 W
2.5 A
1.25 A
0.3 A
27
120 W
-
5A
1.2 A
32
144 W
-
6A
1.44 A
42
264 W
-
11 A
2.64 A
May 2008
Rev 1
1/26
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