DN133 - Low Input Voltage CCFL Power Supply

Low Input Voltage CCFL Power Supply – Design Note 133
Fran Hoffart
Cold cathode fluorescent lamps (CCFLs) are often
used to illuminate liquid crystal displays (LCDs). These
displays appear in laptop computers, gas pumps, automobiles, test equipment, medical equipment and the
like. The lamps themselves are small, relatively efficient
and inexpensive, but they must be driven by specialized power supplies. High AC voltage (significantly
higher than the operating voltage) is needed to start
the lamp. A sinusoidal waveform is desired, the current
must be regulated, efficiency should be high and the
power supply must be self-protecting in the event of
an open-lamp condition.
CCFL power supplies consisting of a Royer class, selfoscillating sine wave converter driven by an LT®1513
switching regulator are shown in Figures 1 and 2. These
circuits are especially suited for low voltage operation,
with guaranteed operation for input voltages as low as
2.7V and as high as 20V. High voltage output regulated
Royer converters, although capable of 90% efficiency,
are not well-suited for low input voltage operation and
C1
2.7V
47μF TO 20V
L1
ELECT
20μH
1
C9
10nF
L2
20μH
5
Q1
4
5
•
IFB
VFB
3
2
GND
4, TAB
D2
15V
D5
R5
10k 1N4148
C7
1μF
R4
20k
C5
4.7nF
Q2
R1
470Ω
LT1513
VC
R7
2k
R6
20k
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
C3
0.082μF
WIMA
D1
3A
1
T1
2
R3
12k
C6
0.1μF
•
6•
3 C4
27pF
3kV
CCFL
PWM
DIMMING
(≈1kHz)
C8
1μF
Two CCFL driver circuits are shown. The first (Figure 1)
drives one end of the lamp, with the other end effectively
grounded. Lamp current is directly sensed at the low
side of the lamp, half-wave rectified by D4, and then
used to develop a feedback voltage across R2. This
voltage, filtered by R3 and C6, drives the VFB pin (2)
to complete the feedback loop. The RMS lamp current
is tightly regulated and is equal to 2.82V/R2.
VSW
VIN
S/S
The LT1513 is a 500kHz current mode switching regulator featuring an internal 3A switch and unique feedback
circuitry. In addition to the Voltage Feedback pin (VFB),
a second feedback node (IFB) provides a simple means
of controlling output current in a flyback or SEPIC
(single-ended primary inductance converter) topology.
C2
4.7μF
CERAMIC
+
7
6
have difficulty operating with input voltages below 5V.
The circuits shown here overcome this limitation while
providing efficiency exceeding 70%.
10
FLUORESCENT LAMP
LAMP CURRENT: 5.6mA
D4
R2
500Ω
D3
1N4148
w2
C2: TOKIN MULTILAYER CERAMIC
C3: MUST BE A LOW LOSS CAPACITOR, WIMA MKP-20 OR EQUIVALENT
L1, L2: COILTRONICS CTX20-4 (MUST BE SEPARATE INDUCTORS)
Q1, Q2: ZETEX ZTX849 OR FZT849
T1: COILTRONICS CTX110605 (67:1)
Figure 1. CCFL Power Supply for Grounded Lamp Configuration Operates on 2.7V
07/96/133_conv
DN133 F01
Because of the high voltage 60kHz lamp drive used
by the CCFL lamps, any stray capacitance from the
lamp and lamp leads to ground will result in unwanted
parasitic current flow, thus lowering efficiency. The
lamp and display housing often have relatively high
stray capacitance, which can dramatically lower the
overall circuit efficiency.
In some displays that exhibit high capacitance, a floating
lamp drive can provide much higher overall efficiency.
The operation of the floating lamp circuit shown in
Figure 2 is similar to that of the grounded lamp, except
for the transformer secondary and the feedback method
used. In this circuit, the lamp current is controlled by
sensing and regulating the Royer input current. This
current is sensed by R2, filtered by R3 and C6 and fed
into the IFB pin (3) of the LT1513, thus completing the
feedback loop. The sense voltage required at the IFB pin
is –100mV. Because the Royer input current rather than
the actual lamp current is regulated, the regulation of
Figure 2 is not as tight as that of Figure 1.
There are three considerations to keep in mind when
laying out a PC board for these supplies. The first is
related to high frequency switcher characteristics. The
500kHz switching frequency allows very small surface
mount components to be used, but it also requires
that PC board traces be kept short (especially the
input capacitor, Schottky diode and LT1513 ground
connections). The second item is the high voltage
C1
2.7V
47μF TO 20V
L1
ELECT
20μH
6
1
C9
1μF
C10
10nF
R7
2k
C8
0.68μF
PWM
DIMMING
(≈1kHz)
D3
1N4148
See Application Notes 49 and 65, Design Note 99 and
the LT1513 Data Sheet for additional information on
driving CCFL lamps.
L2
20μH
Q1
VSW
5
4
•
VC
VFB
R5
4.7k
C7
1μF
IFB
GND
3
R4
4, TAB 20k
D2
15V
D1
3A
C6
0.1μF
C5
4.7nF
Q2
R1
470Ω
LT1513
2
R6
4.7k
The lamp drive is a constant current, and without
protection circuitry, voltages could become very high
in the event of an open-lamp connection, causing
transformer arcing or LT1513 failures. Open-lamp or
high input voltage fault protection is provided by R4,
C5 and the 15V Zener D2, which limit the maximum
voltage available for the Royer converter.
5
VIN
S/S
In both circuits, lamp current can be adjusted downward
to provide lamp dimming. A 5V pulse width modulated
(PWM) 1kHz signal or an adjustable DC voltage can
provide a full range of dimming. In Figure 1, 100% duty
cycle represents minimum lamp brightness, whereas in
Figure 2, 100% duty cycle is maximum lamp brightness.
C2
4.7μF
CERAMIC
+
7
section, which includes T1’s secondary, the ballasting capacitor C3 and the lamp wiring. Lamp starting
voltages can easily exceed 1000V, which can cause a
poorly designed board to arc, resulting in catastrophic
failure. Board leakages can also increase dramatically
with time, resulting in destructive field failures. Third,
surface mount components rely on the PC board copper to conduct the heat away from the components
and dissipate it to the surrounding air. Good thermal
PC board layout practices are necessary.
C3
0.082μF
WIMA
LAMP CURRENT
5.6mA
CCFL
C4
27pF
3kV
10
•6
T1
1
2
•
3
R3
R2
300Ω 0.25Ω
DN133 F02
C2: TOKIN MULTILAYER CERAMIC
C3: MUST BE A LOW LOSS CAPACITOR, WIMA MKP-20 OR EQUIVALENT
L1, L2: COILTRONICS CTX20-4 (MUST BE SEPARATE INDUCTORS)
Q1, Q2: ZETEX ZTX849 OR FZT849
T1: COILTRONICS CTX110605 (67:1)
Figure 2. CCFL Power Supply for Floating Lamp Configuration Operates on 2.7V
Data Sheet Download
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Linear Technology Corporation
For applications help,
call (408) 432-1900
dn133f_conv LT/GP 0796 155K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 1996
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