UCC1972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
Circuit Operation
A current fed push-pull topology is used to power the
CCFL backlight shown in Fig. 1. This topology accommo-
dates a wide input voltage and dimming range while re-
taining sinusoidal operation of the lamp. The converter
consists of a resonant push-pull stage, a high voltage
output stage, and a buck pre-stage used to regulate cur-
rent in the converter.
Referring to Fig. 1, the push-pull stage consists of C
RES
,
Q1, Q2, R
B
, and T1’s primary and auxiliary windings.
The output stage consists of C
BALLAST
, the lamp, the cur-
rent sense resistor R
S
, and T1’s secondary. The reso-
nant frequency of the tank is set by the primary
inductance of T1, along with the resonant capacitor
(C
RES
), and the reflected secondary impedance. The
secondary impedance includes the lamp, the ballast ca-
pacitor (C
BALLAST
), the distributed winding capacitance
of T1, and the stray capacitance which forms between
the lamp, lamp wires, and the backlight reflector. Since
the lamp impedance is nonlinear with operating current,
the tank resonant frequency will vary slightly with load
(typically 1.5:1).
The resonant tank consisting of C
RES
and T1 produces
sinusoidal currents (I
RES
) and voltages and is fed by a
controlled DC current (I
BUCK
) from the buck stage. Note
that the BUCK node voltage is �½ the primary tank volt-
age, as VBAT is located at the center tap of the trans-
former. The high turns ratio transformer (T1) amplifies
the sinusoidal tank voltage to produce a sinusoidal sec-
ondary voltage that is divided between the lamp and bal-
last capacitor.
Transistors Q1 and Q2 are driven out of phase at 50 per-
cent duty cycle with an auxiliary winding on T1. The
winding provides a floating AC voltage source at the res-
onant frequency that is used to drive the transistor bases
alternately on and off. One leg of the auxiliary winding is
tied to the input voltage through base resistor R
B
, which
is sized to provide sufficient base current to the transis-
tors. The transistors channel the buck inductor current
into opposing ends of the tank at the resonant frequency,
supplying energy for the lamp and system losses.
The buck power stage consists of inductor L
BUCK
,
MOSFET switch S
BUCK
, and flyback diode D
BUCK
. In or-
der to prevent interactions between multiple switching
RES ONANT PUS H-PULL S TAGE
T1 PRIMARY
VBAT
VBAT
R
B
C
RES
I
RES
OUTPUT S TAGE
T1 S ECONDARY
I
LAMP
VBAT FB
R
S
C
BALLAS T
CCFL
T1
AUXILIARY
Q1
Q2
VBAT
D
BUCK
V
BUCK
L
BUCK
I
BUCK
Q1
ON
Q2
ON
Q1
ON
Q2
ON
VBAT
S
BUCK
OUT
VBUCK
GND
BUCK S TAGE
UDG-98157
Figure 1. Push-pull, output, and buck stages.
6
TI [ TEXAS INSTRUMENTS ]
