UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
Circuit Operation
The resonant tank consisting of CRES and T1 produces
sinusoidal currents (IRES) and voltages and is fed by a
controlled DC current (IBUCK) 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.
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 CRES
,
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 RB, 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.
Q1, Q2, RB, and T1’s primary and auxiliary windings.
The output stage consists of CBALLAST, the lamp, the cur-
rent sense resistor RS, and T1’s secondary. The reso-
nant frequency of the tank is set by the primary
inductance of T1, along with the resonant capacitor
(CRES), and the reflected secondary impedance. The
secondary impedance includes the lamp, the ballast ca-
pacitor (CBALLAST), 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 buck power stage consists of inductor LBUCK
,
the tank resonant frequency will vary slightly with load MOSFET switch SBUCK, and flyback diode DBUCK. In or-
(typically 1.5:1).
der to prevent interactions between multiple switching
RESONANT PUSH-PULL STAGE
T1 PRIMARY
VBAT
OUTPUT STAGE
C
BALLAST
T1 SECONDARY
CCFL
I
VBAT
LAMP
C
RES
VBAT
FB
R
I
B
RES
T1
AUXILIARY
R
S
Q2
Q1
V
BUCK
VBAT
L
BUCK
VBAT
D
BUCK
I
BUCK
Q1
ON
Q2
ON
Q1
ON
Q2
ON
S
VBUCK
GND
BUCK
OUT
BUCK STAGE
UDG-98157
Figure 1. Push-pull, output, and buck stages.
6
TI [ TEXAS INSTRUMENTS ]
