UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
C
FB
ERROR
AMP LIFIER
PWM
MODULATOR
VBAT
TRANSFORMER
SECONDARY
IMP EDANCE
2 POLE
RLC
FILTER
–
+
2N
S
2
2
R
+X
C
N
LAMP
V
P
SAW
1.5V
R
FB
0.5R
S
Figure 7. Current control loop block diagram.
The resulting gain of the filter is unity below the 15kHz Striking the Lamp
corner frequency, peaking up at the corner frequency
Before the lamp is struck, the lamp presents an imped-
ance much larger than the ballast capacitor and the full
output voltage of the transformer secondary is across the
lamp. Since the buck converter must reverse the
volt-seconds on the buck inductor, the average tank volt-
age at the primary can be no greater than the DC input
voltage. This constraint along with the turns ratio of the
push-pull transformer sets the peak voltage available to
with Q, and rolling off with a 2-pole response above the
corner frequency. As shown in Fig. 7, the transformer
turns ratio provides a voltage gain and the output circuit
(whose impedance includes the lamp and ballast capaci-
tor) converts the voltage into a current. The current
sense resistor produces a voltage on each half cycle,
leaving the error amplifier as the final gain block.
Loop gain is greatest at minimum lamp current and maxi- strike the lamp:
mum input voltage. With a 22V input, a 2V saw-tooth,
VSTRIKE = N S:P ·p·VINPUT
(15)
and 1:67 turns transformer, the low frequency voltage
gain of the PWM, RLC filter, and Transformer is 1500.
With a 375V lamp and 1mA of lamp current (using a
22pF ballast capacitor and 50kHz switching frequency)
the secondary impedance is 400kW. RSENSE at 1mA is
4kW (equation 12), resulting in a low frequency power
loop gain of 7.5. The error amplifier is configured as an
integrator, giving a single pole roll-off and a high gain at
DC. A 68k resistor and 33nF capacitor give a 70Hz
crossover frequency for the feedback network, yielding a
maximum crossover frequency of 500Hz for the total loop
avoiding stability problems with the Q of the resonant
tank. For 5mA of lamp current with a 22V input the total
loop crossover is 200Hz, for low frequency dimming ap-
plications CFB can be reduced to 6.8nF with no instability
(1kHz crossover).
The Coiltronics transformer has a 67:1 turns ratio, giving
2100 peak volts available to strike the lamp with the mini-
mum 10V input. In our example this is more than suffi-
cient for the 1000V required to strike the lamp. With the
22V maximum charger input, the available striking volt-
age could theoretically reach 5000V! The possibility of
breaking down the transformer’s secondary insulation
becomes a real concern at this voltage.
Voltage Clamp Circuit (UCC3972)
An external voltage clamp circuit consisting of D4, Q4,
R7, R8, and R9 can be added to the typical application
circuit as shown in Fig. 8. This circuit limits the maximum
transformer voltage during startup, allowing an extended
time period for striking the lamp while protecting the
transformer from over voltage. For fixed input voltage de-
signs, this circuit is optional since the transformer turns
can be optimized at one voltage.
12
TI [ TEXAS INSTRUMENTS ]
