Datasheet
17
UCC1972/3
UCC2972/3
UCC3972/3
In a practical backlight design, the physical spacing be
-
tween the lamp and high voltage secondary wiring with
respect to the foil reflector and LCD frame can be tight.
With this tight spacing, distributed stray capacitance will
form as shown in Fig. 17. The stray capacitance causes
leakage currents from the high voltage secondary to cir
-
cuit ground. Although the current through stray capaci
-
tance doesn’t directly translate into losses, the extra
current through the transformer, primary resonant tank,
and switching devices does. A poor layout with excessive
stray capacitance can reduce system efficiency by tens
of percent. High frequency harmonics in the secondary
voltage waveform impact efficiency even further, since
capacitive reactance decreases as frequency increases.
This is why a pure sinusoid gives the best electrical to
optical efficiency, minimizing harmonic losses. Sinusoidal
waveforms require more circulating current in the reso
-
nant tank, however, lowering the electrical efficiency of
the converter.
The trade-off of electrical and optical efficiencies must be
optimized to achieve the best design. System electrical
efficiencies of 75-85% are easily achievable in a typical
UCC3972/3 based design while still maintaining good op
-
tical conversion. Efficiencies will vary with external com
-
ponent selection, input voltage, and lamp power. Fig. 18
and 19 show system electrical efficiencies versus input
voltage and output power for the 375V lamp design.
APPLICATION INFORMATION (cont.)
C
BALLAST
SECONDARY
L
A
M
P
C
STRAY
C
STRAY
C
STRAY
C
STRAY
C
STRAY
LAMP POWER 100%
POSITIVE COLUMNS 65%
ELECTRODE
LOSS 35%
UV RADIATION
36%
HEAT LOSS
26%
HEAT LOSS 85%
VISIBLE LIGHT
15%
Hg VISIBLE
LIGHT 3%
Figure 17. Lamp and stray capacitor losses.
50
60
70
80
90
0 5 10 15 20 25 30
INPUT VOLTAGE
EFFICIENCY
Figure 18. Design example efficiency vs. input voltage at
2W.
50
55
60
65
70
75
80
85
0 500 1000 1500 2000 2500
POWER OUT IN MILLIWATTS
EFFICIENCY
Figure 19. Design efficiency vs. output power.
UDG-98165