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