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CCFL leak current at the LCD Module
(2) By reducing the stray capacitance Cs
How to reduce the leak current
Cs = S/d
It is impossible to eliminate the leak current perfectly. There is
an effect that a stray capacitance decreases the lighting start voltage. Widen the distance "d" between the high voltage wiring / lamp
Therefore, it is recommended to decrease the leak current to the and the conductor portion (reflector, chassis etc.).
possible level in consideration of matching with Inverter. For example, a non-conductive reflector should be reviewed.
It is to be noted, however, that the metallic reflector has an
Leak current Is = 2 f CsVL
effect of lowering the lighting start voltage.
(1) By lowering the driving frequency f
If the driving frequency f is made too low, electric discharge (3) By reducing the Lamp Voltage VL
becomes unstable and the luminance is decreased. In consideration (Reducing the lamp impedance)
of the luminance efficiency of the lamp, 50-60 kHz is currently The lamp voltage depends on the CCFL characteristics.
prevalent. As the lamp diameter is smaller, it is necessary to raise As the CCFL is thinner and longer, the lamp voltage becomes
the frequency for stable lighting. higher. As the gas pressure is higher, the impedance tends to be
higher.
[Appendix] How to measure leak current
As a measuring method of the lamp current, please see "Case The measurement point of current is shown.
A" which is recommendable and "Case B" (not recommendable) as
follows.
Case A: recommended Case B: not recommendable
LCD panel LCD Panel
FG FG
Lo Hi Lo Hi
Lamp Lamp
Current probe Current probe
(I LWLo) (I LWHi )
RMS ammeter
(Thermo-couple type) I L (Lo side)
(I LRMS)
Current probe
mA (I LWLo)
I Leak mA
RMS ammeter Inverter current
IL I L (Hi side)
(Thermo-couple type) I L (Hi side)
(Lo side)
(I LeakRMS)
Inverter
GND
Inverter
GND
25
CCFL
Case A
Measure the lamp current and leak current with the RMS
Fig. 27
ammeter (Thermo-couple type). This is because the ammeter
value is more accurate than the calculated value of the
oscilloscope. The oscilloscope is used for checking the lamp
Capacitance component of panel
I Leak = 4.1mA Inverter current = 7.3mA
current waveform (Crest & Imbalance Factor).
(Leak current)
Case B
This is a measuring method in which the difference between high I L (Hi side)
side (IL) and low side (IL) is deemed as leak current {IL(Hi)