CCFL or Inverter? - Page 2

 

pardon, it's a voltage regulator chip on the motherboard, perhaps.

 bymyfault wrote:

The inverter will have a "contrast" input, coming from the motherboard.
It should be one of the few control signals feeding the inverter.

On really old inverters, the contrast control is a simple analog voltage.
As the voltage varies, the light output of the CCFL tube varies. This
is terms an "analog control" method. One way to determine an analog
control is used, is that analog control does not give a very large
adjustment range. It might vary the light level from 70% to 100%, but
not offer any lower adjustment (as the CCFL tube would turn off if you
tried a lower value).

*******

The more modern control method, is PWM based (pulse width modulation).
A digital square wave is sent on the cable. It would look like this,
at perhaps 200Hz or so.

        +---+ +---+ +---+               +-+    +-+
        |   | |   | |   |               | |    | |
     ---+   +-+   +-+   +-           ---+ +----+ +----

     High duty cycle = bright       Low duty cycle = dim

The PWM signal "modulates" the 25KHz oscillation of the inverter.
The inverter runs in "bursts". Since, on average, the CCFL tube
is maintained in the conducting state, it continues to run with the
lower of the output voltages that the inverter offers (like 700VAC).
If the duty cycle were to go so low as to quench the tube, then
the tube would likely not be able to light up again. So the
duty cycle can be varied over a large range. And the adjustment
range using PWM, is larger than with the analog control method.

If you are seeing a dim output, you'd check to see if the PWM
signal was still working. The PWM frequency has to be higher
than the scan rate of the panel, and preferably a frequency
unrelated. Otherwise, there could be a "beat note" or distracting
visual interference, between the CCFL light source, and the
image provided by the LCD panel itself. What I can't tell
you, is what the correct amplitude of the control signal
should be. Perhaps the amplitude is wrong, instead of just
the pulse width. A wrong amplitude might affect the
operation in unpredictable ways.

This picture, shows the CCFL lamp voltage present, during one
of the pulses in the PWM waveform above. So bursts of 700-1000VAC
sine waves are fed to the CCFL, to control the intensity.

http://www.maxim-ic.com/images/appnotes/3997/3997Fig02.gif

And I don't know, what chip on the motherboard, makes that
PWM signal. It would be logical for it to come from the
laptop GPU (graphics processor), but could just as easily
come from some other chip.

You can see in this example, there are five signals feeding
into the inverter (on the ribbon cable). +5V for the logic
gates of the chip. +12V to the MOSFETs pumping the transformer.
Ground signal. And two control signals - an ENABLE signal
to turn the lamp completely ON and OFF, as well as LPWM
to control the intensity while it is ON. Presumably the
allowed logic level on LPWM, is a 5V amplitude signal.

http://circuits.datasheetdir.com/59/OB3302-circuits.jpg

Some inverters will have more signals than that. To
understand inverter operation, you would note the part
number of the IC on the inverter, then dig up a datasheet,
to get a better idea what signals could be on the ribbon cable.

*******

As a "non-electrician", all you can do is verify the ribbon
cable is properly plugged in.

    Paul