ccfl inverters for LCD video

[nomenclator]

According to my understanding, in LCD televisons and computer monitors: the fluoresecent backlight lamp gets AC power from an inverter; the invertor has changed low voltage dc power to high voltage ac power (around 12 volts dc to something like maybe 30,000 vac); the low voltage dc power for the inverter input is provided by a power supply device, which provides dc power for everything in the LCD video device that needs power; and the power supply device converts 120 volts ac, to several different dc voltages, negatively or positively going, relative to ground, as required.

My question is why first convert ac to dc, then back again to ac? Obviously I don't know much about this subject, but common sense would tell me that a device that converts 120 vac to 30,000 vac be simpler to design and manufacture, and require less components to make it work, than a device that not only steps up the voltage, but also has to convert dc to ac? Why is my common sense misleading me? Don't ac inverters require a whole extra section to convert the low vdc to high vac, that could be left out if the high vac power for the ccfl, was obtained from a device that converts low vac to high vac, basicly a transformer with a regulated ac output?

It isn't like the LCD video device is going to be used somewhere where 120 vac isn't available. It is there? If you have ac available, why convert ac to dc, then back again to ac?

 

[betwixt]

The reason is simply that the transformer needed to generate the high voltage would be expensive, large and heavy. The incoming AC power is at 50 or 60Hz, a frequency which requires an iron cored transformer to work efficiently. By converting to DC first, it can be used to generate power at a much higher frequency where a much smaller ferrite cored transformer can be used.

I think your estimate of 30Kv for the backlight is probably excessive. I have no idea what it really is but my guess is that it is more likely to be 1Kv or less.

The other reason for going through DC first is that modern LCD screens use dynamic backlighting, the amount of light is adjusted in proportion to the scene being shown to increase the perceived contrast of the picture. That is relatively eassy to do at low voltage DC but difficult at high voltage AC.

Brian.