Why are LCD TV backlights (CCFL) not driven straight from the mains?

[eem2am]

Hello,

In an LCD TV, a CCFL is used to provide the light source.

Most of the power that is consumed by an LCD TV is consumed by this light source.

The power supply arangement in an LCD TV is that the mains is first stepped down to an isolated 24V DC.

This is then used as the input voltage for a high voltage output inverter which powers the CCFL.

....the CCFL voltage is not isolated from its input voltage (24V DC) to the same degree as the 24V DC supply is isolated from the mains.

Why do they do all this as above?

Why not just drive a CFL backlight directly from the mains?....it'd be much simpler and more efficient.

 

[J_M]

Why not just drive a CFL backlight directly from the mains?....it'd be much simpler and more efficient.

Cold cathodes are not only more efficient than hot cathode (CFL) by a factor of 10 or more they are also much simpler/cheaper to dim (as required in an LCD TV).

 

[RCinFLA]

The CCFL operate at higher voltage then mains. They also operate more efficiently at about 20 kHz to 40 kHz. And finally, you would not like the 50 Hz or 60 Hz flicker if driven directly at mains frequency.

Most TV's now have to make a power factor rating regulations which is why they operate through a PF corrected switching power supply.

 

[eem2am]

J_M: Thanks for the point of dimmability which is very important


I am now seekign info on the web on efficiency of CFL vs CCFL.

I would be surprised if CCFLs were 10 times more efficient than CFL.........since CFLs convert 20% of their received electrical power into light......so if CCFLs are 10 times more efficient, then they are converting around 100% of their received electrical power into light..........which is amazing...............and having worked at Europe's largest lighting company, i am sure they would have been doing a lot more CCFL if they were that great