LCD TV:- LLC converter or Eight flybacks to power backlight?

[eem2am]

Hello,

The following LCD TV contains eight 25W CCFL backlights, (which are powered by eight CCFL inverters...one inverter for each CCFL)

SHARP LC32D12E 32" HD READY LCD TV WITH FREEVIEW - Tesco.Direct


.....each of the CCFL inverters is powered by the same input voltage rail which is 24V.
-This 24V rail is the secondary of a 200W LLC offline resonant converter.
-The input voltage for this LLC resonant converter is the 400V output rail of a boost power factor correction circuit.

.....Now, would it not be more efficient to simply power each CCFL via a 25 Watt flyback with Vout = 24V?.............
-in other words, have eight identical 25W flybacks switching the 400V bus down to 24V.......
-then have the individual CCFL inverters powered by this flyback output voltage rail.

...The point is that conduction losses in SMPS's go as I^2.R........and it is the squared term that means that having the eight flybacks will be more efficient.....since , for example, a halving of the RMS current, means a quartering of the conduction losses.

 

[Orson Cart]

Have you considered the extra RFI generated by eight flybacks running off the 400V rail? - most likely this is the reason they have the 8 x smaller 24VDC input inverters to power the CCFLs, plus the inverters have to generate the strike voltage for the CCFL's (usually several kV) and this might pose problems for a flyback running off 400V (4kV mosfet/igbt devices needed?), the inverters generate symmetric AC into the CCFLs which a flyback might have trouble doing and finally the inverters may have been intended to be controlled by the main TV uP and having the inverters on the mains side gives rise to isolation problems if controlled by a uP on the isolated 24V side. Regards, Orson Cart.