How Backlight Works in LCD TV

*CCFL & EEFL Inverter Board Troubleshooting*

Now that we have a basic understanding of the comparable differences with
CCFL & EEFL lamps, lets get started in troubleshooting the circuitry that
drives both lamp technologies. These circuits are called Inverter boards
(trade Name), and we will start with inverter boards used for CCFL
backlights, which are usually labeled as Master & Slave inverter board
circuitry. The term ‘master/slave’ inverter boards (half-bridge configuration)
are derived from the electrical functionality the two boards perform together
to start-up, balance, and run the CCFL lamps (Master = high side/ Slave =
low side). The master inverter board balances the over-all load condition
between its self (i.e. master) and its slave.

The master inverter board controls the output of the slave inverter board, and the output of the slave
inverter is determined and formatted by the load condition of the master
inverter.

 The slave inverter simply reacts to 3 basic load conditions from the
master inverter, and that is the firing load, start load, and operating load
conditions commanded by the master inverter board. Together both master
and slave inverter boards perform as one CCFL backlight control system.

Inverter boards simply take a low DC input voltage (by using analog circuits
or monolithic control IC), and inverts the DC voltage to an AC sine wave
output to run the florescent lamps, also to separately control the dimming
and brightening affects of the lamps. There are various ways in-which
inverter boards can fail to produce an AC sine output, causing the lamps not
to ignite. If the lamps don’t light up or if there’s a quick flicker of light
across the display screen, this usually due to one of the inverter boards
(master/slave) having a start up fault.

A brief flash of light across the display screen could also be a bad CCFL
lamp, I find this to be a rare fault (through servicing many backlight faults
over long years) but by no means don’t under estimate the fact that lamps do
fail and I have replaced some, and it’s a tedious task just to access the lamps
especially for 50+ inch screens.

85% of the backlight faults I have serviced, stem from faulty components on
inverter boards, 10% have been ‘START’ or INV-ON’ command signals
from the main digital board, 24V or 12V supply missing or reduced from
SMPS power board or D-Dim signals from T-con board.

 The other 5% have
been the actual CCFL lamp its self. T-con boards that have a flat white
ribbon cable connected too the master inverter are mainly used for Digital￾Dim (P-DIM OSD) which is different from A-Dim (Analog Dim). P-Dim
controls the BURST-oscillator and synchronizes with OSD backlight
command options from the customer menu according to room light
conditions (PWM_Sync Dim).

The T-con gets its P-Dim command input from the main digital board, and if
a T-con with this type of OSD P-Dim connection develops a fault (in its P￾Dim output circuitry) it can cause the master inverter to either cancel start￾up, or can cause the master inverter to get stuck in “Reset’ which have
caused the backlights to turn on & off (blinking) in 1+ second intervals with
no picture raster (just blinking backlights only).

There is no picture raster
because the T-con has a fault condition affecting its logic output (D-Dim) to the master inverter, making the master inverter ‘reset’ over and over,
therefore the main digital board has no run-time to boot-up successfully.

When the LVDS cable from the main digital board was disconnected from
the T-con the blinking of the lamps have stopped, and the backlights stayed
on as a result. As stated previously, there are many ways an inverter board
can fail to produce an AC sine output to the lamps, and the inverter failure
can be direct or indirect.

Direct failure meaning the backlight problem is a result of faulty
component(s) directly on the inverter boards, or bad connection points.

Indirect failure meaning the backlight problem is not a result of faulty
component(s) on the inverter board its self, but rather an inadequacy or
absence of a critical input voltage or a crucial input command from another
board that feeds the inverter.

The example given above with D-Dim’ is a descent reference to an indirect
inverter board failure. It wasn’t the master inverter board causing the
blinking of the lamps but instead it was an indirect fault of the T-con’s D￾Dim output too the master-INV board, causing the inverters to strobe the
lamps Note, the screen actually has a running picture, but without any backlights
the picture image can’t be illuminated (viewable).

 This kind of backlight
problem indicates a failure on one of the inverter boards (master/slave).

When inverters fail to drive the lamps, sometimes there are more than 1 or 2
fault scenarios that can cause this to happen. Inverter boards get their power
from the main power supply circuit of the TV; better known as a switch￾mode-power-supply (SMPS) inverter boards usually operate from a 24VDC
input supply source (some use 12VDC input supply).

 If the power board
fails to deliver the inverters supply voltage (24V or 12V) then its quite
obvious the inverter boards simply will not function without a source supply
voltage. Also many inverters receive an ‘INV_ON’ logic-high command
from the main digital board usually labeled INV_ON (or similar). The main
digital board sends this INV_ON command (approximately 3V) to the power
board (through a switching IC after 24V/12V supply is verified), which triggers the master inverter to turn-on. If the input INV_ON signal is not
switched properly from the power board or is not being sent from the main
digital board, then the master inverter wont actuate start up (via inverter
control IC)…. even though the inverters are getting their 24V/12V supply
they wont start up without the INV_ON signal (enable) which turns on the
inverter control IC.

 The burst-oscillating mode from the control IC is
dependent on the PWM and DC external input. There are more failures that
can cause the inverters not to produce a working HV output,