How to repair an LCD inverter
2010-05-05
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Oriented explanation of the steps to repair an inverter of an LCD TV
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An LCD TV has the same circuits as in a conventional TV, but otherwise has circuits that are new, such as the inverter and the whole display (lamps, and matrix liquid crystal display).

The breakdown section that most often is the inverter and power supply, as these elements are those that handle higher voltages and are therefore more circuits get hot.

The inverter is responsible for making calls CCFL lamps illuminate or serve cold cathode lighting or backlight, and can control the screen brightness through these.
We find a circuit that converts alternating voltages in (DC-AC converter).

We usually have a voltage 12-24V DC (12V to 24V about 22 inches and 26 inches and up). This power supply through an oscillator circuit is converted to an AC voltage much higher and will depend on the type of lamp to be exciting, it also depends on the size in inches of the TV for a much larger voltage lamps and higher. Here we have a circuit like the high voltage output of a conventional TV where we had a line transformer that gave us the order of 20KV to turn the picture tube. In the present case we have a high voltage circuit with a corresponding high transformer for each lamp, but with less stress.

The main components in an inverter are the integrated circuit oscillator, which generates a signal or oscillation activated after a power circuit consists of MOS transistors, FETS and transformers, this last set of components eventually excite the lamps.

The inverter main power comes from the power supply, which is present at power-up TV, or at least most work this way, the voltage value depends on the size of the LCD TV which will be between 12 and 24 volts DC.
Normally, these tensions are silkscreened on printed circuit boards as VCC or VBL. The output of the inverter is an AC voltage values from 1200 to 4000 volts. Important not to be measured with the tester or multimeter on these outputs to lamps that can spoil. The best thing to measure here is to use the oscilloscope attenuator.

When we try to repair an inverter should be noted that for this to work properly, you must meet certain conditions that are related to the control signals between the microprocessor and its own inverter. These signals have various functions such as calibration and protection, calibration adjustment are related to the brightness of the lamps.
This handles the brightness signal usually comes as DIMMER silkscreen, and she determined that lighting level should give the lamps to improve the black level.
LCD TVs tend to have a gray level before a neutral black, so the DIMMER should be well regulated.
The control signals are related to the activation and proper functioning of the inverter, inverter most have a line called BL_ON activation (backlight on), OFF or ON ENABLE / DISABLE. Normally OFF = ON = Low level and high level.
We must bear in mind that if this line is not active, the inverter will not work even if you get the right diet. Might also have the right and power in the high state BL_ON the inverter does not work for entering into protected status. The security lines are activated for various reasons, such as a lamp ceases to operate, one or more MOS-FETs have been made are short or open circuit, a transformer is also shorted or open although it may happen that the very line of defense is going to land because of a faulty port on the inverter or the microprocessor.
The line of protection in the inverter can be identified as ERROR, INV_ERROR, errror INV_DISABLE or OUT.

Repairing an inverter can be easy or frustrating depending on the defect.
The most common fault usually output transformers, which either open or cross. They also often fail the MOS-FETS that may be independent or be encapsulated in groups within an integrated circuit (an example FDS8958).
Another cause of failure may be the lamps. To solve this problem, or opens fully to display visual access to them or you must have a CCFL lamp tester, we can do with an old inverter that has an exit at least for a lamp.
To a lesser extent the oscillator circuit failure (often cross over to mass excitation output MOSFET), and almost always breaks down is that the MOSFET circuit that also causes damage to the oscillator.
Sometimes the failure of the inverter are usually filtering problems of food caused by electrolytic capacitors at the source.

Measuring a MOSFET is difficult because there are plenty of them, essentially be measured not in short the drain and source terminals, which typically measure some ohms, but when they are encapsulated as an integrated circuit must resort to the road datasheet for information or technical parameters of these.
I find by experience is better I measure with an oscilloscope that has the components measurement function (such as HAMEG oscilloscopes). With this feature you see the characteristic curves of transistor, as if excited by simply touching a finger in the door of the transistor.

To check the transformer it is easiest to measure its resistance with a tester but can lead to error.
Another way is to swing by injecting a signal and see its output to the oscilloscope. Here we see if it distorts the output signal or lack thereof.

If we believe that the damage is being caused by the integrated oscillator must analyze all voltages (VCC, OUT, etc. ..), it is best to find the technical data and test operation. First of all check that it is fed.


If damaged CCFL lamps only thing possible is replaced by the correct type and form as the original.

If we have problems with inverter control lines to do various tests to find out where the problem.
A control line can go ashore without even having any problem with the inverter, which will cause the inverter not turn on the lights.
It is best to place that line of control has been based on the wrong manual, and proceed to do the following:

With a separate power source, fix it in 5 or 3.3 volts DC and a series resistance between 470 ohms 4.7ky injected into the control pin of the line such as ENABLE / DISABLE or line ERROR, INV_ERROR, INV_DISABLE or errror OUT, if the TV responds to say that this line is grounded and the resistance will return the high state 5v or close to it, although of course the line of control can be activated when the reverse logic pin changes from low (0v) to high (5v).
Then if all went well solder the resistor in series with the standby voltage power supply and control line.

If after many attempts to repair, do not get the desired result, it is best to search the market an inverter that has the same number of outputs of lamps that we want to repair and adjust the pins of one and the other to make the substitution . This method is working best if we adapt well and we're putting a new inverter with which guarantee the repair for quite some time.
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