Backlight Board Typical Case Repair Analysis
Feb 23, 2023
At present, the LCD TV backlight board models are numerous, different manufacturers, different sizes of LCD, the backlight board is different, even the same manufacturer of the same model of LCD using the backlight board is sometimes different. And the backlight board is the largest part of the LCD TV power consumption, but also the largest current, the highest voltage part, so the failure rate is high, because these backlight boards rarely have schematic diagrams, to analyze the failure from the principle, troubleshooting is very difficult, but also very blind. Engineers met with faulty backlight board mostly using the method of replacing the backlight board to solve, but some models of backlight board is difficult to find, so we should be in the actual maintenance as soon as possible to master the overhaul method.
The functional composition of the backlight board
Backlight board components are mainly composed of four parts: oscillator, power amplification, high voltage output, protection detection. The function of each part is as follows.
Backlight panel function introduction
Oscillator: generate 60-100KHZ oscillating square wave signal.
Power amplification: Amplify the signal generated by the oscillator to enough power to light the lamp.
High voltage output: Boost the signal after power amplification and convert it to sine wave output to light up the CCFL lamp.
Protection detection: detect the voltage output of the backlight board and the working current and working status of the CCFL lamp, if there is any abnormality, control the oscillator to stop the output and enter the protection state.
Backlight board working conditions
CPU provides backlight switch control signal (BL-ON) and backlight brightness control signal DIM to the backlight board.
The power supply system of the backlight board includes the power supply of the power amplification part, the oscillation control part, and the protection detection part. For the power amplification part, the current power supply of the backlight board of 32-inch and above screen is 24V, 6-10A, and the backlight board of 26-inch and below screen size mostly adopts 12V power supply, and the power supply of the oscillation control and protection detection part is generally 5V or 6V, which is obtained by 12V or 24V after step-down and voltage regulation.
The three elements of the chip work, namely.
1. chip power supply
2. chip start energy pin (high level)
3. the dimming pin (high level)
Backlight common failures
1. Backlight does not light
2. Backlight flashing protection
3. Backlight bright for a period of time to protect
Backlight board repair method
When repairing the backlight board to master the characteristics of the backlight tube, lighting principle and the composition of the backlight board, working principle. Although the shape of the backlight board, component model composition varies, but the basic principle is the same, as long as the theoretical basis, you can analyze the fault, the backlight board fault types are rarely 3-4 kinds, the backlight board's high-voltage output and power amplifier circuit principle is simple, more than the use of full-bridge / half-bridge drive, generally using MOS tubes or power modules.
Backlight board maintenance first to ensure that the power supply part of the normal work. That is, the standby power supply 3.3V or 5V, 12V, 24V, PFC circuit is working properly, before the backlight system can be overhauled.
Inspection can be removed from the power supply board to the main board of the line, the standby voltage 5V respectively with STB (standby control) BRI (dimming) and BL-ON (backlight control) 3 feet connected together, the normal backlight board should be able to light up, this step can determine whether the main board problems or power supply board problems.
If the backlight tube does not light at all, it means that there is no pulse signal added to the backlight inverter. We can overhaul the backlight circuit when the backlight system can be seen as a line sweep circuit to overhaul.
Backlight control chip has a characteristic, in the chip just power on about 2 seconds is not protected by any feedback pins control, is a pulse output, after starting to get its normal feedback signal from the detection pins before entering the normal working state. If there is no feedback signal or abnormal feedback signal after the start, the chip will enter the protection state and stop outputting excitation signal. We just use this feature to first detect the pulse output pin of the backlight control IC when the power is on, to see if it has a voltage output when the power is on, you can use an oscilloscope to measure the signal waveform of the output pin, you can also use a multimeter AC voltage file to measure whether there is a pulse output.
We determine the fault point based on the voltage value here is in the backlight control chip as the core of the circuit, or in the back-end circuit. (It should be noted that the chip protection after the need to power off and then start after about 2 seconds of output) If there is a signal output, you can determine the fault in the rear drive part. Otherwise, the fault in the backlight chip control part. We can measure whether there is an AC excitation signal output at the output of its backstage excitation transformer (because the operating frequency of the backlight circuit is about 56KHZ beyond the frequency response of our ordinary multimeter, so there is no accurate voltage value, the digital meter is generally ~ 30 volts or so, the mechanical meter in ~ 10 volts or so. (Need to emphasize that no matter how much the voltage value of the 2 windings to obtain the voltage value must be the same, otherwise there is a problem)
Excitation form is not the same, but its maintenance principle is the same the following is its several common circuit cut diagram.
When the output pulse of its excitation transformer is normal, we will continue to measure step by step to the back stage until we find the short circuit point of the pulse.
The above overhaul process is to overhaul the backlight circuit does not light up at all after the power on a fast and effective overhaul method.
Caution.
When measuring the pulse signal, please first put the multimeter test pen first to measure the location and then turn on the power, to prevent the internal resistance of the multimeter to affect the normal working state of the rear stage causing unnecessary losses, or directly disconnect the power supply of the last stage to push the tube first.
Backlight board repair cases
Fault one: a backlight is damaged (or broken)
Failure phenomenon: the moment of power on the screen can be seen to start the light-emitting action, sometimes you can see a dark band, but then extinguished, after extinguished, the LCD screen to the light can see the dark image, the sound is normal.
Failure analysis: When a backlight tube break, its corresponding lamp break detection circuit will sample the signal to the backlight tube break detection input of the oscillation control IC, the internal protection control circuit after a protection delay to cut off the excitation signal output. This is the reason why the screen can be seen to have light-emitting action at the moment of power on, but then extinguished. Start the light because other backlight tubes are normally lit; then extinguished is due to the corresponding protection circuit of the broken backlight tube action protection, the backlight board is closed output caused.
Treatment: For this phenomenon of the power on flashing once and off, to finally confirm the fault, the oscillation control IC backlight tube break detection input (generally OLP) voltage to 2V or more, the purpose is to lift the backlight tube break protection control, there are also IC to reduce the voltage of this foot to cancel the protection, other lamps all lit, two of the damaged backlight tube area is slightly dark.
This is an emergency repair method, because this situation requires the replacement of the LCD screen to deal with, if not replace the LCD screen will have to replace the backlight tube, and replace the lamp maintenance is difficult and risky. At this time, it is recommended that the brightness of the entire LCD screen to adjust to a larger, due to the LCD screen internal light guide, damage to a lamp is not a great impact on the use of the effect. Hands-on ability, if the location of the broken lamp in the middle of the screen, please replace this lamp to the top of the screen, so that the normal viewing is basically invisible. Doing so will not affect the safety of the circuit and can be used with confidence.
When a lamp is damaged, because the lamp does not conduct, there is no voltage drop across its terminals, corresponding to the high-voltage output of the overvoltage protection circuit back to action, then the corresponding step-up transformer input circuit should be disconnected to prevent overvoltage protection action.
Sometimes a lamp aging, its start performance is not good, as shown by: slow start, start time greater than 2S, or even longer, more than the backlight control IC set delay protection time, still can not start normally, the protection control circuit to determine the backlight tube disconnection fault, also close the excitation signal output. This situation is handled in the same way as above.
Fault two: the backlight does not flash, the fuse on the backlight board blown
Failure analysis: fuse blown means that the backlight board has a serious overcurrent, short circuit (mild overcurrent generally will not blow the fuse) has been a short-circuit damage to the components, at this time do not rashly change a fuse power on, otherwise the fault will further expand, and even affect the safety of the other circuits of the machine.
This failure is the power amplifier circuit components, MOS tube or complementary MOS module breakdown, short circuit, which is the most important cause of this failure.
Inspection and repair of such failures, you should first carefully observe the circuit board, especially MOS tubes, MOS tube drive module there is no color yellowing, cracking abnormalities, the general backlight board has several groups of the same drive circuit, you can compare the appearance of the fault found in the general area. This situation can be replaced with the same type of drive MOS tube or drive MOS module, if not the same model, you can choose the parameters are basically the same, especially the on-resistance is similar to the replacement, in order to maintain the balance of the bridge back arm.
Sometimes, the backlight board fuse blown, the power module breakdown short circuit, but the appearance of the power module color does not have any change, which is to use the multimeter resistance file for detection, in the case of the backlight board is not energized, respectively, compared with the detection of each power module feet to ground resistance value, if there is a significantly low resistance value of the module, can be found to have a fault.
Other components are damaged, generally will not cause the fuse to blow.
Treatment: For the backlight is not bright, fuse blown backlight board, check to the power module damage and replacement, you can power on the LCD screen to observe the brightness, and pay attention to observe whether the backlight board has overheating, smoke phenomenon; otherwise, should also check whether the step-up transformer itself short-circuit, because the step-up transformer short-circuit or local short-circuit will eventually also cause the power module breakdown, short-circuit.
If the step-up transformer is damaged, because there are two kinds of backlight boards, one using a step-up transformer backlight board, using EEFL lamps; or the use of balanced inductance method to ensure that the CCFL lamps are driven in parallel. This transformer due to the size of the production process, insulation performance can be guaranteed, generally not easy to damage.
Another is the use of multiple step-up transformers to support multiple CCFL lamps backlight board, a transformer is responsible for a lamp, 16 lamps will require 16 transformers, currently found that this transformer damage a higher percentage. If damaged, must be replaced with the same type of step-up transformer, if not, the emergency treatment method to the failure of the step-up transformer, the primary disconnect, its corresponding backlight tube does not work, but also to lift the backlight board backlight tube disconnect protection, to ensure that the work of other lamps do not receive the impact. To have pieces later is, and then restore the original state.
Fault three: the backlight does not light, the fuse is intact
Failure analysis: this type of failure involves a relatively large range, should first check the backlight board power supply, control interface DC power supply, backlight switch signal, and brightness control level is normal, to confirm that there is no problem before considering the backlight board overhaul, the general backlight board its power supply, control interface and other sockets are clearly marked on the role of the pins, foot by foot detection can determine the fault site.
Secondly, to confirm the oscillation control IC VCC power supply and enable terminal (ENA) start voltage, power amplifier circuit power supply (12V or 24V) is normal, if normal to consider whether the current state of protection, the actual maintenance, found that the backlight control IC damage cases are extremely rare, mostly for the protection of the detection circuit problems.
Treatment: in the maintenance of the backlight does not light, the fuse is intact fault, the most effective way to determine is to lift the protection control function, this is because such failures are generally a part of the backlight board has a fault, which starts the protection control circuit, and then cut off the output of the excitation signal, resulting in the phenomenon of no light.
Different models of oscillation control IC, its over-voltage protection (OVP \ CLAMP) threshold level is also slightly different, generally 2-3V, in general, the over-voltage protection terminal for the low level of normal work, more than the threshold level into the over-voltage protection state, if you want to lift the over-voltage protection control, directly to the over-voltage protection port ground can be.
Backlight tube break protection port OLP, generally low level protection, high level 1.5V when normal operation, if you want to lift the backlight tube break protection, just add a higher level to this pin than the starting control threshold can
In lifting the backlight tube break protection at the same time, must also be over-voltage protection circuit, output current detection feedback circuit is lifted at the same time, all the protection is lifted, power on to observe the backlight board there is no smoke, fire phenomenon, and ready to close the power supply, close the power supply, touch the backlight board drive output components by hand, there is no obvious temperature rise, hot situation, if the temperature is too high, the fault is In this branch, the fault repair, overvoltage, overcurrent detection feedback circuit should be restored to the original state, the lamp break protection does not have to be restored, there is no impact on the circuit.
In the maintenance of the backlight board, more parts of the fault is the power amplifier module (MOS tube) and high-voltage transformer, these two parts are also the most repeated circuit parts on the backlight board.
Note: After the backlight board has been energized to work, do not use a multimeter or oscilloscope to measure its high-voltage output, which will cause "pull arc"
When measuring with an oscilloscope, in the case that the high-voltage output is not connected to the backlight tube, the series resonant output circuit is in an open state, and the measured waveform must not be a sine wave.
The above describes part of the backlight board maintenance experience for reference.
