LCD liquid crystal backlight to protect maintenance ideas

by:Genyu     2020-01-04
In order to remove the protection function, the pin can be grounded when overhauling the LCD backlight. Some chips remove the protection function from the SS soft start. There is also a 3 microampere current source in the chip to charge the capacitor externally connected to the SS terminal. When the charging reaches 3 V, the backlight circuit enters the working state before reaching 3 V, the backlight circuit does not work. It should be noted that the current source current inside the SS and time terminals is extremely small (3 microamps) Therefore, when the voltage of the foot is measured by the meter, the level of the foot will be pulled to a low level through the internal resistance of the meter, resulting in no high level being measured, and the voltage measured repeatedly is a very low voltage, in fact, the circuit is normal at this time. There are two oscillators inside the backlight chip: one is the high-frequency oscillators CT and TR, which generate a 50K high-frequency drive square wave and drive the lamp tube to emit light, which is called the lamp tube operating frequency (This high frequency oscillator is not controlled). Whether the LCD liquid crystal backlight is designed independently or in two-in-one mode, the backlight chip has a timer or Ctime pin, which is familiar to everyone, and some are marked as CP or SCP. This pin is a protective delay external capacitor terminal, when the lamp is over-voltage, over-current, and open circuit fault occurs, it is not immediately protected, but delayed for about 2 seconds. After the power is turned on, the capacitor is charged by the 3 microampere current source in the chip, because the voltage on the capacitor is lower than the threshold within 2 seconds after starting up, the passing door of the protection signal is turned off. Even if overvoltage and overcurrent occur at this time, the protection signal is cut off and cannot trigger the chip to stop working. After 2 seconds, the capacitor will be charged more than 3 V, and the protection door will be opened. At this time, Overvoltage and overcurrent will occur, and protection will appear immediately. Therefore, in order to remove the protection function, the foot can be grounded during maintenance. Some chips remove the protection function from the SS soft start. There is also a 3 microampere current source in the chip to charge the capacitor externally connected to the SS terminal. When the charging reaches 3 V, the backlight circuit enters the working state before reaching 3 V, the backlight circuit does not work. It should be noted that because the current source current inside the SS and time terminals is extremely small (3 microamps) Therefore, when the voltage of the foot is measured by the meter, the level of the foot will be pulled to a low level through the internal resistance of the meter, resulting in no high level being measured, and the voltage measured repeatedly is a very low voltage, in fact, the circuit is normal at this time. There are two oscillators inside the backlight chip: one is the high-frequency oscillators CT and TR, which generate a 50K high-frequency drive square wave and drive the lamp tube to emit light, which is called the lamp tube operating frequency (This high frequency oscillator is not controlled). The other is backlight control: low frequency oscillator LCT, LRT or BCT, BRT, which generates 200K low frequency oscillation to adjust the brightness of the lamp tube. The duty cycle of this oscillator, that is, the width of the square wave, is controlled by the backlight brightness adjustment voltage from the motherboard and the lamp current feedback. When the temperature of the loop mirror rises, the current of the lamp tube will increase, and the current IS fed back into the terminal of the chip, thus narrowing the positive square wave output by the backlight controlled low frequency oscillator and narrowing the duty ratio of the lamp tube's luminous time, the lamp current drops to normal. The voltage at the current feedback end of the lamp tube IS lower than 0. At 4 V, the comparator in the chip will be considered as an open circuit (Positive and negative gate reversal), Enter the lamp open circuit protection. When the pin is higher than 2 V, the comparator inside the chip will think that the lamp current is too large and the winding of the high-voltage transformer is short-circuited and enters overcurrent protection. When the high voltage feedback end VS of the lamp tube is higher than 1. At 2 V, it will be considered that the high voltage is too high to enter the overvoltage protection. For LCD TV, the lamp will be turned off when it is turned on, or it will be turned off after 3 times of flashing. Maintenance ideas--There may be a bad lamp tube; Backlight high voltage transformer short circuit; Lamp overvoltage, overcurrent, lamp open circuit detection circuit is not good; Backlight chip oscillation circuit is poor, etc. At this time, first detect the high-voltage transformer secondary line package, compared with other line packages (Resistance)To find a bad high-voltage transformer. If it is normal, after the screen is turned on and the lamp is turned on, whether the lamp is not bright, the lamp is dark red, the two ends of the lamp are blackened, and the lamp card holder is connected. If the lamp is normal, detect whether there is any abnormality in the over-current and over-voltage detection circuit of the lamp tube. If it is normal, it is necessary to detect the abnormality of the resistance and capacitance, transistors, etc. on the periphery of the backlight chip.
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