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Analysis of the principle and circuit of large screen LCD backlight and high voltage driving circuit

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LCD TV screen is belong to passive luminous display device, liquid crystal screen emits no light of its own, it needs to realize the screen with backlight light, backlight lamp issued light transmitted through liquid crystal screen, using liquid crystal molecules under the action of the electric field is controlled by the light line (the light modulation) to form an image, so a piece of liquid crystal screen imaging must with back light source to become a complete screen, color is rich, high-quality image is to be displayed, the requirements of the backlight spectral range is wide, close to daylight color in order to maximize the now nature of various colors. At present, the liquid crystal screen backlight lamp is generally used as a cathode fluorescent lamp CCFL (cold cathode fluorescent lamp). Large screen LCD TV to ensure that there is sufficient brightness, contrast and uniformity of the entire screen brightness, are using a multi tube system, 32 inch screen generally use 16 light tubes, 47 inch screen generally use 24 light tubes. Power consumption of each tube is about 8W calculation, a 32 inch screen LCD TV backlight power consumption to 130W, a 47 inch LCD TV backlight lamp power consumption reached nearly 200 W (along with the rest of the circuit power consumption, a 32 inch screen LCD TV power consumption in around 200W)

The structure and working principle of cold cathode fluorescent lamp
Cold cathode fluorescent lamp (CCFL is a gas discharge light emitting device and its structure similar to the common fluorescent lamp, different is used instead of tungsten cathode made of Ni, Ta and Zr metal without heating electron emission electrode cold cathode, the tube is filled with low pressure mercury vapor, under strong electric field, cold cathode electron emission enable lamp mercury atomic excitation and ionization, lamp current and radiating 253.7nm UV, UV excited again tube wall of the fluorescent powder coating and luminous.

The characteristics of cold cathode fluorescent lamp
Cold cathode fluorescent lamp is a high non linear load, which trigger (start) voltage is generally three times to work (maintenance) (voltage, voltage and tube length and diameter of the cold cathode fluorescent lamp) in the start, when the voltage has not reached the trigger value (1200 ~ 1600V) when the lamp is positive resistance (number of MOhm), once you reach the trigger value generated inside the discharge lamp ionization current, the current increases, the voltage across the lamp down was a negative resistance characteristic figure 2, so the cold cathode fluorescent lamp trigger lit after limiting device must be in the circuit, the current limit in a tube a rated value, otherwise it will cause excessive current burning lamp, the current is too small and difficult to maintain the light.


                                                  Fig. 1

Fig. 1 is the voltage and current characteristics of the cold cathode fluorescent lamp. The vertical axis indicates the current of the lamp tube. The horizontal axis indicates the voltage of the tube. Before the first lit the lamp, lamp voltage rise at both ends of the horizontal axis, when the tube has not yet reached the trigger voltage (1200V ~ 1600V), lamp current basically no, when reach the trigger voltage (1200V ~ 1600V) tube internal ionization of mercury atoms, generating current, lamp light due to current rise, a sharp decline in voltage both ends of tubes, and maintained at 400V ~ 600V, this time due to the external circuit limiting function of voltage at both ends of the lighting tube basically maintained at about 1/3 of the control voltage of small amplitude voltage at both ends of the lamp will cause changes in the lamp current greatly (current greatly change, directly affect the service life of the lamp tube). It is important to maintain the stability of voltage at both ends of the tube after lighting the lamp.

Cold cathode fluorescent lamp under the good environment of power supply, life can reach 25000 to 50000 hours, which is similar to the life of the CRT), namely the lamp power supply frequency, waveform, trigger voltage, sustaining voltage and the current of the lamp tube to conform to the characteristics of the lamp. To control the brightness of the lamp, the waveform requirements more stringent, otherwise and the service life of the lamp tube is greatly shortened (some screen backlight and LCD screen is made of a whole is not a change of, tube is damaged, the whole screen body has become waste.  
Cold cathode fluorescent lamp requires high efficiency, long life, so the power supply and the lamp excitation part is to meet the lamp characteristics, power supply must be AC sine wave, frequency is 40K ~ 60K, the trigger voltage is 1200 ~ 1600V, the sustain voltage is about 1/3 points (from the trigger voltage of the tube length and diameter of the decision), because each lamp voltage / current characteristics are not exactly the same, the lamp can not be directly used in parallel (although the application of series can be lit, because of the characteristics of the differences in brightness of the lamp tube in series is different, the whole screen will cause uneven brightness), so in a multi tube liquid crystal screen, each of the tubes are all with a single high voltage transformer, figure 2 is a backlight high-voltage Samsung 32 inch screen driver board, the screen has 16 tubes, the driver board has 16 high-voltage output transformer, figure 3 High voltage transformer. 
The backlight lamp high voltage drive board and liquid crystal screen is supporting the factory, the different types and sizes of LCD screen, the pressure driven plate is not interchangeable.


                                                           Fig. 2

                                                               Fig. 1


The brightness of the cold cathode fluorescent lamp control; LCD TV and CRT TV should also like to make brightness control, but because of nonlinear characteristic of cold cathode fluorescent lamp with its unique, depending on the brightness control method of current control to change the voltage of the ordinary, there are certain difficulties, to achieve by increasing the lamp current can be increased although light brightness, but increase the current change brightness is limited, and the excessive current will cause the lamp electrode damage, resulting in shortening the lifespan, also reduce the current control effect of brightness is extremely limited, and reduces the discharge current to maintain the lead out, weak current discharge lamp lamp life is a disadvantage. So the current brightness control of cold cathode fluorescent lamp adopts pulse dimming, the specific method is to use 30 ~ PWM; low frequency 200Hz pulse wave (PWM wave pulse width controlled CPU) modulation of the continuous oscillation of high pressure on the application of cold cathode fluorescent lamp, the continuous oscillation wave into intermittent oscillation, thus to control the brightness, the control principle is intermittent; to stop power supply of cold cathode fluorescent lamp in a very short period, the stop time is very short, not enough to make the ionization state of lamp disappears, but the intensity of ultraviolet radiation decreased excitation fluorescent powder on the wall of the tube is reduced, the brightness is decreased, as long as the control PWM pulse duty cycle, can change the lamp on / off cycle time than in a guide, so as to achieve the purpose of controlling the average brightness of the lamp tube.