How to solve the current in the backlight applicat

2022-08-16
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How to solve the current distribution problem in the backlight application of large screen LCD

because the cost of cold cathode fluorescent lamp (CCFL) is very low, usually large screen LCD uses CCFL as the backlight to produce uniform white light. However, the use of light emitting diodes (LEDs) as backlights is attracting the attention of major manufacturers. LED is better than C in size, energy efficiency, spectral purity, mechanical strength, reliability and elimination of harmful substances such as mercury. CFL is also becoming more and more popular

white light can come from a single white LED; It can also be produced by three independent R-G LEDs whose chromatograms are well matched with LCD pixel color filters. In the printing process, the friction between the equipment and the substrate (film) accounts for an important element -b led. This technology can greatly improve the luminous efficiency and color range, so that the display effect is clearer and brighter. At present, LCD with CCFL backlight can only produce% of NTSC colors, while the new display with LED backlight can produce all colors defined in NTSC system, and even colors outside the scope of NTSC definition. Using the ultra fast switching time of LED, the backlight intensity can be adjusted, so as to further enhance the image contrast and reduce the tailing phenomenon caused by fast moving images

to replace CCFL in small screen LCD monitors (usually 19 inches), three color LED devices can be deployed around the housing to replace CCFL tubes. Usually, only the light source is replaced (from CCFL to LED string), and the housing, light guide and light film can remain unchanged. For larger LCD (more than 20 inches), due to the high luminous flux requirements, the LED moment can be directly deployed behind the LCD panel. Its left end is a rotatable collet array, and the necessary diffusion layer and light film are sandwiched between these led matrices. The size of the LED matrix varies with the panel size, generally hundreds of LEDs. In order to ensure uniform illumination, it is necessary to use a special diffraction scattering layer on the standard light film. With the progress of technology, the output brightness of semiconductor chips will be higher and higher, so the number of LEDs in series or matrix LEDs can be reduced, thereby further reducing the cost of materials and systems

of course, designers must also face many challenges, such as maintaining spectral consistency in the case of temperature changes and led aging. However, this technology is promising. Some major notebook computer manufacturers are planning to launch more products with LED backlights. Large screen TV manufacturers have also invested a lot of resources in this area, so LED backlight is expected to become more and more popular in the consumer market

recently, there has been a breakthrough in backlight panel technology, and the newly launched high brightness white LED can be used in the backlight panel of LCD. These new LEDs need a 4V DC power supply with a very small package size and a power of 200W. This design adopts the patented technology invented by a non listed company, which is committed to developing and selling innovative high dynamic range (HDR) image technology. This technology can be used to make higher brightness displays. By using enhanced video processing algorithm to adjust the brightness of LED, the brightness can be 10 times higher than that of traditional LCD. Each LED in the backlight is independently addressable, so the light intensity can be dynamically changed frame by frame or even micro region by region. This technology can achieve a higher dynamic range of the display. Compared with previous technologies, it can make the dark places darker and the bright places brighter, thus significantly improving the clarity of the image. This technology has been applied in large screen displays (37 and 46 inch high-definition displays) used in some high-end imaging devices

these new LEDs need a 4V DC power supply with a very small package size and a power of up to 200W. Previous products used a 500A 5.5V power supply, so it was very difficult to handle such a large current distribution in the display. Transit bus converter (BCM) is a V · I chip module designed for high-power LED applications to solve the current distribution problem. BCM adopts the patented sinusoidal amplitude converter (SAC) topology of VICOR company, which has advanced power density, efficiency and low noise performance. The BCM has an overall dimension of only 1.1 square inches and a typical weight of 15 grams. It can provide an isolated and step-down voltage for non isolated load point converter (Nipol). Because of its fast response and low output noise, the aluminum electrolytic or tantalum capacitors with limited service life commonly used at the load end can be reduced or eliminated, thereby effectively saving the circuit board area, materials and overall system cost

now, the robot transports multiple preforms on a plate spring to the multi cavity lower mold of hp-rtm press, which can distribute 48V voltage to any position with lower current, and then locally reduce the voltage to 4V of high current. There are 4 small boards on each large board to form a system, so there are a total of 16 BCM in each system. The result is that each board operates on a 200A, 4V power supply. On the contrary, if the system is supported by a large power supply, it needs an 800A power supply. Of course, such a power supply is very dangerous. This is why the 48V voltage of 20a is distributed to each board. This power supply has higher manageability and fairly good fusibility

bcm has become an appropriate solution due to the following factors. The first is the size and efficiency of BCM, which does not need to use any special radiator. Secondly, it works at 48V safety voltage (SELV). BCM can also provide output voltages of different standards optimized for different applications. Generally, the system input voltage in the above applications needs to be adjusted to the working voltage of 4.1V to 4.2V. Since BCM is a converter rather than a voltage regulator, designers can use up to 48V input voltage to obtain the specified output voltage they need

other backlight schemes require constant current to drive high-power LED arrays connected in series. In general, constant current is used to ensure predictable luminous brightness and chromaticity values. Although V · I chip non isolated pre regulator module (PRM) regulator and multiplier current voltage conversion module (VTM) voltage converter are mainly designed to provide stable voltage by using adaptive loop voltage stabilization method, they can also achieve constant output current through simple circuit modification

compared with traditional methods, using PRM and VTM to provide constant current has many advantages. Using VTM in the system can multiply the current at the load point, and the output current of VTM is proportional to its input current (as shown in Formula 1 below)

iout=iin/k

where k is the K coefficient of VTM, or simply called the step-down ratio

therefore, in controlled current applications, the output current of VTM can be controlled by detecting and adjusting its input current. Detecting lower current requires smaller sensors, which consumes lower power and improves overall efficiency. In addition, the V · I chip itself also has high efficiency and power density, which makes the whole LED system small in size, low in temperature, and maximizes the output lumens per watt of power consumption

most known led types can be driven with a single prm+vtm pair. The PRM is pre configured with an internal voltage loop to adjust the output voltage of the PRM to a set value. The internal working principle of PRM should be well understood, because the external constant current circuit is designed to work with the internal voltage control loop, and the output current of VTM can be adjusted by changing the PRM voltage reference value

The simplified block diagram of PRM internal voltage control loop is shown in Figure 1. The internal reference is connected to the SC port of PRM through a 10k resistor and a 0.22uf capacitor to realize the soft start function. SC voltage can be adjusted by increasing external resistance or applying external voltage. The voltage applied at the SC port should not exceed 6vdc

Figure 1: functional diagram of PRM internal error amplifier

sc voltage is buffered and fed back to the error amplifier through the resistance voltage divider, which is represented as a gain block of 0.961. R68 forms the upper half of the voltage detection resistance voltage divider. This resistance is fixed for each PRM. The lower half of the voltage divider is formed by adding a resistor between the OS pin and SG (ROS). Equation 1 defines PRM output as a function of VSC and ROS. It can be seen from Formula 1 that for a given ROS resistance, adjusting the SC voltage can determine the PRM output voltage. This is the method used by the external current control circuit to control the output

where: VSC is the voltage at the SC pin of PRM, ROS is the resistance between OS and SG of PRM, and R68 is the internal resistance of PRM

the recommended current control circuit is shown in Figure 2. Since VTM is a current multiplier, the output current of VTM can be adjusted by its input current. The advantage of this method is that the current can be detected before the VTM current multiplication circuit (at a higher voltage point), thereby reducing the I2R power consumption of the external shunt circuit. In addition, the control circuit remains on the main circuit (PRM) side, so there is no need to isolate the feedback signal

Figure 2: recommended current control circuit, which is composed of voltage reference, shunt resistance, differential amplifier and error amplifier

the above circuit is composed of voltage reference, shunt resistance, differential amplifier and error amplifier. The low-end detection circuit is realized at the PRM output with an op amp configured as a differential amplifier. The voltage on the shunt resistor (R1) is detected and amplified by the gain multiple determined by resistors R2 to R5. The reference voltage is generated using an accurately adjustable shunt reference and connected to the in-phase terminal of the error amplifier. This is the voltage used by the error amplifier to compare with the differential amplifier output (VSENSE). The output of the error amplifier (veao) is connected to the SC through resistors R7 and R8, thereby realizing the adjustment of the PRM output set point. The error amplifier will adjust the PRM output voltage until VSENSE is equal to the reference voltage Vref. This will force the VTM input current and VTM output current to become constants determined by VREF

PRM with a simple external current detection circuit can be used as a constant current source. VTM converts the fractional bus voltage into a voltage suitable for different colors of LEDs from 0.8 to 55v (for example, 6V for blue LEDs, 14V for amber, 24V for green)

the flexibility of the fractional power architecture (FPA) allows the same PRM to drive different VTMS (different K factors) for different colors of LEDs. At the same time, because only another PRM model is used, the same VTM can remain unchanged under different input source voltages. In addition, VTM can be placed next to the load point with high current to minimize voltage drop and power consumption

after adding a PFC front end with a high-voltage BCM bus converter (380V) in the upstream, a 48V bus can be provided to the prm/vtm or BCM in the downstream to drive LED drivers for different colors (low-power LEDs). This will become a PFC AC to DC power supply, which can supply power to 0.8V to 55v high-power LED arrays

48v to 4V BCM is a high efficiency (>94%) sinusoidal amplitude converter with narrow input range. It adopts a new DC DC converter topology, which can be used to supply power to non isolated pol converters or as an independent source. BCM is very small, with an area of only 1.1in2, power of 210W per square inch, and light weight, only 0.5oz, but power density of 876w/in3

prm is a very efficient non isolated voltage regulator, which can accept a wide range of input voltage and provide stable but adjustable output voltage or "fractional ratio bus" through boost or step-down technology. PRM can be used as a non isolated regulator alone, or it can be used together with VTM to achieve complete, high efficiency and power

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